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Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (/ˌæstəˈrɔɪdiə/). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on the seabed in all the world’s oceans, from warm, tropical zones to frigid, polar regions. They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.

Starfish are marine invertebrates. They typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of defense. The Asteroidea occupy several significant ecological roles. Starfish, such as the ochre sea star (Pisaster ochraceus) and the reef sea star (Stichaster australis), have become widely known as examples of the keystone species concept in ecology. The tropical crown-of-thorns starfish (Acanthaster planci) is a voracious predator of coral throughout the Indo-Pacific region, and the Northern Pacific seastar is on the list of the World’s 100 Worst Invasive Alien Species.

The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather sparse, as starfish tend to disintegrate after death. Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture. They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.

Anatomy

Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms.^[3] The Antarctic Labidiaster annulatus can have over fifty.^[4]

Mapping the expression patterns of genes that express differently across the body axes suggests that one could think of the body of a starfish as a disembodied head walking about the sea floor on its lips. The known markers for trunk structures are expressed only in internal tissues rather than on the surface. Only the front part of the axis, which specifies head-related structures, is represented on the body surface.^[5]

Body wall

The body wall consists of a thin cuticle, an epidermis consisting of a single layer of cells, a thick dermis formed of connective tissue and a thin coelomic myoepithelial layer, which provides the longitudinal and circular musculature. The dermis contains an endoskeleton of calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice.^[6] They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in a tessellated manner and form the main covering of the aboral surface.^[7] Some are specialised structures such as the madreporite (the entrance to the water vascular system), pedicellariae and paxillae.^[6] Pedicellariae are compound ossicles with forceps-like jaws. They remove debris from the body surface and wave around on flexible stalks in response to physical or chemical stimuli while continually making biting movements. They often form clusters surrounding spines.^[8][9] Paxillae are umbrella-like structures found on starfish that live buried in sediment. The edges of adjacent paxillae meet to form a false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected. All the ossicles, including those projecting externally, are covered by the epidermal layer.^[6]

Several groups of starfish, including Valvatida and Forcipulatida, possess pedicellariae.^[8] In Forcipulatida, such as Asterias and Pisaster, they occur in pompom-like tufts at the base of each spine, whereas in the Goniasteridae, such as Hippasteria phrygiana, the pedicellariae are scattered over the body surface. Some are thought to assist in defence, while others aid in feeding or in the removal of organisms attempting to settle on the starfish’s surface.^[10] Some species like Labidiaster annulatus, Rathbunaster californicus and Novodinia antillensis use their large pedicellariae to capture small fish and crustaceans.^[11]

There may also be papulae, thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water. These serve a respiratory function.^[12] The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices. This arrangement enables both easy flexion of the arms by the starfish and the rapid onset of stiffness and rigidity required for actions performed under stress.^[13]

Water vascular system

The water vascular system of the starfish is a hydraulic system made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and gas exchange. Water enters the system through the madreporite, a porous, often conspicuous, sieve-like ossicle on the aboral surface. It is linked through a stone canal, often lined with calcareous material, to a ring canal around the mouth opening. A set of radial canals leads off this; one radial canal runs along the ambulacral groove in each arm. There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla. These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove. There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows. The interior of the whole canal system is lined with cilia.^[14]

When longitudinal muscles in the ampullae contract, valves in the lateral canals close and water is forced into the tube feet. These extend to contact the substrate. Although the tube feet resemble suction cups in appearance, the gripping action is a function of adhesive chemicals rather than suction.^[15] Other chemicals and relaxation of the ampullae allow for release from the substrate. The tube feet latch on to surfaces and move in a wave, with one arm section attaching to the surface as another releases.^[16][17] Some starfish turn up the tips of their arms while moving which gives maximum exposure of the sensory tube feet and the eyespot to external stimuli.^[18]

Having descended from bilateral organisms, starfish may move in a bilateral fashion, particularly when hunting or in danger. When crawling, certain arms act as the leading arms, while others trail behind.^[3][19][9] Most starfish cannot move quickly, a typical speed being that of the leather star (Dermasterias imbricata), which can manage just 15 cm (6 in) in a minute.^[20] Some burrowing species from the genera Astropecten and Luidia have points rather than suckers on their long tube feet and are capable of much more rapid motion, “gliding” across the ocean floor. The sand star (Luidia foliolata) can travel at a speed of 2.8 m (9 ft 2 in) per minute.^[21] When a starfish finds itself upside down, two adjacent arms are bent backwards to provide support, the opposite arm is used to stamp the ground while the two remaining arms are raised on either side; finally the stamping arm is released as the starfish turns itself over and recovers its normal stance.^[19]

Apart from their function in locomotion, the tube feet act as accessory gills. The water vascular system serves to transport oxygen from, and carbon dioxide to, the tube feet and also nutrients from the gut to the muscles involved in locomotion. Fluid movement is bidirectional and initiated by cilia.^[14] Gas exchange also takes place through other gills known as papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Oxygen is transferred from these to the coelomic fluid, which acts as the transport medium for gasses. Oxygen dissolved in the water is distributed through the body mainly by the fluid in the main body cavity; the circulatory system may also play a minor role.^[22]

Digestive system and excretion

The gut of a starfish occupies most of the disc and extends into the arms. The mouth is located in the centre of the oral surface, where it is surrounded by a tough peristomial membrane and closed with a sphincter. The mouth opens through a short oesophagus into a stomach divided by a constriction into a larger, eversible cardiac portion and a smaller pyloric portion. The cardiac stomach is glandular and pouched, and is supported by ligaments attached to ossicles in the arms so it can be pulled back into position after it has been everted. The pyloric stomach has two extensions into each arm: the pyloric caeca. These are elongated, branched hollow tubes that are lined by a series of glands, which secrete digestive enzymes and absorb nutrients from the food. A short intestine and rectum run from the pyloric stomach to open at a small anus at the apex of the aboral surface of the disc.^[23]

Primitive starfish, such as Astropecten and Luidia, swallow their prey whole, and start to digest it in their cardiac stomachs. Shell valves and other inedible materials are ejected through their mouths. The semi-digested fluid is passed into their pyloric stomachs and caeca where digestion continues and absorption ensues.^[23] In more advanced species of starfish, the cardiac stomach can be everted from the organism’s body to engulf and digest food. When the prey is a clam or other bivalve, the starfish pulls with its tube feet to separate the two valves slightly, and inserts a small section of its stomach, which releases enzymes to digest the prey. The stomach and the partially digested prey are later retracted into the disc. Here the food is passed on to the pyloric stomach, which always remains inside the disc.^[24] The retraction and contraction of the cardiac stomach is activated by a neuropeptide known as NGFFYamide.^[25]

Because of this ability to digest food outside the body, starfish can hunt prey much larger than their mouths. Their diets include clams and oysters, arthropods, small fish and gastropod molluscs. Some starfish are not pure carnivores, supplementing their diets with algae or organic detritus. Some of these species are grazers, but others trap food particles from the water in sticky mucus strands that are swept towards the mouth along ciliated grooves.^[23]

The main nitrogenous waste product is ammonia. Starfish have no distinct excretory organs; waste ammonia is removed by diffusion through the tube feet and papulae.^[22] The body fluid contains phagocytic cells called coelomocytes, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to the tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water. Some waste may also be excreted by the pyloric glands and voided with the faeces.^[22]

Starfish do not appear to have any mechanisms for osmoregulation, and keep their body fluids at the same salt concentration as the surrounding water. Although some species can tolerate relatively low salinity, the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine environments.^[22]

Sensory and nervous systems

Although starfish do not have many well-defined sense organs, they are sensitive to touch, light, temperature, orientation and the status of the water around them. The tube feet, spines and pedicellariae are sensitive to touch. The tube feet, especially those at the tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food.^[24] There are eyespots at the ends of the arms, each one made of 80–200 simple ocelli. These are composed of pigmented epithelial cells that respond to light and are covered by a thick, transparent cuticle that both protects the ocelli and acts to focus light. Many starfish also possess individual photoreceptor cells in other parts of their bodies and respond to light even when their eyespots are covered. Whether they advance or retreat depends on the species.^[26]

While a starfish lacks a centralized brain, it has a complex nervous system with a nerve ring around the mouth and a radial nerve running along the ambulacral region of each arm parallel to the radial canal. The peripheral nerve system consists of two nerve nets: a sensory system in the epidermis and a motor system in the lining of the coelomic cavity. Neurons passing through the dermis connect the two.^[26] The ring nerves and radial nerves have sensory and motor components and coordinate the starfish’s balance and directional systems.^[12] The sensory component receives input from the sensory organs while the motor nerves control the tube feet and musculature. The starfish does not have the capacity to plan its actions. If one arm detects an attractive odour, it becomes dominant and temporarily over-rides the other arms to initiate movement towards the prey. The mechanism for this is not fully understood.^[26]

Circulatory system

The body cavity contains the circulatory or haemal system. The vessels form three rings: one around the mouth (the hyponeural haemal ring), another around the digestive system (the gastric ring) and the third near the aboral surface (the genital ring). The heart beats about six times a minute and is at the apex of a vertical channel (the axial vessel) that connects the three rings. At the base of each arm are paired gonads; a lateral vessel extends from the genital ring past the gonads to the tip of the arm. This vessel has a blind end and there is no continuous circulation of the fluid within it. This liquid does not contain a pigment and has little or no respiratory function but is probably used to transport nutrients around the body.^[27]

Secondary metabolites

Starfish produce a large number of secondary metabolites in the form of lipids, including steroidal derivatives of cholesterol, and fatty acid amides of sphingosine. The steroids are mostly saponins, known as asterosaponins, and their sulphated derivatives. They vary between species and are typically formed from up to six sugar molecules (usually glucose and galactose) connected by up to three glycosidic chains. Long-chain fatty acid amides of sphingosine occur frequently and some of them have known pharmacological activity. Various ceramides are also known from starfish and a small number of alkaloids have also been identified. The functions of these chemicals in the starfish have not been fully investigated but most have roles in defence and communication. Some are feeding deterrents used by the starfish to discourage predation. Others are antifoulants and supplement the pedicellariae to prevent other organisms from settling on the starfish’s aboral surface. Some are alarm pheromones and escape-eliciting chemicals, the release of which trigger responses in conspecific starfish but often produce escape responses in potential prey.^[28] Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.^[29]

Life cycle

Sexual reproduction

Most species of starfish are gonochorous, there being separate male and female individuals. These are usually not distinguishable externally as the gonads cannot be seen, but their sex is apparent when they spawn.^[30] Some species are simultaneous hermaphrodites, producing eggs and sperm at the same time, and in a few of these the same gonad, called an ovotestis, produces both eggs and sperm.^[31] Other starfish are sequential hermaphrodites. Protandrous individuals of species like Asterina gibbosa start life as males before changing sex into females as they grow older. In some species such as Nepanthia belcheri, a large female can split in half and the resulting offspring are males. When these grow large enough they change back into females.^[32]

Each starfish arm contains two gonads that release gametes through openings called gonoducts, located on the central disc between the arms. Fertilization is generally external but in a few species, internal fertilization takes place. In most species, the buoyant eggs and sperm are simply released into the water (free spawning) and the resulting embryos and larvae live as part of the plankton. In others, the eggs may be stuck to the undersides of rocks.^[33] In certain species of starfish, the females brood their eggs – either by simply enveloping them^[33] or by holding them in specialised structures. Brooding may be done in pockets on the starfish’s aboral surface,^[34][30] inside the pyloric stomach (Leptasterias tenera)^[35] or even in the interior of the gonads themselves.^[31] Those starfish that brood their eggs by “sitting” on them usually assume a humped posture with their discs raised off the substrate.^[36] Pteraster militaris broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch.^[34] In these brooding species, the eggs are relatively large, and supplied with yolk, and they generally develop directly into miniature starfish without an intervening larval stage.^[31] The developing young are called lecithotrophic because they obtain their nutrition from the yolk as opposed to “planktotrophic” larvae that feed in the water column. In Parvulastra parvivipara, an intragonadal brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch.^[37] Brooding is especially common in polar and deep-sea species that live in environments unfavourable for larval development^[30] and in smaller species that produce just a few eggs.^[38][39]

In the tropics, a plentiful supply of phytoplankton is continuously available for starfish larvae to feed on. Spawning takes place at any time of year, each species having its own characteristic breeding season.^[40] In temperate regions, the spring and summer brings an increase in food supplies. The first individual of a species to spawn may release a pheromone that serves to attract other starfish to aggregate and to release their gametes synchronously.^[41] In other species, a male and female may come together and form a pair.^[42][43] This behaviour is called pseudocopulation^[44] and the male climbs on top, placing his arms between those of the female. When she releases eggs into the water, he is induced to spawn.^[41] Starfish may use environmental signals to coordinate the time of spawning (day length to indicate the correct time of the year,^[42] dawn or dusk to indicate the correct time of day), and chemical signals to indicate their readiness to breed. In some species, mature females produce chemicals to attract sperm in the sea water.^[45]

Larval development

Most starfish embryos hatch at the blastula stage. The original ball of cells develops a lateral pouch, the archenteron. The entrance to this is known as the blastopore and it will later develop into the anus—together with chordates, echinoderms are deuterostomes, meaning the second (deutero) invagination becomes the mouth (stome); members of all other phyla are protostomes, and their first invagination becomes the mouth. Another invagination of the surface will fuse with the tip of the archenteron as the mouth while the interior section will become the gut. At the same time, a band of cilia develops on the exterior. This enlarges and extends around the surface and eventually onto two developing arm-like outgrowths. At this stage the larva is known as a bipinnaria. The cilia are used for locomotion and feeding, their rhythmic beat wafting phytoplankton towards the mouth.^[8]

The next stage in development is a brachiolaria larva and involves the growth of three short, additional arms. These are at the anterior end, surround a sucker and have adhesive cells at their tips. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk formed from the ventral arms and sucker. Metamorphosis now takes place with a radical rearrangement of tissues. The left side of the larval body becomes the oral surface of the juvenile and the right side the aboral surface. Part of the gut is retained, but the mouth and anus move to new positions. Some of the body cavities degenerate but others become the water vascular system and the visceral coelom. The starfish is now pentaradially symmetrical. It casts off its stalk and becomes a free-living juvenile starfish about 1 mm (0.04 in) in diameter. Starfish of the order Paxillosida have no brachiolaria stage, with the bipinnaria larvae settling on the seabed and developing directly into juveniles.^[8]

Asexual reproduction

Main article: Asexual reproduction in starfish

Some species of starfish in the three families Asterinidae, Asteriidae and Solasteridae are able to reproduce asexually as adults either by fission of their central discs^[46] or by autotomy of one or more of their arms.^[47] Which of these processes occurs depends on the genus. Among starfish that are able to regenerate their whole body from a single arm, some can do so even from fragments just 1 cm (0.4 in) long.^[48] Single arms that regenerate a whole individual are called comet forms. The division of the starfish, either across its disc or at the base of the arm, is usually accompanied by a weakness in the structure that provides a fracture zone.^[49]

The larvae of several species of starfish can reproduce asexually before they reach maturity.^[50] They do this by autotomising some parts of their bodies or by budding.^[51] When such a larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development.^[52] Though this costs it time and energy and delays maturity, it allows a single larva to give rise to multiple adults when the conditions are appropriate.^[51]

Regeneration

Main article: Starfish regeneration

Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.^[48] A few can regrow a complete new disc from a single arm, while others need at least part of the central disc to be attached to the detached part.^[22] Regrowth can take several months or years,^[48] and starfish are vulnerable to infections during the early stages after the loss of an arm. A separated limb lives off stored nutrients until it regrows a disc and mouth and is able to feed again.^[48] Other than fragmentation carried out for the purpose of reproduction, the division of the body may happen inadvertently due to part being detached by a predator, or part may be actively shed by the starfish in an escape response.^[22] The loss of parts of the body is achieved by the rapid softening of a special type of connective tissue in response to nervous signals. This type of tissue is called catch connective tissue and is found in most echinoderms.^[53] An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms.^[54]

Lifespan

The lifespan of a starfish varies considerably between species, generally being longer in larger forms and in those with planktonic larvae. For example, Leptasterias hexactis broods a small number of large-yolked eggs. It has an adult weight of 20 g (0.7 oz), reaches sexual maturity in two years and lives for about ten years.^[8] Pisaster ochraceus releases a large number of eggs into the sea each year and has an adult weight of up to 800 g (28 oz). It reaches maturity in five years and has a maximum recorded lifespan of 34 years.^[8] The average lifespan of a starfish is 35 years, and larger starfish species typically live longer than their smaller counterparts.^[55]

Ecology

Distribution and habitat

Echinoderms, including starfish, maintain a delicate internal electrolyte balance that is in equilibrium with sea water, making it impossible for them to live in a freshwater habitat.^[16] Starfish species inhabit all of the world’s oceans. Habitats range from tropical coral reefs, rocky shores, tidal pools, mud, and sand to kelp forests, seagrass meadows^[56] and the deep-sea floor down to at least 6,000 m (20,000 ft).^[57] The greatest diversity of species occurs in coastal areas.^[56]

Diet

Most species are generalist predators, eating microalgae, sponges, bivalves, snails and other small animals.^[24][58] The crown-of-thorns starfish consumes coral polyps,^[59] while other species are detritivores, feeding on decomposing organic material and faecal matter.^[58][60] A few are suspension feeders, gathering in phytoplankton; Henricia and Echinaster often occur in association with sponges, benefiting from the water current they produce.^[61] Various species have been shown to be able to absorb organic nutrients from the surrounding water, and this may form a significant portion of their diet.^[61]

The processes of feeding and capture may be aided by special parts; Pisaster brevispinus, the short-spined pisaster from the West Coast of America, can use a set of specialized tube feet to dig itself deep into the soft substrate to extract prey (usually clams).^[62] Grasping the shellfish, the starfish slowly pries open the prey’s shell by wearing out its adductor muscle, and then inserts its everted stomach into the crack to digest the soft tissues. The gap between the valves need only be a fraction of a millimetre wide for the stomach to gain entry.^[16] Cannibalism has been observed in juvenile sea stars as early as four days after metamorphosis.^[63]

Ecological impact

Starfish are keystone species in their respective marine communities. Their relatively large sizes, diverse diets and ability to adapt to different environments makes them ecologically important.^[64] The term “keystone species” was in fact first used by Robert Paine in 1966 to describe a starfish, Pisaster ochraceus.^[65] When studying the low intertidal coasts of Washington state, Paine found that predation by P. ochraceus was a major factor in the diversity of species. Experimental removals of this top predator from a stretch of shoreline resulted in lower species diversity and the eventual domination of Mytilus mussels, which were able to outcompete other organisms for space and resources.^[66] Similar results were found in a 1971 study of Stichaster australis on the intertidal coast of the South Island of New Zealand. S. australis was found to have removed most of a batch of transplanted mussels within two or three months of their placement, while in an area from which S. australis had been removed, the mussels increased in number dramatically, overwhelming the area and threatening biodiversity.^[67]

The feeding activity of the omnivorous starfish Oreaster reticulatus on sandy and seagrass bottoms in the Virgin Islands appears to regulate the diversity, distribution and abundance of microorganisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles.^[68] Organisms that dislike this disturbance are replaced by others better able to rapidly recolonise “clean” sediment. In addition, foraging by these migratory starfish creates diverse patches of organic matter, which may play a role in the distribution and abundance of organisms such as fish, crabs and sea urchins that feed on the sediment.^[69]

Starfish sometimes have negative effects on ecosystems. Outbreaks of crown-of-thorns starfish have caused damage to coral reefs in Northeast Australia and French Polynesia.^[59][70] A study in Polynesia found that coral cover declined drastically with the arrival of migratory starfish in 2006, dropping from 50% to under 5% in three years. This had a cascading effect on the whole benthic community and reef-feeding fish.^[59] Asterias amurensis is one of a few echinoderm invasive species. Its larvae likely arrived in Tasmania from central Japan via water discharged from ships in the 1980s. The species has since grown in numbers to the point where they threaten commercially important bivalve populations. As such, they are considered pests,^[71] and are on the Invasive Species Specialist Group’s list of the world’s 100 worst invasive species.^[72]

Sea Stars (starfish) are the main predators of kelp-eating sea urchins. Satellite imagery shows that sea urchin populations have exploded due to starfish mass deaths, and that by 2021, sea urchins have destroyed 95% of California’s kelp forests.^[73]

Threats

Starfish may be preyed on by conspecifics, sea anemones,^[74] other starfish species, tritons, crabs, fish, gulls and sea otters.^{[38][71][75][76]} Their first lines of defence are the saponins present in their body walls, which have unpleasant flavours.^[77] Some starfish such as Astropecten polyacanthus also include powerful toxins such as tetrodotoxin among their chemical armoury, and the slime star can ooze out large quantities of repellent mucus. They also have body armour in the form of hard plates and spines.^[78] The crown-of-thorns starfish is particularly unattractive to potential predators, being heavily defended by sharp spines, laced with toxins and sometimes with bright warning colours.^[79] Other species protect their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities.^[78]

Several species sometimes suffer from a wasting condition caused by bacteria in the genus Vibrio;^[75] however, a more widespread wasting disease, causing mass mortalities among starfish, appears sporadically. A paper published in November 2014 revealed the most likely cause of this disease to be a densovirus the authors named sea star-associated densovirus (SSaDV).^[80] The protozoan Orchitophrya stellarum is known to infect the gonads of starfish and damage tissue.^[75] Starfish are vulnerable to high temperatures. Experiments have shown that the feeding and growth rates of P. ochraceus reduce greatly when their body temperatures rise above 23 °C (73 °F) and that they die when their temperature rises to 30 °C (86 °F).^[81][82] This species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide.^[83] It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs like the stomach.^[84]

Starfish and other echinoderms are sensitive to marine pollution.^[85] The common starfish is considered to be a bioindicator for marine ecosystems.^[86] A 2009 study found that P. ochraceus is unlikely to be affected by ocean acidification as severely as other marine animals with calcareous skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when the pH is lowered. Researchers found that when P. ochraceus were exposed to 21 °C (70 °F) and 770 ppm carbon dioxide (beyond rises expected in the next century), they were relatively unaffected. Their survival is likely due to the nodular nature of their skeletons, which are able to compensate for a shortage of carbonate by growing more fleshy tissue.^[87]

Evolution

Fossil record

Echinoderms first appeared in the fossil record in the Cambrian. The first known asterozoans were the Somasteroidea, which exhibit characteristics of both groups.^[88] Starfish are infrequently found as fossils, possibly because their hard skeletal components separate as the animal decays. Despite this, there are a few places where accumulations of complete skeletal structures occur, fossilized in place in Lagerstätten – so-called “starfish beds”.^[89]

By the late Paleozoic, the crinoids and blastoids were the predominant echinoderms, and some limestones from this period are made almost entirely from fragments from these groups. In the two major extinction events that occurred during the late Devonian and late Permian, the blastoids were wiped out and only a few species of crinoids survived.^[88] Many starfish species also became extinct in these events, but afterwards the surviving few species diversified rapidly within about sixty million years during the Early Jurassic and the beginning of the Middle Jurassic.^[90][91] A 2012 study found that speciation in starfish can occur rapidly. During the last 6,000 years, divergence in the larval development of Cryptasterina hystera and Cryptasterina pentagona has taken place, the former adopting internal fertilization and brooding and the latter remaining a broadcast spawner.^[92]

Diversity

See also: List of echinodermata ordersVideo showing the tube feet movement of a starfish

The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830.^[93] It is derived from the Greek aster, ἀστήρ (a star) and the Greek eidos, εἶδος (form, likeness, appearance).^[94] The class Asteroidea belongs to the phylum Echinodermata. As well as the starfish, the echinoderms include sea urchins, sand dollars, brittle and basket stars, sea cucumbers and crinoids. The larvae of echinoderms have bilateral symmetry, but during metamorphosis this is replaced with radial symmetry, typically pentameric.^[12] Adult echinoderms are characterized by having a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres.^[95] Starfish are included in the subphylum Asterozoa, the characteristics of which include a flattened, star-shaped body as adults consisting of a central disc and multiple radiating arms. The subphylum includes the two classes of Asteroidea, the starfish, and Ophiuroidea, the brittle stars and basket stars. Asteroids have broad-based arms with skeletal support provided by calcareous plates in the body wall^[90] while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed “vertebrae”.^[96]

The starfish are a large and diverse class with over 1,900 living species. There are seven extant orders, Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida and Velatida^[1] and two extinct ones, Calliasterellidae and Trichasteropsida.^[2] Living asteroids, the Neoasteroidea, are morphologically distinct from their forerunners in the Paleozoic. The taxonomy of the group is relatively stable but there is ongoing debate about the status of the Paxillosida, and the deep-water sea daisies, though clearly Asteroidea and currently included in Velatida, do not fit easily in any accepted lineage. Phylogenetic data suggests that they may be a sister group, the Concentricycloidea, to the Neoasteroidea, or that the Velatida themselves may be a sister group.^[91]

Living groups

Brisingida (2 families, 17 genera, 111 species)^[97]Species in this order have a small, inflexible disc and 6–20 long, thin arms, which they use for suspension feeding. They have a single series of marginal plates, a fused ring of disc plates, a reduced number of aboral plates, crossed pedicellariae, and several series of long spines on the arms. They live almost exclusively in deep-sea habitats, although a few live in shallow waters in the Antarctic.^[98][99] In some species, the tube feet have rounded tips and lack suckers.^[100]

Forcipulatida (6 families, 63 genera, 269 species)^[101]Species in this order have distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. They tend to have robust bodies^[102] and have tube feet with flat-tipped suckers usually arranged in four rows.^[100] The order includes well-known species from temperate regions, including the common starfish of North Atlantic coasts and rock pools, as well as cold-water and abyssal species.^[103]Notomyotida (1 family, 8 genera, 75 species)^[104]These starfish are deep-sea dwelling and have particularly flexible arms. The inner dorso-lateral surfaces of the arms contain characteristic longitudinal muscle bands.^[1] In some species, the tube feet lack suckers.^[100]

Paxillosida (7 families, 48 genera, 372 species)^[105]This is a primitive order and members do not extrude their stomach when feeding, lack an anus and have no suckers on their tube feet. Papulae are plentiful on their aboral surface and they possess marginal plates and paxillae. They mostly inhabit soft-bottomed areas of sand or mud.^[8] There is no brachiolaria stage in their larval development.^[106] The comb starfish (Astropecten polyacanthus) is a member of this order.^[107]

Spinulosida (1 family, 8 genera, 121 species)^[108]Most species in this order lack pedicellariae and all have a delicate skeletal arrangement with small or no marginal plates on the disc and arms. They have numerous groups of short spines on the aboral surface.^[109][110] This group includes the red starfish Echinaster sepositus.^[111]Valvatida (16 families, 172 genera, 695 species)^[112]Most species in this order have five arms and two rows of tube feet with suckers. There are conspicuous marginal plates on the arms and disc. Some species have paxillae and in some, the main pedicellariae are clamp-like and recessed into the skeletal plates.^[110] This group includes the cushion stars,^[113] the leather star^[114] and the sea daisies.^[115]Velatida (4 families, 16 genera, 138 species)^[116]This order of starfish consists mostly of deep-sea and other cold-water starfish often with a global distribution. The shape is pentagonal or star-shaped with five to fifteen arms. They mostly have poorly developed skeletons with papulae widely distributed on the aboral surface and often spiny pedicellariae.^[117] This group includes the slime star.^[118]

Extinct groups

Extinct groups within the Asteroidea include:^[2]

• † Calliasterellidae, with the type genus Calliasterella from the Devonian and Carboniferous^[119]
• † Palastericus, a Devonian genus^[120]
• † Trichasteropsida, with the Triassic genus Trichasteropsis (at least 2 species)^[2]

Phylogeny

External

Starfish are deuterostome animals, like the chordates. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree.^[121] The times at which the clades diverged are shown under the labels in millions of years ago (mya).

Bilateria	Xenacoelomorpha NephrozoaDeuterostomiaChordata EchinodermataEleutherozoaEchinozoaHolothuroidea Echinoidea AsterozoaOphiuroidea Asteroidea Crinoidea c. 500 mya>540 myaProtostomiaEcdysozoa Spiralia 610 mya650 mya

Internal

The phylogeny of the Asteroidea has been difficult to resolve, with visible (morphological) features proving inadequate, and the question of whether traditional taxa are clades in doubt.^[2] The phylogeny proposed by Gale in 1987 is:^[2][122]

	† Palaeozoic AsteroidsPaxillosidaValvatida, including Velatida, Spinulosida (not a clade)[2]Forcipulatida, including Brisingida

The phylogeny proposed by Blake in 1987 is:^[2][123]

	† Palaeozoic Asteroids† Calliasterellidae† Compasteridae† TrichasteropsidaBrisingidaForcipulatidaSpinulosidaVelatidaNotomyotidaValvatidaPaxillosida

Later work making use of molecular evidence, with or without the use of morphological evidence, had by 2000 failed to resolve the argument.^[2] In 2011, on further molecular evidence, Janies and colleagues noted that the phylogeny of the echinoderms “has proven difficult”, and that “the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods”. They presented a phylogenetic tree for the living Asteroidea only; using the traditional names of starfish orders where possible, and indicating “part of” otherwise, the phylogeny is shown below. The Solasteridae are split from the Velatida, and the old Spinulosida is broken up.^[124]

	Solasteridae and part of Spinulosida, e.g. Stegnaster and part of Valvatida, e.g. AsterinaOdontasteridae, which was a part of ValvatidaPaxillosidapart of Spinulosida, e.g. Echinaster, part of Valvatida, e.g. ArchasterForcipulatidaBrisingida with part of Velatida, e.g. Caymanostella and part of Forcipulatida, e.g. StichasterVelatida except for Solasteridae

	Notomyotida (not analysed)

Human relations

In research

Starfish are deuterostomes, closely related, together with all other echinoderms, to chordates, and are used in reproductive and developmental studies. Female starfish produce large numbers of oocytes that are easily isolated; these can be stored in a pre-meiosis phase and stimulated to complete division by the use of 1-methyladenine.^[125] Starfish oocytes are well suited for this research as they are large and easy to handle, transparent, simple to maintain in sea water at room temperature, and they develop rapidly.^[126] Asterina pectinifera, used as a model organism for this purpose, is resilient and easy to breed and maintain in the laboratory.^[127]

Another area of research is the ability of starfish to regenerate lost body parts. The stem cells of adult humans are incapable of much differentiation and understanding the regrowth, repair and cloning processes in starfish may have implications for human medicine.^[128]

Starfish also have an unusual ability to expel foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.^[129]

In legend and culture

An aboriginal Australian fable retold by the Welsh school headmaster William Jenkyn Thomas (1870–1959)^[130] tells how some animals needed a canoe to cross the ocean. Whale had one but refused to lend it, so Starfish kept him busy, telling him stories and grooming him to remove parasites, while the others stole the canoe. When Whale realized the trick he beat Starfish ragged, which is how Starfish still is today.^[131]

In 1900, the scholar Edward Tregear documented The Creation Song, which he describes as “an ancient prayer for the dedication of a high chief” of Hawaii. Among the “uncreated gods” described early in the song are the male Kumulipo (“Creation”) and the female Poele, both born in the night, a coral insect, the earthworm, and the starfish.^[132]

Georg Eberhard Rumpf‘s 1705 The Ambonese Curiosity Cabinet describes the tropical varieties of Stella Marina or Bintang Laut, “Sea Star”, in Latin and Malay respectively, known in the waters around Ambon. He writes that the Histoire des Antilles reports that when the sea stars “see thunder storms approaching, [they] grab hold of many small stones with their little legs, looking to … hold themselves down as if with anchors”.^[133]

Starfish is the title of novels by Peter Watts^[134] and Jennie Orbell,^[135] and in 2012, Alice Addison wrote a non-fiction book titled Starfish: A Year in the Life of Bereavement and Depression.^[136] The Starfish and the Spider is a 2006 business management book by Ori Brafman and Rod Beckstrom; its title alludes to the ability of the starfish to regenerate itself because of its decentralized nervous system, and the book suggests ways that a decentralized organisation may flourish.^[137]

In the Nickelodeon animated television series SpongeBob SquarePants, the eponymous character’s best friend is a dim-witted starfish, Patrick Star.^[138]

As food

Starfish are widespread in the oceans, but are only occasionally used as food. There may be good reason for this: the bodies of numerous species are dominated by bony ossicles, and the body wall of many species contains saponins, which have an unpleasant taste,^[77] and others contain tetrodotoxins which are poisonous.^[139] Some species that prey on bivalve molluscs can transmit paralytic shellfish poisoning.^[140] Georg Eberhard Rumpf found few starfish being used for food in the Indonesian archipelago, other than as bait in fish traps, but on the island of “Huamobel” [sic] the people cut them up, squeeze out the “black blood” and cook them with sour tamarind leaves; after resting the pieces for a day or two, they remove the outer skin and cook them in coconut milk.^[133] Starfish are sometimes eaten in China,^[141] Japan^[142][143] and in Micronesia.^[144]

As collectables

Starfish are in some cases taken from their habitat and sold to tourists as souvenirs, ornaments, curios or for display in aquariums. In particular, Oreaster reticulatus, with its easily accessed habitat and conspicuous coloration, is widely collected in the Caribbean. In the early to mid 20th century, this species was common along the coasts of the West Indies, but collection and trade have severely reduced its numbers. In the State of Florida, O. reticulatus is listed as endangered and its collection is illegal. Nevertheless, it is still sold throughout its range and beyond.^[76] A similar phenomenon exists in the Indo-Pacific for species such as Protoreaster nodosus.^[145]

In industry and military history

With its multiple arms, the starfish provides a popular metaphor for computer networks,^[146] companies^[147][148] and software tools.^[149] It is also the name of a seabed imaging system and company.^[150]

Starfish has repeatedly been chosen as a name in military history. Three ships of the Royal Navy have borne the name HMS Starfish: an A-class destroyer launched in 1894;^[151] an R-class destroyer launched in 1916;^[152] and an S-class submarine launched in 1933 and lost in 1940.^[153] In World War II, Starfish sites were large-scale night-time decoys created during The Blitz to simulate burning British cities.^[154] Starfish Prime was a high-altitude nuclear test conducted by the United States on 9 July 1962.^[155]

Cichlids (/ˈsɪklɪdz/)^[a] are a large, diverse, and widespread family of percomorph fish in the family Cichlidae, order Cichliformes. At least 1,760 species have been scientifically described, making it one of the largest vertebrate families, with only the Cyprinidae being more speciose.^[3] New species are discovered annually, and many species remain undescribed. The actual number of species is therefore unknown, with estimates varying between 2,000 and 3,000.^[4] They are native to the Neotropics, Africa (including Madagascar), the Middle East, and the Indian subcontinent, although some species have been introduced worldwide.

Many cichlids, particularly tilapia, are important food fishes, while others, such as the Cichla species, are valued game fish. The family also includes many popular freshwater aquarium fish kept by hobbyists, including the angelfish, oscars, and discus.^[5][6] Cichlids have the largest number of endangered species among vertebrate families, most in the haplochromine group.^[7] Cichlids are particularly well known for having evolved rapidly into many closely related but morphologically diverse species within large lakes, particularly Lakes Tanganyika, Victoria, Malawi, and Edward.^[8][9] Their diversity in the African Great Lakes is important for the study of speciation in evolution.^[10] Many cichlids introduced into waters outside of their natural range have become nuisances.^[11]

All cichlids practice some form of parental care for their eggs and fry, usually in the form of guarding the eggs and fry or mouthbrooding.

Anatomy and appearance

[edit]

Cichlids span a wide range of body sizes, from species as small as 2.5 cm (1 in) in length (e.g., female Neolamprologus multifasciatus) to much larger species approaching 1 m (3 ft) in length (Boulengerochromis and Cichla). As a group, cichlids exhibit a similar diversity of body shapes, ranging from strongly laterally compressed species (such as Altolamprologus, Pterophyllum, and Symphysodon) to species that are cylindrical and highly elongated (such as Julidochromis, Teleogramma, Teleocichla, Crenicichla, and Gobiocichla).^[5] Generally, however, cichlids tend to be of medium size, ovate in shape, and slightly laterally compressed, and generally similar to the North American sunfishes in morphology, behavior, and ecology.^[12]

Cichlids share a single key trait – the fusion of the lower pharyngeal bones into a single tooth-bearing structure. A complex set of muscles allows the upper and lower pharyngeal bones to be used as a second set of jaws for processing food, allowing a division of labor between the “true jaws” (mandibles) and the “pharyngeal jaws“. Cichlids are efficient and often highly specialized feeders that capture and process a very wide variety of food items. This is assumed to be one reason why they are so diverse.^[5]

Taxonomy

[edit]

Internal taxonomy

[edit]

The following consensus taxonomy is based on the Catalog of Fishes (2025)^[13]

• Family CichlidaeBonaparte, 1835
  • Subfamily Etroplinae Kullander, 1998 (Indian and Madagascan cichlids)
  • Subfamily Ptychochrominae Sparks, 2004 (Malagasy cichlids)
  • Subfamily Pseudocrenilabrinae Fowler, 1934 (African cichlids)
  • Subfamily Cichlinae Bonaparte, 1835 (American cichlids)

In the past, cichlid taxonomy has varied depending on the author. Kullander (1998) recognized eight subfamilies of cichlids: the Astronotinae, Cichlasomatinae, Cichlinae, Etroplinae, Geophaginae, Heterochromidinae, Pseudocrenilabrinae, and Retroculinae.^[14] A ninth subfamily, the Ptychochrominae, was later recognized by Sparks and Smith.^[15] Cichlid taxonomy is still debated, and classification of genera cannot yet be definitively given. A comprehensive system of assigning species to monophyletic genera is still lacking, and there is not complete agreement on what genera should be recognized in this family.^[16]

As an example of the classification problems, Kullander^[17] placed the African genus Heterochromis phylogenetically within Neotropical cichlids, although later papers^{[citation needed]} concluded otherwise. Other problems center upon the identity of the putative common ancestor for the Lake Victoria superflock (many closely related species sharing a single habitat), and the ancestral lineages of Lake Tanganyikan cichlids.^{[citation needed]}

Phylogeny derived from morphological characters shows differences at the genus level with phylogeny based on genetic loci.^[18] A consensus remains that the Cichlidae as a family are monophyletic.^[19]

In cichlid taxonomy, dentition was formerly used as a classifying characteristic, but this was complicated because in many cichlids, tooth shapes change with age, due to wear, and cannot be relied upon. Genome sequencing and other technologies transformed cichlid taxonomy.

Alternatively, all cichlid species native to the New World, can be classified under the subfamily Cichlinae, while Etroplinae can classify all cichlid species native to the Old World.

External taxonomy

[edit]

The taxonomic placement of cichlids has long been disputed and variable, and has only recently been largely resolved. In the past, based on morphological characteristics, cichlids were classed in a suborder, the Labroidei, along with the wrasses (Labridae), in the order Perciformes.^[20] However, studies incorporating molecular phylogenetics have contradicted this grouping.^[21]

More recent phylogenetic studies support the creation of a distinct order, the Cichliformes, to contain the cichlids and their close relatives, which are no longer thought to be closely related to wrasses. The closest living relative of cichlids has been found to be the marine convict blenny, and both families are classified in the 5th edition of Fishes of the World as the two families in the Cichliformes, part of the subseries Ovalentaria.^[22] The Catalog of Fishes adopts the same placement, although the leaffishes (which have a similar African and South American distribution) are now also placed in the Cichliformes.^[23] Although these interrelationships are now generally well-supported, other authors have interpreted these relationships in differing ways, such as instead placing the cichlids, leaffish, and convict blenny as the most basal members of an expanded Blenniiformes.^[24]

Evolution

[edit]

Modern cichlids have a disjunct distribution consisting of Africa (including Madagascar), the Neotropics (including Cuba and Hispaniola), the Levant, southern Iran, and the southern Indian subcontinent. This distribution has become the subject of much scientific dispute, with it being debated whether modern cichlid distribution is a consequence of the breakup of Gondwana (which would make cichlids a particularly ancient group dating to the Early Cretaceous), or if it is instead based on more recent oceanic dispersal by the cichlids (despite modern members of the group being largely restricted to freshwater).^[25]

Proponents of the Gondwanan theory, which saw more support in the past, have noted that the cichlids display the precise sister relationships predicted by Gondwanan distribution: Africa-South America and India-Madagascar, and that with the exception of the species from Cuba, Hispaniola and Madagascar, cichlids have not reached any oceanic island. The dispersal hypothesis, in contrast, requires cichlids to have negotiated thousands of kilometers of open ocean between India and Madagascar without colonizing any other island, or for that matter, crossing the Mozambique Channel to Africa.^[25]

However, more recent studies incorporating phylogenetic evidence have found that the divergences within the cichlids are far too young for cichlids to have even been present for the breakup of Gondwana. Molecular clock estimates have placed the family’s origin only to the Late Cretaceous period, and the divergences within the family to have occurred anywhere between the Late Cretaceous to the Eocene (depending on the study). This suggests that only dispersal can support modern cichlid distribution. However, the factors that may have allowed prehistoric cichlids to make migrations over entire oceans remains a mystery.^{[2][26][27][28]} It is known that during the Paleogene, the Atlantic Ocean between South America and Africa was significantly narrower, and it has been suggested that either now-submerged islands or a large plume from the Congo River may have allowed for a shallower or less saline environment that was conducive for cichlids to disperse from Africa to South America. Under the dispersal hypothesis, it is generally accepted that Africa was the ancestral home for cichlids, from which they dispersed to attain their present distribution.^[28]

Fossil record

[edit]

The fossil record of cichlids is comprehensive, although it only starts in the Eocene, well after the family is thought to have undergone significant evolutionary diversification. Fossil cichlids appear in both South America and Africa at roughly the same time in the Eocene, with fossil cichlids known from the Early Eocene (48.6 mya)-aged Lumbrera Formation of Argentina,^[1] as well as the Middle Eocene (46 mya)-aged Mahenge Formation of Tanzania,^[29] suggesting that the divergence between Old and New World cichlids must have occurred prior to this point.

Several African fossil sites that contain cichlids (including the Eocene-aged Mahenge Formation of Tanzania and the Miocene-aged Ngorora Formation of Kenya)^[30] appear to represent former maars or rift lakes, and the fossil cichlids present in them appear to represent species flocks akin to those in the modern African rift lakes. This suggests that rapid diversification within enclosed ecosystems is a longstanding trait of cichlids.^[29][30]

Fossil remains also suggest that cichlids ranged further north in the geologic past, with the extinct tilapia Oreochromis lorenzoi being known from the Late Miocene of Italy.^[31]

Distribution and habitat

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Cichlids are one of the largest vertebrate families in the world. They are most diverse in Africa and South America. Africa alone is host to at least an estimated 1,600 species.^[16] Central America and Mexico have about 120 species, as far north as the Rio Grande in South Texas. Madagascar has its own distinctive species (Katria, Oxylapia, Paratilapia, Paretroplus, Ptychochromis, and Ptychochromoides), only distantly related to those on the African mainland.^[33][34] Native cichlids are largely absent in Asia, except for 9 species in Israel, Lebanon, and Syria (Astatotilapia flaviijosephi, Oreochromis aureus, O. niloticus, Sarotherodon galilaeus, Coptodon zillii, and Tristramella spp.), two in Iran (Iranocichla), and three in India and Sri Lanka (Etroplus and Pseudetroplus).^[16] If disregarding Trinidad and Tobago (where the few native cichlids are members of genera that are widespread in the South American mainland), the three species from the genus Nandopsis are the only cichlids from the Antilles in the Caribbean, specifically Cuba and Hispaniola. Europe, Australia, Antarctica, and North America north of the Rio Grande drainage have no native cichlids, although in Florida, Hawaii, Japan, northern Australia, and elsewhere, feral populations of cichlids have become established as exotics.^{[32][35][36][37][38][39][40]} Although no longer present in Europe except as introductions, tilapias are known to have ranged as far north as Italy during the Miocene.^[31]

Although most cichlids are found at relatively shallow depths, several exceptions do exist. The deepest known occurrences are Trematocara at more than 300 m (1,000 ft) below the surface in Lake Tanganyika.^[41] Others found in relatively deep waters include species such as Alticorpus macrocleithrum and Pallidochromis tokolosh down to 150 m (500 ft) below the surface in Lake Malawi,^[42][43] and the whitish (nonpigmented) and blind Lamprologus lethops, which is believed to live as deep as 160 m (520 ft) below the surface in the Congo River.^[44]

Cichlids are less commonly found in brackish and saltwater habitats, though many species tolerate brackish water for extended periods; Mayaheros urophthalmus, for example, is equally at home in freshwater marshes and mangrove swamps, and lives and breeds in saltwater environments such as the mangrove belts around barrier islands.^[5] Several species of Tilapia, Sarotherodon, and Oreochromis are euryhaline and can disperse along brackish coastlines between rivers.^[16] Only a few cichlids, however, inhabit primarily brackish or salt water, most notably Etroplus maculatus, Etroplus suratensis, and Sarotherodon melanotheron.^[45] The perhaps most extreme habitats for cichlids are the warm hypersaline lakes where the members of the genera Alcolapia and Danakilia are found. Lake Abaeded in Eritrea encompasses the entire distribution of D. dinicolai, and its temperature ranges from 29 to 45 °C (84 to 113 °F).^[46] Although the vast majority of Malagasy cichlids are entirely restricted to fresh water, Ptychochromis grandidieri and Paretroplus polyactis are commonly found in coastal brackish water and apparently are salt tolerant,^[47][48] as is also the case for Etroplus maculatus and E. suratensis from India and Sri Lanka.^[49][50]

Ecology

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Feeding

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Within the cichlid family, carnivores, herbivores, omnivores, planktivores, and detritivores are known, meaning the Cichlidae encompass essentially the full range of food consumption possible in the animal kingdom. Various species have morphological adaptations for specific food sources,^[51] but most cichlids consume a wider variety of foods based on availability. Carnivorous cichlids can be further divided into piscivorous and molluscivorous, since the morphology and hunting behavior differ greatly between the two categories. Piscivorous cichlids eat other fish, fry, larvae, and eggs. Some species eat the offspring of mouthbrooders by head-ramming, wherein the hunter shoves its head into the mouth of a female to expel her young and eat them.^[52] Molluscivorous cichlids have several hunting strategies amongst the varieties within the group. Lake Malawi cichlids consume substrate and filter it out through their gill rakers to eat the mollusks that were in the substrate. Gill rakers are finger-like structures that line the gills of some fish to catch any food that might escape through their gills.^[53]

Many cichlids are primarily herbivores, feeding on algae (e.g. Petrochromis) and plants (e.g. Etroplus suratensis). Small animals, particularly invertebrates, are only a minor part of their diets.

Other cichlids are detritivores and eat organic material, called Aufwuchs (offal); among these species are the tilapiines of the genera Oreochromis, Sarotherodon, and Tilapia.

Other cichlids are predatory and eat little or no plant matter. These include generalists that catch a variety of small animals, including other fishes and insect larvae (e.g. Pterophyllum), as well as variety of specialists. Trematocranus is a specialized snail-eater, while Pungu maclareni feeds on sponges. A number of cichlids feed on other fish, either entirely or in part. Crenicichla species are stealth predators that lunge from concealment at passing small fish, while Rhamphochromis species are open-water pursuit predators that chase down their prey.^[55] Paedophagous cichlids such as the Caprichromis species eat other species’ eggs or young, in some cases ramming the heads of mouthbrooding species to force them to disgorge their young.^{[56][57][58][59]} Among the more unusual feeding strategies are those of Corematodus, Docimodus evelynae, Plecodus, Perissodus, and Genyochromis spp., which feed on scales and fins of other fishes, a behavior known as lepidophagy,^[60][61][62] along with the death-mimicking behaviour of Nimbochromis and Parachromis species, which lay motionless, luring small fish to their side prior to ambush.^[63][64]

This variety of feeding styles has helped cichlids to inhabit similarly varied habitats. Its pharyngeal teeth (in the throat) afford cichlids so many “niche” feeding strategies, because the jaws pick and hold food, while the pharyngeal teeth crush the prey.

Behavior

[edit]

Aggression

[edit]

Aggressive behavior in cichlids is ritualized and consists of multiple displays used to seek confrontation while being involved in evaluation of competitors,^[65] coinciding with temporal proximity to mating. Displays of ritualized aggression in cichlids include a remarkably rapid change in coloration, during which a successfully dominant^[65] territorial male assumes a more vivid and brighter coloration, while a subordinate or “nonterritorial” male assumes a dull-pale coloration.^[66] In addition to color displays, cichlids employ their lateral lines to sense movements of water around their opponents to evaluate the competing male for physical traits/fitness.^[67] Male cichlids are very territorial due to the pressure of reproduction, and establish their territory and social status by physically driving out^[68] challenging males (novel intruders)^[69] through lateral displays (parallel orientation, uncovering gills),^[70] biting, or mouth fights (head-on collisions of open mouths, measuring jaw sizes, and biting each other’s jaws). The cichlid social dichotomy is composed of a single dominant with multiple subordinates, where the physical aggression of males becomes a contest for resources^[68] (mates, territory, food). Female cichlids prefer to mate with a successfully alpha male with vivid coloration, whose territory has food readily available.

Mating

[edit]

Cichlids mate either monogamously or polygamously.^[5] The mating system of a given cichlid species is not consistently associated with its brooding system. For example, although most monogamous cichlids are not mouthbrooders, Chromidotilapia, Gymnogeophagus, Spathodus, and Tanganicodus all include – or consist entirely of – monogamous mouthbrooders. In contrast, numerous open- or cave-spawning cichlids are polygamous; examples include many Apistogramma, Lamprologus, Nannacara, and Pelvicachromis species.^[5][71]

Most adult male cichlids, specifically in the cichlid tribe Haplochromini, exhibit a unique pattern of oval-shaped color dots on their anal fins. These phenomena, known as egg spots, aid in the mouthbrooding mechanisms of cichlids. The egg spots consist of carotenoid-based pigment cells, which indicate a high cost to the organism, when considering that fish are not able to synthesize their own carotenoids.^[72]

The mimicry of egg spots is used by males for the fertilization process. Mouthbrooding females lay eggs and immediately snatch them up with their mouths. Over millions of years, male cichlids have evolved egg spots to initiate the fertilization process more efficiently.^[73] When the females are snatching up the eggs into their mouth, the males gyrate their anal fins, which illuminates the egg spots on his tail. Afterwards, the female, believing these are her eggs, places her mouth to the anal fin (specifically the genital papilla) of the male, which is when he discharges sperm into her mouth and fertilizes the eggs.^[72]

The genuine color of egg spots is a yellow, red, or orange inner circle with a colorless ring surrounding the shape. Through phylogenetic analysis, using the mitochondrial ND2 gene, the true egg spots are thought to have evolved in the common ancestor of the Astatoreochromis lineage and the modern Haplochrominis species. This ancestor was most likely riverine in origin, based on the most parsimonious representation of habitat type in the cichlid family.^[74] The presence of egg spots in a turbid riverine environment would seem particularly beneficial and necessary for intraspecies communication.^[74]

Two pigmentation genes are found to be associated with egg-spot patterning and color arrangement. These are fhl2-a and fhl2-b, which are paralogs.^[73] These genes aid in pattern formation and cell-fate determination in early embryonic development. The highest expression of these genes was temporally correlated with egg-spot formation. A short, interspersed, repetitive element was also seen to be associated with egg spots. Specifically, it was evident upstream of the transcriptional start site of fhl2 in only Haplochrominis species with egg spots^[73]

Self-fertilization

[edit]

The cichlid Benitochromis nigrodorsalis from Western Africa ordinarily undergoes biparental reproduction, but is also able to undergo facultative (optional) selfing (self-fertilization).^[75] Facultative selfing may be an adaptive option when a mating partner is unavailable.^[75]

Brood care

[edit]

Further information: List of fish species that protect their young

Pit spawning in cichlids

[edit]

Pit spawning, also referred to as substrate breeding, is a behavior in cichlid fish in which a fish builds a pit in the sand or ground, where a pair court and consequently spawn.^[76] Many different factors go into this behavior of pit spawning, including female choice of the male and pit size, as well as the male defense of the pits once they are dug in the sand.^[77]

Cichlids are often divided into two main groups: mouthbrooders and substrate brooders. Different parenting investment levels and behaviors are associated with each type of reproduction.^[78] As pit spawning is a reproductive behavior, many different physiological changes occur in the cichlid while this process is occurring that interfere with social interaction.^[79] Different kinds of species that pit spawn, and many different morphological changes occur because of this behavioral experience.^[76]

Pit spawning is an evolved behavior across the cichlid group. Phylogenetic evidence from cichlids in Lake Tanganyika could be helpful in uncovering the evolution of their reproductive behaviors.^[80] Several important behaviors are associated with pit spawning, including parental care, food provisioning,^[81] and brood guarding.^[82]

Mouth brooding vs. pit spawning

[edit]

One of the differences studied in African cichlids is reproductive behavior. Some species pit spawn and some are known as mouth brooders. Mouthbrooding is a reproductive technique where the fish scoop up eggs and fry for protection.^[78] While this behavior differs from species to species in the details, the general basis of the behavior is the same. Mouthbrooding also affects how they choose their mates and breeding grounds. In a 1995 study, Nelson found that in pit-spawning females choose males for mating based on the size of the pit that they dig, as well as some of the physical characteristics seen in the males.^[77] Pit spawning also differs from mouth brooding in the size and postnatal care exhibited. Eggs that have been hatched from pit-spawning cichlids are usually smaller than those of mouthbrooders. Pit-spawners’ eggs are usually around 2 mm, while mouthbrooders are typically around 7 mm. While different behaviors take place postnatally between mouthbrooders and pit spawners, some similarities exist. Females in both mouthbrooders and pit-spawning cichlids take care of their young after they are hatched. In some cases, both parents exhibit care, but the female always cares for the eggs and newly hatched fry.^[83]

Pit spawning process

[edit]

Many species of cichlids use pit spawning, but one of the less commonly studied species that exhibits this behavior is the Neotropical Cichlasoma dimerus. This fish is a substrate breeder that displays biparental care after the fry have hatched from their eggs. One study^[76] examined reproductive and social behaviors of this species to see how they accomplished their pit spawning, including different physiological factors such as hormone levels, color changes, and plasma cortisol levels. The entire spawning process could take about 90 minutes and 400~800 eggs could be laid. The female deposits about 10 eggs at a time, attaching them to the spawning surface, which may be a pit constructed on the substrate or another surface. The number of eggs laid was correlated to the space available on the substrate. Once the eggs were attached, the male swam over the eggs and fertilized them. The parents would then dig pits in the sand, 10–20 cm wide and 5–10 cm deep, where larvae were transferred after hatching. Larvae began swimming 8 days after fertilization and parenting behaviors and some of the physiological factors measured changed.

Color changes

[edit]

In the same study, color changes were present before and after the pit spawning occurred. For example, after the larvae were transferred and the pits were beginning to be protected, their fins turned a dark grey color.^[76] In another study, of the rainbow cichlid, Herotilapia multispinosa,^[79] color changes occurred throughout the spawning process. Before spawning, the rainbow cichlid was an olive color with grey bands. Once spawning behaviors started, the body and fins of the fish became a more golden color. When the eggs were finished being laid, the pelvic fin all the way back to the caudal fin turned to a darker color and blackened in both the males and the females.^[79]

Pit sizes

[edit]

Females prefer a bigger pit size when choosing where to lay eggs.^[77] Differences are seen in the sizes of pits that created, as well as a change in the morphology of the pits.^[84] Evolutionary differences between species of fish may cause them to either create pits or castles when spawning. The differences were changes in the way that each species fed, their macrohabitats, and the abilities of their sensory systems.^[84]

Evolution

[edit]

Cichlids are renowned for their recent, rapid evolutionary radiation, both across the entire clade and within different communities across separate habitats.^{[78][80][84][85][86][87]} Within their phylogeny, many parallel instances are seen of lineages evolving to the same trait and multiple cases of reversion to an ancestral trait.

The family Cichlidae arose between 80 and 100 million years ago within the order Perciformes (perch-like fishes).^[85] Cichlidae can be split into a few groups based on their geographic location: Madagascar, Indian, African, and Neotropical (or South American). The most famous and diverse group, the African cichlids, can be further split either into Eastern and Western varieties, or into groups depending on which lake the species is from: Lake Malawi, Lake Victoria, or Lake Tanganyika.^[85][86] Of these subgroups, the Madagascar and Indian cichlids are the most basal and least diverse.^{[citation needed]}

Of the African cichlids, the West African or Lake Tanganyika cichlids are the most basal.^[80][85] Cichlids’ common ancestor is believed to have been a spit-spawning species.^[86] Both Madagascar and Indian cichlids retain this feature. However, of the African cichlids, all extant substrate brooding species originate solely from Lake Tanganyika.^[78][86] The ancestor of the Lake Malawi and Lake Victoria cichlids were mouthbrooders. Similarly, only around 30% of South American cichlids are thought to retain the ancestral substrate-brooding trait. Mouthbrooding is thought to have evolved individually up to 14 times, and a return to substrate brooding as many as three separate times between both African and Neotropical species.^[86]

Associated behaviors

[edit]

Cichlids have a great variety of behaviors associated with substrate brooding, including courtship and parental care alongside the brooding and nest-building behaviors needed for pit spawning. Cichlids’ behavior typically revolves around establishing and defending territories when not courting, brooding, or raising young. Encounters between males and males or females and females are agonistic, while an encounter between a male and female leads to courtship.^[88] Courtship in male cichlids follows the establishment of some form of territory, sometimes coupled with building a bower to attract mates.^[77][84][88] After this, males may attempt to attract female cichlids to their territories by a variety of lekking display strategies or otherwise seek out females of their species.^[77] However, cichlids, at the time of spawning, undergo a behavioral change such that they become less receptive to outside interactions.^[88] This is often coupled with some physiological change in appearance.^[76][79][88]

Brood care

[edit]

Cichlids can have maternal, paternal, or biparental care. Maternal care is most common among mouthbrooders, but cichlids’ common ancestor is thought to exhibit paternal-only care.^[86] Other individuals outside of the parents may also play a role in raising young; in the biparental daffodil cichlid (Neolamprologus pulcher), closely related satellite males, those males that surround other males’ territories and attempt to mate with female cichlids in the area, help rear the primary males’ offspring and their own.^[89]

A common form of brood care involves food provisioning. For example, females of lyretail cichlids (Neolamprologus modabu) dig at sandy substrate more to push nutritional detritus and zooplankton into the surrounding water. Adult of N. modabu perform this strategy to collect food for themselves, but dig more when offspring are present, likely to feed their fry.^[82][90] This substrate-disruption strategy is rather common and can also be seen in convict cichlids (Cichlasoma nigrofasciatum).^[81][90] Other cichlids have an ectothermal mucus that they grow and feed to their young, while still others chew and distribute caught food to offspring. These strategies, however, are less common in pit-spawning cichlids.^[90]

Cichlids have highly organized breeding activities.^[16] All species show some form of parental care for both eggs and larvae, often nurturing free-swimming young until they are weeks or months old. Communal parental care, where multiple monogamous pairs care for a mixed school of young have also been observed in multiple cichlid species, including Amphilophus citrinellus, Etroplus suratensis, and Tilapia rendalli.^[91][92][93] Comparably, the fry of Neolamprologus brichardi, a species that commonly lives in large groups, are protected not only by the adults, but also by older juveniles from previous spawns.^[94] Several cichlids, including discus (Symphysodon spp.), some Amphilophus species, Etroplus, and Uaru species, feed their young with a skin secretion from mucous glands.^[5][95]

The species Neolamprologus pulcher uses a cooperative breeding system, in which one breeding pair has many helpers that are subordinate to the dominant breeders.

Parental care falls into one of four categories:^[95] substrate or open brooders, secretive cave brooders (also known as guarding speleophils^[96]), and at least two types of mouthbrooders, ovophile mouthbrooders and larvophile mouthbrooders.^[97]

Open brooding

[edit]

Open- or substrate-brooding cichlids lay their eggs in the open, on rocks, leaves, or logs. Examples of open-brooding cichlids include Pterophyllum and Symphysodon species and Anomalochromis thomasi. Male and female parents usually engage in differing brooding roles. Most commonly, the male patrols the pair’s territory and repels intruders, while the female fans water over the eggs, removing the infertile ones, and leading the fry while foraging. Both sexes are able to perform the full range of parenting behaviours.^[97]

Cave brooding

[edit]

Secretive cave-spawning cichlids lay their eggs in caves, crevices, holes, or discarded mollusc shells, frequently attaching the eggs to the roof of the chamber. Examples include Pelvicachromis spp., Archocentrus spp., and Apistogramma spp.^[95] Free-swimming fry and parents communicate in captivity and in the wild. Frequently, this communication is based on body movements, such as shaking and pelvic fin flicking. In addition, open- and cave-brooding parents assist in finding food resources for their fry. Multiple neotropical cichlid species perform leaf-turning and fin-digging behaviors.^[97]

Ovophile mouthbrooding

[edit]

Ovophile mouthbrooders incubate their eggs in their mouths as soon as they are laid, and frequently mouthbrood free-swimming fry for several weeks. Examples include many East African Rift lakes (Lake Malawi, Lake Tanganyika, and Lake Victoria) endemics, e.g.: Maylandia, Pseudotropheus, Tropheus, and Astatotilapia burtoni, along with some South American cichlids such as Geophagus steindachneri.

Larvophile mouthbrooding

[edit]

Larvophile mouthbrooders lay eggs in the open or in a cave and take the hatched larvae into the mouth. Examples include some variants of Geophagus altifrons, and some Aequidens, Gymnogeophagus, and Satanoperca, as well as Oreochromis mossambicus and Oreochromis niloticus.^[5][95] Mouthbrooders, whether of eggs or larvae, are predominantly females. Exceptions that also involve the males include eretmodine cichlids (genera Spathodus, Eretmodus, and Tanganicodus), some Sarotherodon species (such as Sarotherodon melanotheron^[98]), Chromidotilapia guentheri, and some Aequidens species.^[5][97][99] This method appears to have evolved independently in several groups of African cichlids.^[16]

Speciation

[edit]

Cichlids provide scientists with a unique perspective of speciation, having become extremely diverse in the recent geological past, those of Lake Victoria actually within the last 10,000 to 15,000 years, a small fraction of the millions taken for Galápagos finch speciation in Darwin’s textbook case.^[101] Some of the contributing factors to their diversification are believed to be the various forms of prey processing displayed by cichlid pharyngeal jaw apparatuses. These different jaw apparatuses allow for a broad range of feeding strategies, including algae scraping, snail crushing, planktivory, piscivory, and insectivory.^[102] Some cichlids can also show phenotypic plasticity in their pharyngeal jaws, which can also help lead to speciation. In response to different diets or food scarcity, members of the same species can display different jaw morphologies that are better suited to different feeding strategies. As species members begin to concentrate around different food sources and continue their lifecycle, they most likely spawn with like individuals. This can reinforce the jaw morphology and given enough time, create new species.^[103] Such a process can happen through allopatric speciation, whereby species diverge according to different selection pressures in different geographical areas, or through sympatric speciation, by which new species evolve from a common ancestor while remaining in the same area. In Lake Apoyo in Nicaragua, Amphilophus zaliosus and its sister species Amphilophus citrinellus display many of the criteria needed for sympatric speciation.^[104] In the African rift lake system, cichlid species in numerous distinct lakes evolved from a shared hybrid swarm.^[100]

Population status

[edit]

In 2010, the International Union for Conservation of Nature classified 184 species as vulnerable, 52 as endangered, and 106 as critically endangered.^[105] At present, the IUCN only lists Yssichromis sp. nov. argens as extinct in the wild, and six species are listed as entirely extinct, but many more possibly belong in these categories (for example, Haplochromis aelocephalus, H. apogonoides, H. dentex, H. dichrourus, and numerous other members of the genus Haplochromis have not been seen since the 1980s, but are maintained as critically endangered on the small chance that tiny –but currently unknown– populations survive).^[105]

Lake Victoria

[edit]

Main article: Lake Victoria § Cichlid fish

Because of the introduced Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus), and water hyacinth, deforestation that led to water siltation, and overfishing, many Lake Victoria cichlid species have become extinct or been drastically reduced. By around 1980, lake fisheries yielded only 1% cichlids, a drastic decline from 80% in earlier years.^[107]

By far the largest Lake Victoria group is the haplochromine cichlids, with more than 500 species, but at least 200 of these (about 40%) have become extinct,^{[108][109][110]} and many others are seriously threatened.^[111] Initially it was feared that the percentage of extinct species was even higher,^[112] but some species have been rediscovered after the Nile perch started to decline in the 1990s.^[109][113] Some species have survived in nearby small satellite lakes,^[113] or in refugia among rocks or papyrus sedges (protecting them from the Nile perch),^[114] or have adapted to the human-induced changes in the lake itself.^[109][110] The species were often specialists and these were not affected to the same extent. For example, the piscivorous haplochromines were particularly hard hit with a high number of extinctions,^[115] while the zooplanktivorous haplochromines reached densities in 2001 that were similar to before the drastic decline, although consisting of fewer species and with some changes in their ecology.^[109]

Food and game fish

[edit]

Although cichlids are mostly small- to medium-sized, many are notable as food and game fishes. With few thick rib bones and tasty flesh, artisan fishing is not uncommon in Central America and South America, as well as areas surrounding the African rift lakes.^[107]

Tilapia

[edit]

The most important food cichlids, however, are the tilapiines of North Africa. Fast growing, tolerant of stocking density, and adaptable, tilapiine species have been introduced and farmed extensively in many parts of Asia and are increasingly common aquaculture targets elsewhere.

Farmed tilapia production is about 1,500,000 tonnes (1,700,000 short tons) annually, with an estimated value of US$1.8 billion,^[116] about equal to that of salmon and trout.

Unlike those carnivorous fish, tilapia can feed on algae or any plant-based food. This reduces the cost of tilapia farming, reduces fishing pressure on prey species, avoids concentrating toxins that accumulate at higher levels of the food chain, and makes tilapia the preferred “aquatic chickens” of the trade.^[107]

Game fish

[edit]

Many large cichlids are popular game fish. The peacock bass (Cichla species) of South America is one of the most popular sportfish. It was introduced in many waters around the world.^[where?] In Florida, this fish generates millions of hours of fishing and sportfishing revenue of more than US$8 million a year.^[117] Other cichlids preferred by anglers include the oscar, Mayan cichlid (Cichlasoma urophthalmus), and jaguar cichlid (Parachromis managuensis).^[117]

Aquarium fish

[edit]

Further information: List of cichlid fish of South America

Since 1945, cichlids have become increasingly popular as aquarium fish.^{[5][95][97][118][119][120][121]}

The most common species in hobbyist aquaria is Pterophyllum scalare from the Amazon River basin in tropical South America, known in the trade as the “angelfish“. Other popular or readily available species include the oscar (Astronotus ocellatus), convict cichlid (Archocentrus nigrofasciatus) and discus fish (Symphysodon).^[5]

Hybrids and selective breeding

[edit]

Some cichlids readily hybridize with related species, both in the wild and under artificial conditions.^[122] Other groups of fishes, such as European cyprinids, also hybridize.^[123] Unusually, cichlid hybrids have been put to extensive commercial use, in particular for aquaculture and aquaria.^[6][124] The hybrid red strain of tilapia, for example, is often preferred in aquaculture for its rapid growth. Tilapia hybridization can produce all-male populations to control stock density or prevent reproduction in ponds.^[6]

Aquarium hybrids

[edit]

The most common aquarium hybrid is perhaps the blood parrot cichlid, which is a cross of several species, especially from species in the genus Amphilophus. (There are many hypotheses, but the most likely is: Amphilophus labiatus × Vieja synspillus ^{[citation needed]} With a triangular-shaped mouth, an abnormal spine, and an occasionally missing caudal fin (known as the “love heart” parrot cichlid), the fish is controversial among aquarists. Some have called blood parrot cichlids “the Frankenstein monster of the fish world”.^[125] Another notable hybrid, the flowerhorn cichlid, was very popular in some parts of Asia from 2001 until late 2003, and is believed to bring good luck to its owner.^[126] The popularity of the flowerhorn cichlid declined in 2004.^[127] Owners released many specimens into the rivers and canals of Malaysia and Singapore, where they threaten endemic communities.^[128]

Numerous cichlid species have been selectively bred to develop ornamental aquarium strains. The most intensive programs have involved angelfish and discus, and many mutations that affect both coloration and fins are known.^{[5][129][130]} Other cichlids have been bred for albino, leucistic, and xanthistic pigment mutations, including oscars, convict cichlid and Pelvicachromis pulcher.^[5][95] Both dominant and recessive pigment mutations have been observed.^[131] In convict cichlids, for example, a leucistic coloration is recessively inherited,^[132] while in Oreochromis niloticus niloticus, red coloration is caused by a dominant inherited mutation.^[133]

This selective breeding may have unintended consequences. For example, hybrid strains of Mikrogeophagus ramirezi have health and fertility problems.^[134] Similarly, intentional inbreeding can cause physical abnormalities, such as the notched phenotype in angelfish.^[135]

Genera

[edit]

The genus list is as per FishBase. Studies are continuing, however, on the members of this family, particularly the haplochromine cichlids of the African rift lakes.^[33]

Abactochromis Oliver & Arnegard 2010Acarichthys Eigenmann 1912Acaronia Myers 1940Alcolapia Thys van den Audenaerde 1969Alticorpus Stauffer & McKaye 1988Altolamprologus Poll 1986Amatitlania Schmitter-Soto, 2007Amphilophus Agassiz, 1859Andinoacara Musilova, Rican, & Novak 2009Anomalochromis Greenwood 1985Apistogramma Regan 1913Apistogrammoides Meinken 1965Aristochromis Trewavas, 1935Astatoreochromis Pellegrin 1904Astatotilapia Pellegrin 1904Astronotus Swainson 1839Aulonocara Regan 1922Aulonocranus Regan 1920Australoheros Rican & Kullander 2006Baileychromis Poll 1986Bathybates Boulenger 1898Benitochromis Lamboj 2001Benthochromis Poll 1986Biotodoma Eigenmann & Kennedy 1903Biotoecus Eigenmann & Kennedy 1903Boulengerochromis Pellegrin 1904Buccochromis Eccles & Trewavas 1989Bujurquina Kullander 1986Callochromis Regan 1920Caprichromis Eccles & Trewavas 1989Caquetaia Fowler 1945Cardiopharynx Poll 1942Chaetobranchopsis Steindachner, 1875Chaetobranchus Heckel, 1840Chalinochromis Poll 1974Champsochromis Boulenger 1915Cheilochromis Eccles & Trewavas 1989Chetia Trewavas 1961Chilochromis Boulenger 1902Chilotilapia Boulenger 1908Chromidotilapia Boulenger 1898Cichla Bloch & Schneider 1801Cichlasoma Swainson 1839Cleithracara Kullander & Nijssen 1989Coelotilapia[136]Congochromis Stiassny & Schliewen 2007CongolapiaCopadichromis Eccles & Trewavas 1989Coptodon[136]Corematodus Boulenger 1897Crenicara Steindachner 1875Crenicichla Heckel 1840Cryptoheros Allgayer 2001Ctenochromis Pfeffer 1893Ctenopharynx Eccles & Trewavas 1989Cunningtonia Boulenger 1906Cyathochromis Trewavas 1935Cyathopharynx Regan 1920Cyclopharynx Poll 1948Cynotilapia Regan 1922Cyphotilapia Regan 1920Cyprichromis Scheuermann 1977Cyrtocara Boulenger 1902Danakilia Thys van den Audenaerde 1969Dicrossus Steindachner 1875Dimidiochromis Eccles & Trewavas 1989Diplotaxodon Trewavas 1935Divandu Lamboj & Snoeks 2000Docimodus Boulenger 1897Eclectochromis Eccles & Trewavas 1989Ectodus Boulenger 1898Enigmatochromis Lamboj 2009Eretmodus Boulenger 1898Etia Schliewen & Stiassny 2003

Etroplus Cuvier 1830Exochochromis Eccles & Trewavas 1989Fossorochromis Eccles & Trewavas 1989Genyochromis Trewavas 1935Geophagus Heckel 1840Gephyrochromis Boulenger 1901Gnathochromis Poll 1981Gobiocichla Kanazawa 1951Grammatotria Boulenger 1899Greenwoodochromis Poll 1983Guianacara Kullander & Nijssen 1989Gymnogeophagus Miranda Ribeiro 1918Haplochromis Hilgendorf 1888Haplotaxodon Boulenger 1906Hemibates Regan 1920Hemichromis Peters 1857Hemitaeniochromis Eccles & Trewavas 1989Hemitilapia Boulenger 1902Herichthys Baird & Girard 1854Heroina Kullander, 1996Heros Heckel, 1840Heterochromis Regan 1922Heterotilapia[136]Hoplarchus Kaup,1860Hypselecara Kullander 1986Hypsophrys Agassiz 1859Interochromis Yamaoka, Hori & Kuwamura 1988Iodotropheus Oliver & Loiselle 1972Iranocichla Coad 1982Julidochromis Boulenger 1898Katria Stiassny & Sparks 2006Konia Trewavas 1972Krobia Kullander & Nijssen 1989Labeotropheus Ahl 1926Labidochromis Trewavas 1935Laetacara Kullander 1986Lamprologus Schilthuis 1891Lepidiolamprologus Pellegrin 1904Lestradea Poll 1943Lethrinops Regan 1922Lichnochromis Trewavas 1935Limbochromis Greenwood 1987Limnochromis Regan 1920Limnotilapia Regan 1920Lobochilotes Boulenger 1915†Mahengechromis Murray 2001[137]Maylandia Meyer & Foerster 1984Mazarunia Kullander 1990Mchenga Stauffer & Konings, 2006Melanochromis Trewavas, 1935Mesonauta Günther, 1862Microchromis Johnson 1975Mikrogeophagus Meulengracht-Madson 1968Myaka Trewavas 1972Mylochromis Regan 1920Naevochromis Eccles & Trewavas 1989Nandopsis Gill, 1862Nannacara Regan 1905Nanochromis Pellegrin 1904Neolamprologus Colombe & Allgayer 1985Nimbochromis Eccles & Trewavas 1989Nosferatu De la Maza-Benignos, Ornelas-García, Lozano-Vilano, García Ramírez & Doadrio, 2015[138]Nyassachromis Eccles & Trewavas 1989Ophthalmotilapia Pellegrin 1904Oreochromis Günther 1889Orthochromis Greenwood 1954Otopharynx Regan 1920Oxylapia Kiener & Maugé 1966Pallidochromis Turner 1994Parachromis Agassiz 1859Paracyprichromis Poll 1986Parananochromis Greenwood 1987Paraneetroplus Regan 1905Paratilapia Bleeker, 1868Paretroplus Bleeker, 1868Pelmatochromis Steindachner 1894Pelmatolapia[136]

Pelvicachromis Thys van den Audenaerde 1968Perissodus Boulenger 1898Petenia Günther, 1862Petrochromis Boulenger 1898Petrotilapia Trewavas 1935Pharyngochromis Greenwood 1979Placidochromis Eccles & Trewavas 1989Plecodus Boulenger 1898†Plesioheros Malabarba, Zuleta & del Papa 2006[1]†Proterocara Perez, Malabarba, & del Papa 2010[1]Protomelas Eccles & Trewavas 1989Pseudocrenilabrus Fowler 1934Pseudosimochromis Nelissen 1977Pseudotropheus Regan 1922Pterochromis Trewavas 1973Pterophyllum Heckel 1840Ptychochromis Steindachner 1880Ptychochromoides Kiener & Mauge 1966Pungu Trewavas 1972Reganochromis Whitley 1929Retroculus Eigenmann & Bray 1894Rhamphochromis Regan 1922Rocio Schmitter-Soto, 2007Sargochromis Regan 1920Sarotherodon Rppell 1852Satanoperca Günther, 1862Schwetzochromis Poll 1948Sciaenochromis Eccles & Trewavas 1989Serranochromis Regan 1920Simochromis Boulenger 1898Spathodus Boulenger 1900Steatocranus Boulenger 1899Stigmatochromis Eccles & Trewavas 1989Stomatepia Trewavas 1962Symphysodon Heckel 1840Taeniacara Myers 1935Taeniochromis Eccles & Trewavas 1989Taeniolethrinops Eccles & Trewavas 1989Tahuantinsuyoa Kullander 1991Tangachromis Poll 1981Tanganicodus Poll 1950Teleocichla Kullander 1988Teleogramma Boulenger 1899Telmatochromis Boulenger 1898Theraps Günther, 1862Thoracochromis Greenwood 1979Thorichthys Meek 1904Thysochromis Daget 1988Tilapia Smith, 1840 See also: Tilapiine cichlidsTomocichla Regan 1908Tramitichromis Eccles & Trewavas 1989Trematocara Boulenger 1899Trematocranus Trewavas 1935†Tremembichthys Malabarba & Malabarba 2008[1]Triglachromis Poll & Thys van den Audenaerde 1974Tristramella Trewavas 1942Tropheops Trewavas 1984Tropheus Boulenger 1898Tylochromis Regan 1920Tyrannochromis Eccles & Trewavas 1989Uaru Heckel 1840Variabilichromis Colombe & Allgayer 1985†Warilochromis Altner, Ruthensteiner & Reichenbacher 2008Xenochromis Boulenger 1899Xenotilapia Boulenger 1899

Gallery

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• The oscar (Astronotus ocellatus) is one of the most popular cichlids in the fishkeeping hobby.
• The butterfly peacock bass (Cichla ocellaris) was introduced intentionally in Florida as gamefish.
• The Nile tilapia (Oreochromis niloticus) is farmed extensively as food fish in many parts of the world.
• The angelfish (Pterophyllum scalare) has long been commercially bred for the aquarium trade.
• Sexual dimorphism is common in cichlids. Shown here are a male (front, with egg spots) and a female (rear) Maylandia lombardoi.
• A pair of blue rams (Mikrogeophagus ramirezi), male in front, female behind. Many cichlids form strong pair bonds while breeding.
• A discus (Symphysodon spp.) is guarding its eggs. Advanced broodcare is one of the defining characteristics of cichlids.
• Lake Malawi, Eastern Africa, is home to numerous cichild species including this Livingston’s cichlid (Nimbochromis livingstonii).
• Also from Lake Malawi
• Also from Lake Malawi
• A shell-brooding cichlid of the genus Lamprologus from Lake Tanganyika in East Africa
• The Texas cichlid (Herichthys cyanoguttatus) is the only cichlid native to the United States.
• Pelvicachromis pulcher is a West African riverine cichlid, and part of the aquarists dwarf cichlid group.
• The flowerhorn cichlid is a man-made hybrid that has recently gained popularity among aquarists, particularly in Asia.
• Ivanacara adoketa, a dwarf cichlid from Brazil
• The red terror cichlid is a highly aggressive species from the rivers of Northeast South America.
• A juvenile female Maylandia lombardoi with faint stripes
• A juvenile Aequidens diadema