Starfish orsea stars arestar-shapedechinoderms belonging to theclassAsteroidea (/ˌæstəˈrɔɪdiə/). Common usage frequently finds these names being also applied toophiuroids, which are correctly referred to asbrittle stars or basket stars. Starfish are also known asasteroids due to being in the class Asteroidea. About 1,900 species of starfish live on theseabed in all the world'soceans, from warm,tropical zones to frigid,polar regions. They are found from theintertidal zone down toabyssal depths, at 6,000 m (20,000 ft) below the surface.
Starfish aremarine 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 havetube feet operated by ahydraulic system and a mouth at the centre of the oral or lower surface. They areopportunistic feeders and are mostlypredators onbenthic invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs andsuspension feeding. They have complexlife cycles and can reproduce both sexually andasexually. Most canregenerate damaged parts or lost arms and they can shed arms as a means of defense. The Asteroidea occupy several significantecological roles. Starfish, such as theochre sea star (Pisaster ochraceus) and thereef sea star (Stichaster australis), have become widely known as examples of thekeystone species concept in ecology. The tropicalcrown-of-thorns starfish (Acanthaster planci) is a voracious predator ofcoral throughout the Indo-Pacific region, and theNorthern Pacific seastar is on the list of theWorld's 100 Worst Invasive Alien Species.
Thefossil record for starfish is ancient, dating back to theOrdovician around 450 million years ago, but it is rather sparse, as starfish tend to disintegrate after death. Only theossicles 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 ascurios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.
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 AntarcticLabidiaster 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]
The body wall consists of a thin cuticle, anepidermis consisting of a single layer of cells, a thickdermis formed ofconnective tissue and a thincoelomicmyoepithelial layer, which provides the longitudinal and circular musculature. The dermis contains anendoskeleton ofcalcium carbonate components known as ossicles. These are honeycombed structures composed ofcalcite 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 atessellated manner and form the main covering of the aboral surface.[7] Some are specialised structures such as themadreporite (the entrance to the water vascular system),pedicellariae andpaxillae.[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, includingValvatida andForcipulatida, possesspedicellariae.[8] In Forcipulatida, such asAsterias andPisaster, they occur inpompom-like tufts at the base of each spine, whereas in theGoniasteridae, such asHippasteria 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 likeLabidiaster annulatus,Rathbunaster californicus andNovodinia antillensis use their large pedicellariae to capture small fish and crustaceans.[11]
There may also bepapulae, thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water. These serve arespiratory 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 theinterstices. 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]
The watervascular system of the starfish is ahydraulic system made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation andgas exchange. Water enters the system through themadreporite, 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 theambulacral 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 withcilia.[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 thesubstrate. 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 frombilateral 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 theleather star (Dermasterias imbricata), which can manage just 15 cm (6 in) in a minute.[20] Some burrowing species from the generaAstropecten andLuidia have points rather than suckers on their longtube 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 transportoxygen 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 bycilia.[14] Gas exchange also takes place through othergills known as papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Oxygen is transferred from these to thecoelomic 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]
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 toughperistomial membrane and closed with asphincter. The mouth opens through a shortoesophagus into astomach 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 byligaments 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 digestiveenzymes and absorb nutrients from the food. A shortintestine andrectum run from the pyloric stomach to open at a smallanus at the apex of the aboral surface of the disc.[23]
Primitive starfish, such asAstropecten andLuidia, swallow theirprey 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 otherbivalve, 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 aneuropeptide 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 andoysters,arthropods, smallfish andgastropod molluscs. Some starfish are not purecarnivores, supplementing their diets withalgae or organic detritus. Some of these species aregrazers, but others trap food particles from the water in stickymucus strands that are swept towards the mouth alongciliated grooves.[23]
The main nitrogenous waste product isammonia. Starfish have no distinct excretory organs; waste ammonia is removed by diffusion through the tube feet and papulae.[22] The body fluid containsphagocytic 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 thefaeces.[22]
Starfish do not appear to have any mechanisms forosmoregulation, and keep their body fluids at the same salt concentration as the surrounding water. Although some species can tolerate relatively lowsalinity, the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in manyestuarine 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 simpleocelli. These are composed ofpigmented 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 individualphotoreceptor 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 acentralized brain, it has a complexnervous 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 thecirculatory 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 pairedgonads; 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 ofsecondary metabolites in the form oflipids, includingsteroidal derivatives ofcholesterol, andfatty acidamides ofsphingosine. The steroids are mostlysaponins, known as asterosaponins, and theirsulphated derivatives. They vary between species and are typically formed from up to six sugar molecules (usuallyglucose andgalactose) connected by up to threeglycosidic chains. Long-chain fatty acid amides of sphingosine occur frequently and some of them have knownpharmacological activity. Variousceramides are also known from starfish and a small number ofalkaloids 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 areantifoulants and supplement the pedicellariae to prevent other organisms from settling on the starfish's aboral surface. Some are alarmpheromones and escape-eliciting chemicals, the release of which trigger responses inconspecific 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 aregonochorous, 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 theyspawn.[30] Some species aresimultaneous hermaphrodites, producing eggs and sperm at the same time, and in a few of these the same gonad, called anovotestis, produces both eggs and sperm.[31] Other starfish aresequential hermaphrodites.Protandrous individuals of species likeAsterina gibbosa start life as males before changing sex into females as they grow older. In some species such asNepanthia belcheri, a large female cansplit 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 releasegametes 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 resultingembryos andlarvae live as part of theplankton. In others, the eggs may be stuck to the undersides of rocks.[33] In certain species of starfish, the femalesbrood 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 withyolk, 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 thewater column. InParvulastra parvivipara, anintragonadal 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 apheromone 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 calledpseudocopulation[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]
Most starfish embryos hatch at theblastula stage. The original ball of cells develops a lateral pouch, thearchenteron. The entrance to this is known as theblastopore and it will later develop into the anus—together withchordates,echinoderms aredeuterostomes, meaning the second (deutero) invagination becomes the mouth (stome); members of all otherphyla areprotostomes, 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 ofcilia 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 abipinnaria. The cilia are used for locomotion and feeding, their rhythmic beat waftingphytoplankton towards the mouth.[8]
The next stage in development is abrachiolaria 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
"Comet" ofLinckia guildingi, showing starfish body regrowing from a single arm
Some species of starfish in the three families Asterinidae, Asteriidae and Solasteridae are able to reproduceasexually as adults either byfission of their central discs[46] or byautotomy 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 bybudding.[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]
Some species of starfish have the ability toregenerate 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 calledcatch 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 internalelectrolyte balance that is in equilibrium with sea water, making it impossible for them to live in afreshwater habitat.[16] Starfish species inhabit all of the world's oceans. Habitats range from tropicalcoral reefs, rocky shores,tidal pools, mud, and sand tokelp 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]
Most species are generalist predators, eatingmicroalgae,sponges,bivalves,snails and other small animals.[24][58] Thecrown-of-thorns starfish consumescoral polyps,[59] while other species aredetritivores, feeding on decomposing organic material and faecal matter.[58][60] A few are suspension feeders, gathering inphytoplankton;Henricia andEchinaster 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 theWest Coast of America, can use a set of specialized tube feet to dig itself deep into the soft substrate to extract prey (usuallyclams).[62] Grasping the shellfish, the starfish slowly pries open the prey's shell by wearing out itsadductor 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 on a wood pierPisaster ochraceus consuming a mussel in centralCalifornia
Starfish arekeystone species in their respective marinecommunities. 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 byRobert Paine in 1966 to describe a starfish,Pisaster ochraceus.[65] When studying the low intertidal coasts ofWashington state, Paine found that predation byP. 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 ofMytilus mussels, which were able to outcompete other organisms for space and resources.[66] Similar results were found in a 1971 study ofStichaster australis on the intertidal coast of theSouth Island ofNew 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 whichS. australis had been removed, the mussels increased in number dramatically, overwhelming the area and threateningbiodiversity.[67]
The feeding activity of theomnivorous starfishOreaster reticulatus on sandy and seagrass bottoms in theVirgin 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 andFrench 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 wholebenthic community and reef-feeding fish.[59]Asterias amurensis is one of a few echinoderminvasive species. Its larvae likely arrived inTasmania 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 importantbivalve populations. As such, they are considered pests,[71] and are on the Invasive Species Specialist Group'slist 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]
Starfish may be preyed on by conspecifics, sea anemones,[74] other starfish species,tritons, crabs, fish,gulls andsea otters.[38][71][75][76] Their first lines of defence are thesaponins present in their body walls, which have unpleasant flavours.[77] Some starfish such asAstropecten polyacanthus also include powerful toxins such astetrodotoxin among their chemical armoury, and theslime star can ooze out large quantities of repellent mucus. They also have body armour in the form of hard plates and spines.[78] Thecrown-of-thorns starfish is particularly unattractive to potential predators, being heavily defended by sharp spines, laced with toxins and sometimes with brightwarning 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 awasting condition caused by bacteria in the genusVibrio;[75] however, a more widespreadwasting disease, causingmass 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 protozoanOrchitophrya 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 ofP. 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 tomarine pollution.[85] Thecommon starfish is considered to be abioindicator for marine ecosystems.[86] A 2009 study found thatP. ochraceus is unlikely to be affected byocean acidification as severely as other marine animals withcalcareous skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when thepH is lowered. Researchers found that whenP. ochraceus were exposed to 21 °C (70 °F) and 770 ppmcarbon 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]
Echinoderms first appeared in thefossil record in theCambrian. The first known asterozoans were theSomasteroidea, 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 inLagerstätten – so-called "starfish beds".[89]
By the latePaleozoic, thecrinoids andblastoids were the predominant echinoderms, and some limestones from this period are made almost entirely from fragments from these groups. In the two majorextinction events that occurred during the lateDevonian and latePermian, 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 theEarly Jurassic and the beginning of theMiddle Jurassic.[90][91] A 2012 study found thatspeciation in starfish can occur rapidly. During the last 6,000 years, divergence in the larval development ofCryptasterina hystera andCryptasterina pentagona has taken place, the former adopting internal fertilization and brooding and the latter remaining a broadcast spawner.[92]
Video showing the tube feet movement of a starfishClose up starfish atWakatobi National Park, 2018
The scientific name Asteroidea was given to starfish by the French zoologistde Blainville in 1830.[93] It is derived from theGreekaster, ἀστήρ (a star) and the Greekeidos, εἶδος (form, likeness, appearance).[94] The class Asteroidea belongs to the phylumEchinodermata. As well as the starfish, the echinoderms includesea urchins,sand dollars,brittle andbasket stars,sea cucumbers andcrinoids. The larvae of echinoderms have bilateral symmetry, but duringmetamorphosis this is replaced withradial symmetry, typicallypentameric.[12] Adult echinoderms are characterized by having awater vascular system with externaltube feet and acalcareous endoskeleton consisting ofossicles connected by a mesh ofcollagen fibres.[95] Starfish are included in the subphylumAsterozoa, 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 twoclasses of Asteroidea, the starfish, andOphiuroidea, 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 sevenextant orders,Brisingida,Forcipulatida,Notomyotida,Paxillosida,Spinulosida,Valvatida andVelatida[1] and two extinct ones,Calliasterellidae andTrichasteropsida.[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 thePaxillosida, and the deep-water sea daisies, though clearly Asteroidea and currently included inVelatida, do not fit easily in any accepted lineage.Phylogenetic data suggests that they may be asister group, the Concentricycloidea, to the Neoasteroidea, or that the Velatida themselves may be a sister group.[91]
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]
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 thecommon starfish of North Atlantic coasts and rock pools, as well as cold-water and abyssal species.[103]
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]
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]
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 starfishEchinaster sepositus.[111]
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 havepaxillae and in some, the main pedicellariae are clamp-like and recessed into the skeletal plates.[110] This group includes thecushion stars,[113] the leather star[114] and thesea daisies.[115]
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 theslime star.[118]
Starfish aredeuterostome animals, like thechordates. A 2014 analysis of 219 genes from all classes of echinoderms gives the followingphylogenetic tree.[121] The times at which theclades diverged are shown under the labels in millions of years ago (mya).
The phylogeny of the Asteroidea has been difficult to resolve, with visible (morphological) features proving inadequate, and the question of whether traditionaltaxa areclades in doubt.[2] The phylogeny proposed by Gale in 1987 is:[2][122]
† Palaeozoic Asteroids
Paxillosida
Valvatida, including Velatida, Spinulosida(not a clade)[2]
Forcipulatida, including Brisingida
The phylogeny proposed by Blake in 1987 is:[2][123]
† Palaeozoic Asteroids
† Calliasterellidae
† Compasteridae
† Trichasteropsida
Brisingida
Forcipulatida
Spinulosida
Velatida
Notomyotida
Valvatida
Paxillosida
Later work making use ofmolecular 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.Asterina
Odontasteridae, which was a part of Valvatida
Paxillosida
part of Spinulosida, e.g.Echinaster, part of Valvatida, e.g.Archaster
Forcipulatida
Brisingida with part of Velatida, e.g.Caymanostella and part of Forcipulatida, e.g.Stichaster
Starfish aredeuterostomes, closely related, together with all other echinoderms, tochordates, and are used in reproductive and developmental studies. Female starfish produce large numbers ofoocytes that are easily isolated; these can be stored in apre-meiosis phase and stimulated to complete division by the use of1-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 amodel 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. Thestem 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
A starfish with five legs. Used as an illustration of "Hope in God", a poem by Lydia Sigourney which appeared inPoems for the Sea, 1850
Anaboriginal Australian fable retold by the Welsh school headmasterWilliam 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 scholarEdward Tregear documentedThe Creation Song, which he describes as "an ancient prayer for the dedication of a high chief" ofHawaii. 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]
1860 watercolor painting of a starfish by Jacques Burkhardt
Georg Eberhard Rumpf's 1705The Ambonese Curiosity Cabinet describes the tropical varieties ofStella Marina orBintang Laut, "Sea Star", in Latin andMalay respectively, known in the waters aroundAmbon. He writes that theHistoire 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 byPeter Watts[134] and Jennie Orbell,[135] and in 2012, Alice Addison wrote a non-fiction book titledStarfish: A Year in the Life of Bereavement and Depression.[136]The Starfish and the Spider is a 2006business management book by Ori Brafman andRod 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]
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 containssaponins, which have an unpleasant taste,[77] and others containtetrodotoxins which are poisonous.[139] Some species that prey onbivalve molluscs can transmitparalytic shellfish poisoning.[140] Georg Eberhard Rumpf found few starfish being used for food in theIndonesian 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 sourtamarind leaves; after resting the pieces for a day or two, they remove the outer skin and cook them incoconut 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 assouvenirs,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 theState of Florida,O. reticulatus is listed asendangered 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 asProtoreaster nodosus.[145]
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