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Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (Template:IPAc-en). 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 Template:Cvt 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.
AnatomyEdit
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.<ref name=arms/> The Antarctic Labidiaster annulatus can have over fifty.<ref>Template:Cite book</ref>
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.<ref>Lacalli, T. (2023). A radical evolutionary makeover gave echinoderms their unusual body plan. Nature. https://doi.org/10.1038/d41586-023-03123-1</ref>
Body wallEdit
The body wall layers include a thin cuticle, an epidermis consisting of a single layer of cells, a thick dermis formed of connective tissue and thin coelomic myoepithelial layer for the muscles, and a peritoneum. The dermis contains an endoskeleton of calcium carbonate components known as ossicles. These are honeycomb-like structures composed of calcite microcrystals arranged in a lattice.<ref name="Ruppert876">Ruppert et al., 2004. pp. 876–880</ref> 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.<ref name="Glossary">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Some are specialised structures such as the madreporite (the entrance to the water vascular system), pedicellariae and paxillae. Paxillae are umbrella-like structures found on starfish that live buried in substrate. 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. The ossicles are located under the epidermal layer, even those emerging externally.<ref name=Ruppert876/>
Several groups of starfish, including Valvatida and Forcipulatida, possess pedicellariae.<ref name="Ruppert889" /> 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.<ref name="synthesis">Template:Cite book</ref> Some species like Labidiaster annulatus and Novodinia antillensis use their pedicellariae to catch prey.<ref>Template:Cite book in Lawrence (2013)</ref>
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.<ref name="Lander" /> 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.<ref>Template:Cite journal</ref>
Water vascular systemEdit
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.<ref name="Ruppert883">Ruppert et al., 2004. pp. 879–883, 889</ref>
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.<ref>Template:Cite journal</ref> 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.<ref name=Dorit/><ref>Template:Cite journal</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref name="arms">Template:Cite arXiv</ref><ref name="Chengcheng2012">Template:Cite journal</ref><ref name="Carefoot">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Most starfish cannot move quickly, a typical speed being that of the leather star (Dermasterias imbricata), which can manage just Template:Convert in a minute.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 Template:Convert per minute.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="Chengcheng2012"/>
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.<ref name="Ruppert883" />
Digestive system and excretionEdit
The gut of a starfish fills most of the disc and extends into the arms. The mouth is occupies the centre of the oral surface, where it is surrounded by a tough peristomial membrane and closed with a sphincter. A short oesophagus connects the mouth to a stomach which consists of an 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 long, 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 the anus at the apex of the aboral surface of the disc.<ref name="Ruppert885">Ruppert et al., 2004. p. 885</ref>
Primitive starfish, such as Astropecten and Luidia, shallow their prey whole, and start to digest it in their cardiac stomachs, spitting out hard material like shells. The semi-digested fluid flows into the caeca for more digestion as well as absorption.<ref name="Ruppert885" /> 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.<ref name="Carefootfeeding">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The retraction and contraction of the cardiac stomach is activated by a neuropeptide known as NGFFYamide.<ref>Template:Cite journal</ref>
The main nitrogenous waste product is ammonia, which is removed diffusion through the tube feet, papulae and other thin-walled areas. Other waste material include urates. Their 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.<ref name="Ruppert887">Ruppert et al., 2004. pp. 886–887</ref>
Starfish keep their body fluids at the same salt concentration as the surrounding water, the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine environments.<ref name=Ruppert887/>
Sensory and nervous systemsEdit
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.<ref name=Carefootfeeding/> There are eyespots at the ends of the arms, each one made of 80–200 simple ocelli composed of pigmented epithelial cells. Individual photoreceptor cells are present in other parts of their bodies and respond to light. Whether they advance or retreat depends on the species.<ref name="Ruppert884">Ruppert et al., 2004. pp. 883–884</ref>
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: one in the epidermis and the other in the lining of the coelomic cavity, which are the sensory and motor systems respectively. Neurons passing through the dermis join the two.<ref name="Ruppert884" /> The ring nerves and radial nerves have sensory and motor components and coordinate the starfish's balance and directional systems.<ref name="Lander"/> The sensory component is supplied with information from the sensory organs while the motor nerves control the tube feet and musculature. If one arm detects something, it becomes dominant and temporarily over-rides the other arms to initiate movement towards it.<ref name="Ruppert884"/>
Circulatory and gas exchange systemEdit
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. Blood does not contain a pigment but is probably used to transport nutrients around the body.<ref name="Ruppert887"/> Gas exchange aminly takes place through gills known as papulae, which are thin-walled bulges along the aboral surface of the arms. Oxygen is transferred from these to the coelomic fluid, which moves gas around the body.<ref name="Ruppert883" />
Secondary metabolitesEdit
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 with some having known biological activity. Starfish also contain various ceramides and a small number of alkaloids. These these chemicals in the starfish may function 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 stimulate flight in potential prey.<ref>Template:Cite book</ref> Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.<ref>Template:Cite journal</ref>
Life cycleEdit
Sexual reproductionEdit
Most species of starfish are gonochorous, there being separate male and female individuals.<ref name="Ruppert888"/> 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.<ref name="Byrne2005">Template:Cite journal</ref> 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.<ref name="Ottesen1982">Template:Cite journal</ref>
Each starfish arm contains two gonads that release gametes through openings called gonoducts, located on the central disc between the arms.<ref name="Ruppert888"/> 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.<ref name="Crump1983">Template:Cite journal</ref> In certain species of starfish, the females brood their eggs – either by simply enveloping them<ref name=Crump1983/> or by holding them in specialised structures. Brooding may be done in pockets on the starfish's aboral surface,<ref name="McClary1989">Template:Cite journal</ref><ref name=Ruppert888/> inside the pyloric stomach (Leptasterias tenera)<ref name="Hendler1982">Template:Cite journal</ref> or even in the interior of the gonads themselves.<ref name=Byrne2005/> Those starfish that brood their eggs by "sitting" on them usually assume a humped posture with their discs raised off the substrate.<ref name="Chia1966">Template:Cite journal</ref> Pteraster militaris broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch.<ref name=McClary1989/> 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.<ref name=Byrne2005/> 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.<ref name="Byrne1996">Template:Cite journal</ref> Brooding is especially common in species that live in colder waters.<ref name="Ruppert888">Ruppert et al., 2004. pp. 887–888</ref> and in smaller species that produce just a few eggs.<ref name="Gaymer2013"/><ref>Template:Cite book in Lawrence (2013)</ref>
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.<ref>Template:Cite journal</ref> 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.<ref name="Beach">Template:Cite journal</ref> In other species, a male and female may come together and form a pair.<ref name="Bos2013">Template:Cite journal</ref><ref name="Run1988">Template:Cite journal</ref> This behaviour is called pseudocopulation<ref name="Keesing2011">Template:Cite journal</ref> 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.<ref name=Beach/> Starfish may use environmental signals to coordinate the time of spawning (day length to indicate the correct time of the year,<ref name=Bos2013/> 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.<ref name="Miller1989">Template:Cite journal</ref>
Larval developmentEdit
Starfish embryos typically hatch as blastulas. Invaginations take place, the first creating the anus is created from blastopore, while a second, taking place in the ectodermic layer, creates the mouth. The archenteron stretches towards the mouth and connects with it, forming the gut.<ref>Ruppert et al., 2004. pp. 875</ref> 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.<ref name=Ruppert889/>
The next stage in development is a brachiolaria larva and involves the growth of three short ventral-anterior arms with adhesive tips and surrounding a sucker. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk made from the ventral arms and sucker. Metamorphosis now takes place with a radical rearrangement of tissues. The larvae develops an oral surface on the left and a aboral surface on the right. While the gut remains, the mouth and anus move to new positions. Some of the body cavities disappear 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 up to Template:Convert in diameter.<ref name="Ruppert889" />
Asexual reproductionEdit
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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<ref>Template:Cite journal</ref> or by autotomy of one or more of their arms.<ref>Template:Cite journal</ref> 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 Template:Convert long.<ref name="Edmondson1935">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
The larvae of several species of starfish can reproduce asexually before they reach maturity.<ref name="Eaves2003">Template:Cite journal</ref> They do this by autotomising some parts of their bodies or by budding.<ref name="Jaeckle1994">Template:Cite journal</ref> When such a larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development.<ref name="Vickery200">Template:Cite journal</ref> 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.<ref name=Jaeckle1994/>
RegenerationEdit
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Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.<ref name=Edmondson1935/> 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.<ref name=Ruppert887/> Regrowth can take several months or years,<ref name=Edmondson1935/> 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.<ref name=Edmondson1935/> Other than fragmentation carried out for the purpose of reproduction, the division of the body may happen as a defense mechanism.<ref name=Ruppert887/> 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.<ref name="Hayashi1986">Template:Cite journal</ref> An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms.<ref name="Mladenov1989">Template:Cite journal</ref>
LifespanEdit
The lifespan of a starfish varies considerably between species. For example, Leptasterias hexactis at reaches sexual maturity at Template:Convert and in two years and lives for about ten years. Pisaster ochraceus matures at Template:Convert, and in five years and has a maximum recorded lifespan of 34 years.<ref name="Ruppert889" /> The average lifespan of a starfish is 35 years, and larger starfish species typically live longer than their smaller counterparts.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
EcologyEdit
Distribution and habitatEdit
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.<ref name=Dorit/> 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<ref name="grizmek">Template:Cite encyclopedia</ref> and the deep-sea floor down to at least Template:Convert.<ref>Template:Cite journal</ref> The greatest diversity of species occurs in coastal areas.<ref name=grizmek/>
DietEdit
Most species are generalist predators, eating microalgae, sponges, bivalves, snails and other small animals.<ref name=Carefootfeeding/><ref name=Lawrence124/> The crown-of-thorns starfish consumes coral polyps,<ref name=outbreak/> while other species are detritivores, feeding on decomposing organic material and faecal matter.<ref name="Lawrence124">Template:Cite book in Lawrence (2013)</ref><ref>Template:Cite book in Lawrence (2012)</ref> A few are suspension feeders, gathering in phytoplankton; Henricia and Echinaster often occur in association with sponges, benefiting from the water current they produce.<ref name="Florkin">Template:Cite book</ref> 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.<ref name=Florkin/>
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).<ref>Template:Cite book</ref> 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.<ref name="Dorit">Template:Cite book</ref> Cannibalism has been observed in juvenile sea stars as early as four days after metamorphosis.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Ecological impactEdit
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.<ref>Template:Cite book in Lawrence (2013)</ref> The term "keystone species" was in fact first used by Robert Paine in 1966 to describe a starfish, Pisaster ochraceus.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
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.<ref name="outbreak">Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref name=outbreak/> 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,<ref name="Byrne177">Template:Cite book in Lawrence (2013)</ref> and are on the Invasive Species Specialist Group's list of the world's 100 worst invasive species.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref>Template:Cite news</ref>
ThreatsEdit
Starfish may be preyed on by conspecifics, sea anemones,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> other starfish species, tritons, crabs, fish, gulls and sea otters.<ref name="Gaymer2013">Template:Cite book in Lawrence (2013)</ref><ref name=Byrne177/><ref name="Robles166">Template:Cite book in Lawrence (2013)</ref><ref name="Scheibling150">Template:Cite book in Lawrence (2013)</ref> Their first lines of defence are the saponins present in their body walls, which have unpleasant flavours.<ref name=AnderssonSaponins/> 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.<ref name="Mahdefence">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Other species protect their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities.<ref name=Mahdefence/>
Several species sometimes suffer from a wasting condition caused by bacteria in the genus Vibrio;<ref name=Robles166/> 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).<ref>Template:Cite journal</ref> The results of a study published in 2025 suggest that lower temperatures and higher salinity in fjords off the coast of central British Columbia are providing refuges for populations of sea stars affected by the virus.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref> The protozoan Orchitophrya stellarum is known to infect the gonads of starfish and damage tissue.<ref name=Robles166/> 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 Template:Convert and that they die when their temperature rises to Template:Convert.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> This species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide.<ref>Template:Cite journal</ref> It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs like the stomach.<ref>Template:Cite journal</ref>
Starfish and other echinoderms are sensitive to marine pollution.<ref>Template:Cite journal</ref> The common starfish is considered to be a bioindicator for marine ecosystems.<ref>Template:Cite journal</ref> 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 Template:Convert 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.<ref>Template:Cite journal</ref>
EvolutionEdit
Fossil recordEdit
Echinoderms first appeared in the fossil record in the Cambrian. The first known asterozoans were the Somasteroidea, which exhibit characteristics of both groups.<ref name="Waggonerfossil">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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".<ref>Template:Cite book</ref>
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.<ref name=Waggonerfossil/> 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.<ref name=treeoflife/><ref name="plosoneMah">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
DiversityEdit
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The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830.<ref>Template:Cite WoRMS</ref> It is derived from the Greek aster, ἀστήρ (a star) and the Greek eidos, εἶδος (form, likeness, appearance).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="Lander">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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<ref name="treeoflife">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed "vertebrae".<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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<ref name="Asterozoa"/> and two extinct ones, Calliasterellidae and Trichasteropsida.<ref name=Knott/> 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.<ref name=plosoneMah/>
Living groupsEdit
- Brisingida (2 families, 17 genera, 111 species)<ref>Template:Cite WoRMS</ref>
- 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.<ref>Template:Cite journal</ref><ref>Template:Cite encyclopedia</ref> In some species, the tube feet have rounded tips and lack suckers.<ref name="Vickery">Template:Cite journal</ref>
- Forcipulatida (6 families, 63 genera, 269 species)<ref>Template:Cite WoRMS</ref>
- Species in this order have distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. They tend to have robust bodies<ref name="Barnes">Template:Cite book</ref> and have tube feet with flat-tipped suckers usually arranged in four rows.<ref name=Vickery/> 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.<ref>Template:Cite encyclopedia</ref>
- Notomyotida (1 family, 8 genera, 75 species)<ref>Template:Cite WoRMS</ref>
- These starfish are deep-sea dwelling and have particularly flexible arms. The inner dorso-lateral surfaces of the arms contain characteristic longitudinal muscle bands.<ref name="Asterozoa"/> In some species, the tube feet lack suckers.<ref name=Vickery/>
- Paxillosida (7 families, 48 genera, 372 species)<ref>Template:Cite WoRMS</ref>
- This is a primitive order and members do not extrude their stomach when feeding,<ref name="Ruppert885"/> lack an anus and have no suckers on their tube feet. Papulae are present on their aboral surface and they possess marginal plates and paxillae. They mostly inhabit soft substrates.<ref name="Ruppert889">Ruppert et al, 2004. pp. 887–889</ref> There is no brachiolaria stage in their larval development.<ref>Template:Cite journal</ref> The comb starfish (Astropecten polyacanthus) is a member of this order.<ref>Template:Cite WoRMS</ref>
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- Spinulosida (1 family, 8 genera, 121 species)<ref>Template:Cite WoRMS</ref>
- 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.<ref>Template:Cite encyclopedia</ref><ref name=Blake/> This group includes the red starfish Echinaster sepositus.<ref>Template:Cite WoRMS</ref>
- Valvatida (16 families, 172 genera, 695 species)<ref>Template:Cite WoRMS</ref>
- 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.<ref name="Blake">Template:Cite journal</ref> This group includes the cushion stars,<ref>Template:Cite WoRMS</ref> the leather star<ref>Template:Cite WoRMS</ref> and the sea daisies.<ref>Template:Cite WoRMS</ref>
- Velatida (4 families, 16 genera, 138 species)<ref>Template:Cite WoRMS</ref>
- 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.<ref>Template:Cite encyclopedia</ref> This group includes the slime star.<ref>Template:Cite WoRMS</ref>
Extinct groupsEdit
Extinct groups within the Asteroidea include:<ref name="Knott">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
- † Calliasterellidae, with the type genus Calliasterella from the Devonian and Carboniferous<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref>
- † Palastericus, a Devonian genus<ref>Walker, Cyril, Ward, DavidFossils : Smithsonian Handbook, Template:ISBN (2002, paperback, revisited), Template:ISBN (1992, 1st edition). Page 186</ref>
- † Trichasteropsida, with the Triassic genus Trichasteropsis (at least 2 species)<ref name=Knott/>
PhylogenyEdit
ExternalEdit
Starfish are deuterostome animals, like the chordates. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree.<ref name="TelfordLowe2014">Template:Cite journal</ref> The times at which the clades diverged are shown under the labels in millions of years ago (mya).
InternalEdit
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.<ref name=Knott/> The phylogeny proposed by Gale in 1987 is:<ref name=Knott/><ref>Template:Cite journal</ref>
The phylogeny proposed by Blake in 1987 is:<ref name=Knott/><ref>Template:Cite journal</ref>
Later work making use of molecular evidence, with or without the use of morphological evidence, had by 2000 failed to resolve the argument.<ref name=Knott/> 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.<ref name="Janies">Template:Cite journal</ref>
Human relationsEdit
In researchEdit
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.<ref>Template:Cite journal</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Asterina pectinifera, used as a model organism for this purpose, is resilient and easy to breed and maintain in the laboratory.<ref name="Davydov">Template:Cite book</ref>
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.<ref>Template:Cite journal</ref>
Starfish also have an unusual ability to expel foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
In legend and cultureEdit
An aboriginal Australian fable retold by the Welsh school headmaster William Jenkyn Thomas (1870–1959)<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>Template:Cite book</ref>
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.<ref name="Tregear">Template:Cite journal</ref>
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".<ref name="Rumpf">Template:Cite book</ref>
As foodEdit
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,<ref name="AnderssonSaponins">Template:Cite journal</ref> and others contain tetrodotoxins which are poisonous.<ref>Template:Cite journal</ref> Some species that prey on bivalve molluscs can transmit paralytic shellfish poisoning.<ref>Template:Cite journal</ref> 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" Template: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.<ref name=Rumpf/> Starfish are sometimes eaten in China,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Japan<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and in Micronesia.<ref>Template:Cite book</ref>
As collectablesEdit
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.<ref name=Scheibling150/> A similar phenomenon exists in the Indo-Pacific for species such as Protoreaster nodosus.<ref>Template:Cite journal</ref>
ReferencesEdit
BibliographyEdit
External linksEdit
- {{#invoke:citation/CS1|citation
|CitationClass=web }} A blog about sea stars by a passionate and professional specialist.