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88-535: Pisaster (from Greek πίσος , "pea", and ἀστήρ , "star") is a genus of Pacific sea stars that includes three species, P. brevispinus , P. giganteus , and P. ochraceus . Their range extends along the Pacific coast from Alaska to southern California in the intertidal zone. The largest individuals of Pisaster can reach diameters of up to 70 cm (28 in) across; they all develop five arms, but some may be lost from injury or disease, and occasionally

176-435: 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

264-609: A tessellated manner and form the main covering of the aboral surface. Some are specialised structures such as the madreporite (the entrance to the water vascular system), pedicellariae and paxillae . 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. Paxillae are umbrella-like structures found on starfish that live buried in sediment. The edges of adjacent paxillae meet to form

352-570: A wasting condition caused by bacteria in the genus Vibrio ; 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). The protozoan Orchitophrya stellarum is known to infect the gonads of starfish and damage tissue. Starfish are vulnerable to high temperatures. Experiments have shown that

440-490: 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 . The feeding activity of

528-471: A cascading effect on the whole benthic community and reef-feeding fish. 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, and are on

616-411: 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 while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed "vertebrae". The starfish are

704-411: 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. Regrowth can take several months or years, 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. Other than fragmentation carried out for

792-399: 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. Several groups of starfish, including Valvatida and Forcipulatida , possess pedicellariae . In Forcipulatida, such as Asterias and Pisaster , they occur in pompom -like tufts at

880-497: 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. 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 or by autotomy of one or more of their arms. Which of these processes occurs depends on

968-545: A large and diverse class with over 1,900 living species. There are seven extant orders, Brisingida , Forcipulatida , Notomyotida , Paxillosida , Spinulosida , Valvatida and Velatida and two extinct ones, Calliasterellidae and Trichasteropsida . Living asteroids, the Neoasteroidea, are morphologically distinct from their forerunners in the Paleozoic. The taxonomy of the group is relatively stable but there

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1056-443: A large female can split in half and the resulting offspring are males. When these grow large enough they change back into females. Each starfish arm contains two gonads that release gametes through openings called gonoducts, located on the central disc between the arms. Fertilization is generally external but in a few species, internal fertilization takes place. In most species, the buoyant eggs and sperm are simply released into

1144-400: A larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development. 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. Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time. A few can regrow

1232-546: A male and female may come together and form a pair. This behaviour is called pseudocopulation 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. Starfish may use environmental signals to coordinate the time of spawning (day length to indicate the correct time of the year, 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

1320-546: A maximum recorded lifespan of 34 years. The average lifespan of a starfish is 35 years, and larger starfish species typically live longer than their smaller counterparts. 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. 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 and

1408-475: 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

1496-432: 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. 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. The Antarctic Labidiaster annulatus can have over fifty. Mapping

1584-408: A protective external false skin. Beneath this is a water-filled cavity which contains the madreporite and delicate gill structures known as papillae. Sea urchins are covered with plates which are usually fused together to give a rigid test , but in the order Echinothurioida , the test is leathery because the plates are separate. The test is divided into five segments that extend from the apex to

1672-437: A protective function and are also used for locomotion. Pedicellariae are compound ossicles that articulate with other ossicles and protrude from the aboral (upper) surface of some sea stars (and also the test of sea urchins). They usually have short fleshy stalks and either two or three moveable ossicles forming a set of pincer-like jaws. They may be scattered over the surface or may be grouped around spines. Their function

1760-411: 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

1848-592: A role in the distribution and abundance of organisms such as fish, crabs and sea urchins that feed on the sediment. 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 . 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

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1936-456: A small number of alkaloids have also been identified. The functions of these chemicals in the starfish have not been fully investigated but most have roles in defence and communication. Some are feeding deterrents used by the starfish to discourage predation. Others are antifoulants and supplement the pedicellariae to prevent other organisms from settling on the starfish's aboral surface. Some are alarm pheromones and escape-eliciting chemicals,

2024-504: 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. 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

2112-537: A thin cuticle, an epidermis consisting of a single layer of cells, a thick dermis formed of connective tissue and a thin coelomic myoepithelial layer, which provides the longitudinal and circular musculature. The dermis contains an endoskeleton of calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice. They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in

2200-454: A tough peristomial membrane and closed with a sphincter . The mouth opens through a short oesophagus into a stomach divided by a constriction into a larger, eversible cardiac portion and a smaller pyloric portion. The cardiac stomach is glandular and pouched, and is supported by ligaments attached to ossicles in the arms so it can be pulled back into position after it has been everted. The pyloric stomach has two extensions into each arm:

2288-584: A variety of forms including flat plates, spines, rods and crosses, and specialised compound structures including pedicellariae and paxillae . Plates are tabular ossicles that fit neatly together in a tessellated manner. They form the main skeletal covering for sea urchins and sea stars. Spines are ossicles that project from the body wall and articulate with other ossicles through ball and socket joints mounted on tubercles . They are formed from crystals of calcite and can be solid or hollow, long or short, thick or thin and sharp or blunt. The spines serve

2376-446: Is apparent when they spawn . 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. 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 ,

2464-440: Is at the apex of a vertical channel (the axial vessel) that connects the three rings. At the base of each arm are paired gonads ; a lateral vessel extends from the genital ring past the gonads to the tip of the arm. This vessel has a blind end and there is no continuous circulation of the fluid within it. This liquid does not contain a pigment and has little or no respiratory function but is probably used to transport nutrients around

2552-494: Is light but tough and collagenous ligaments connect the ossicles together. The ossicles are embedded in a tough connective tissue which is also part of the endoskeleton. When an ossicle becomes redundant, specialised cells known as phagocytes are able to reabsorb the calcareous material. All the ossicles, even those that protrude from the body wall, are covered by a thin layer of epidermis but functionally they act more like an exoskeleton than an endoskeleton. Ossicles have

2640-752: 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. Extinct groups within the Asteroidea include: Starfish are deuterostome animals, like

2728-418: Is passed into their pyloric stomachs and caeca where digestion continues and absorption ensues. 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

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2816-401: Is replaced with radial symmetry , typically pentameric . 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. Starfish are included in the subphylum Asterozoa , the characteristics of which include a flattened, star-shaped body as adults consisting of

2904-453: Is to pick off debris so as to keep the surface clean and to prevent larvae of other invertebrates from settling and growing there. Paxillae are small pillar-shaped ossicles with flat tops sometimes found covering the aboral surface of sea stars such as Luidia , Astropecten and Goniaster that live underneath sediment. Their stalks emerge from the body wall and their tops, each fringed with short spines, and abut each other to form

2992-415: Is transferred from these to the coelomic fluid , which acts as the transport medium for gasses. Oxygen dissolved in the water is distributed through the body mainly by the fluid in the main body cavity; the circulatory system may also play a minor role. 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

3080-616: 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

3168-481: 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

3256-412: The chordates . A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree . The times at which the clades diverged are shown under the labels in millions of years ago (mya). Xenacoelomorpha [REDACTED] Chordata [REDACTED] Ossicle (echinoderm) Ossicles are created intracellularly by specialised secretory cells known as sclerocytes in

3344-451: The class Asteroidea ( / ˌ æ s t ə ˈ r ɔɪ d i ə / ). 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

3432-428: The dermis of the body wall of echinoderms. Each ossicle is composed of microcrystals of calcite arranged in a three-dimensional lattice known as a stereom . Under polarized light the ossicle behaves as if it were a single crystal because the axes of all the crystals are parallel. The space between the crystals is known as the stroma and allows entry to sclerocytes for enlargement and repair. The honeycomb structure

3520-609: The fossil record in the Cambrian . The first known asterozoans were the Somasteroidea , which exhibit characteristics of both groups. 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". By

3608-495: The intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface. Starfish are marine invertebrates . They typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by

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3696-576: 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. 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

3784-409: The pH is lowered. Researchers found that when P. ochraceus were exposed to 21 °C (70 °F) and 770  ppm carbon dioxide (beyond rises expected in the next century), they were relatively unaffected. Their survival is likely due to the nodular nature of their skeletons, which are able to compensate for a shortage of carbonate by growing more fleshy tissue. Echinoderms first appeared in

3872-724: The saponins present in their body walls, which have unpleasant flavours. 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. 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 . Other species protect their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities. Several species sometimes suffer from

3960-599: The Invasive Species Specialist Group's list of the world's 100 worst invasive species . 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. Starfish may be preyed on by conspecifics, sea anemones, other starfish species, tritons , crabs, fish, gulls and sea otters . Their first lines of defence are

4048-419: The aboral pole consisting of five genital plates, one of which is the madreporite , and five smaller ocular plates. Other large specialist plates surround the mouth in a set of jaws known as Aristotle's lantern . Sea stars have separate plates giving flexibility to the disc and arms. They are arranged into interambulacral and ambulacral regions and the arms have an ambulacral groove on the underside from which

4136-535: The articulating element that joins it to the next. Several types of small ossicles are found in the body wall of sea cucumbers . Baskets are cup-shaped and usually have four projections. Buttons are disc-shaped and pierced by four holes and may be smooth or knobbed. Perforated plates are sieve-like and often widely distributed and rods provide support for the tube feet and tentacles . In the order Apodida , members of which lack tube feet, there are anchor-shaped ossicles attached to anchor plates. The flukes project from

4224-585: 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. Some species like Labidiaster annulatus , Rathbunaster californicus and Novodinia antillensis use their large pedicellariae to capture small fish and crustaceans. There may also be papulae , thin-walled protrusions of

4312-496: The body cavity that reach through the body wall and extend into the surrounding water. These serve a respiratory function. 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. The water vascular system of

4400-622: The body. Starfish produce a large number of secondary metabolites in the form of lipids , including steroidal derivatives of cholesterol , and fatty acid amides of sphingosine . The steroids are mostly saponins , known as asterosaponins, and their sulphated derivatives. They vary between species and are typically formed from up to six sugar molecules (usually glucose and galactose ) connected by up to three glycosidic chains. Long-chain fatty acid amides of sphingosine occur frequently and some of them have known pharmacological activity . Various ceramides are also known from starfish and

4488-555: The deep-sea floor down to at least 6,000 m (20,000 ft). The greatest diversity of species occurs in coastal areas. Most species are generalist predators, eating microalgae , sponges , bivalves , snails and other small animals. The crown-of-thorns starfish consumes coral polyps, while other species are detritivores , feeding on decomposing organic material and faecal matter. A few are suspension feeders, gathering in phytoplankton ; Henricia and Echinaster often occur in association with sponges, benefiting from

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4576-443: 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. The body wall consists of

4664-435: The feeding and growth rates of P. ochraceus reduce greatly when their body temperatures rise above 23 °C (73 °F) and that they die when their temperature rises to 30 °C (86 °F). This species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide. It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs like

4752-578: 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. 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. The larvae of several species of starfish can reproduce asexually before they reach maturity. They do this by autotomising some parts of their bodies or by budding . When such

4840-573: The interior of the gonads themselves. Those starfish that brood their eggs by "sitting" on them usually assume a humped posture with their discs raised off the substrate. Pteraster militaris broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch. 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. The developing young are called lecithotrophic because they obtain their nutrition from

4928-404: The lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine environments. 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

5016-429: 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. Many starfish species also became extinct in these events, but afterwards

5104-703: The latter remaining a broadcast spawner. The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830. It is derived from the Greek aster , ἀστήρ (a star) and the Greek eidos , εἶδος (form, likeness, appearance). 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

5192-406: The leading arms, while others trail behind. Most starfish cannot move quickly, a typical speed being that of the leather star ( Dermasterias imbricata ), which can manage just 15 cm (6 in) in a minute. 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

5280-459: The mouth. The next stage in development is a brachiolaria larva and involves the growth of three short, additional arms. These are at the anterior end, surround a sucker and have adhesive cells at their tips. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk formed from the ventral arms and sucker. Metamorphosis now takes place with

5368-463: The mouth. Another invagination of the surface will fuse with the tip of the archenteron as the mouth while the interior section will become the gut. At the same time, a band of cilia develops on the exterior. This enlarges and extends around the surface and eventually onto two developing arm-like outgrowths. At this stage the larva is known as a bipinnaria . The cilia are used for locomotion and feeding, their rhythmic beat wafting phytoplankton towards

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5456-427: The mouth. Each contains two ambulacral rows of plates alternating with two interambulacral rows. The ambulacral plates are each pierced by a pair of pores through which the active tube feet are connected to the water vascular system . Ossicles in the form of spines connect to tubercles on some of the plates. Sea urchins have several types of pedicellariae, some of which are toxic. A ring of specialised plates surround

5544-471: The ocean floor. The sand star ( Luidia foliolata ) can travel at a speed of 2.8 m (9 ft 2 in) per minute. 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. Apart from their function in locomotion,

5632-404: The other arms to initiate movement towards the prey. The mechanism for this is not fully understood. 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

5720-668: 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. The retraction and contraction of the cardiac stomach is activated by a neuropeptide known as NGFFYamide. Because of this ability to digest food outside the body, starfish can hunt prey much larger than their mouths. Their diets include clams and oysters , arthropods , small fish and gastropod molluscs . Some starfish are not pure carnivores , supplementing their diets with algae or organic detritus. Some of these species are grazers , but others trap food particles from

5808-559: 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. 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. An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms. The lifespan of

5896-541: The pyloric caeca. These are elongated, branched hollow tubes that are lined by a series of glands, which secrete digestive enzymes and absorb nutrients from the food. A short intestine and rectum run from the pyloric stomach to open at a small anus at the apex of the aboral surface of the disc. Primitive starfish, such as Astropecten and Luidia , swallow their prey whole, and start to digest it in their cardiac stomachs. Shell valves and other inedible materials are ejected through their mouths. The semi-digested fluid

5984-553: The re-growth of an injured arm will result in an individual with more than five arms. Sea stars in the genus Pisaster are all predators; the ochre sea star is the best-characterized of these, and is considered a "keystone" predator that controls the relative abundance of many other species in the ecosystem. This article about a starfish is a stub . You can help Misplaced Pages by expanding it . Asteroidea † Calliasterellidae † Trichasteropsida Starfish or sea stars are star-shaped echinoderms belonging to

6072-420: The release of which trigger responses in conspecific starfish but often produce escape responses in potential prey. Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide. Most species of starfish are gonochorous , there being separate male and female individuals. These are usually not distinguishable externally as the gonads cannot be seen, but their sex

6160-452: The sea water. Most starfish embryos hatch at the blastula stage. The original ball of cells develops a lateral pouch, the archenteron . The entrance to this is known as the blastopore and it will later develop into the anus—together with chordates , echinoderms are deuterostomes , meaning the second ( deutero ) invagination becomes the mouth ( stome ); members of all other phyla are protostomes , and their first invagination becomes

6248-675: The soft substrate to extract prey (usually clams ). 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. Cannibalism has been observed in juvenile sea stars as early as four days after metamorphosis. 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. The term "keystone species"

6336-417: The species. While a starfish lacks a centralized brain , it has a complex nervous system with a nerve ring around the mouth and a radial nerve running along the ambulacral region of each arm parallel to the radial canal. The peripheral nerve system consists of two nerve nets: a sensory system in the epidermis and a motor system in the lining of the coelomic cavity. Neurons passing through the dermis connect

6424-467: 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

6512-412: The stomach. Starfish and other echinoderms are sensitive to marine pollution . The common starfish is considered to be a bioindicator for marine ecosystems. 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

6600-452: 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. 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. Having descended from bilateral organisms, starfish may move in a bilateral fashion, particularly when hunting or in danger. When crawling, certain arms act as

6688-551: The surviving few species diversified rapidly within about sixty million years during the Early Jurassic and the beginning of the Middle Jurassic . 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

6776-408: The tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water. Some waste may also be excreted by the pyloric glands and voided with the faeces . Starfish do not appear to have any mechanisms for osmoregulation , and keep their body fluids at the same salt concentration as the surrounding water. Although some species can tolerate relatively low salinity ,

6864-569: The tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food. There are eyespots at the ends of the arms, each one made of 80–200 simple ocelli . These are composed of pigmented epithelial cells that respond to light and are covered by a thick, transparent cuticle that both protects the ocelli and acts to focus light. Many starfish also possess individual photoreceptor cells in other parts of their bodies and respond to light even when their eyespots are covered. Whether they advance or retreat depends on

6952-473: 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. 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. In other species,

7040-415: 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 . Gas exchange also takes place through other gills known as papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Oxygen

7128-572: The tube feet project. Other ossicles that may be present include pedicellariae and paxillae. There is often a large row of marginal plates adjoining the ambulacral groove, sometimes bearing spines. Brittle stars do not have pedicellariae, and the plates that cover their surface are known as shields. On the arms these are in four rows with each segment having an aboral and oral shield and two lateral shields, usually with fringing spines. Other ossicles include spines, tubercles, small scales and vertebrae. The large central vertebrae in each arm segment provides

7216-412: The two. The ring nerves and radial nerves have sensory and motor components and coordinate the starfish's balance and directional systems. 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

7304-438: 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. In certain species of starfish, the females brood their eggs – either by simply enveloping them or by holding them in specialised structures. Brooding may be done in pockets on the starfish's aboral surface, inside the pyloric stomach ( Leptasterias tenera ) or even in

7392-464: The water current they produce. 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. 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

7480-455: The water in sticky mucus strands that are swept towards the mouth along ciliated grooves. The main nitrogenous waste product is ammonia . Starfish have no distinct excretory organs; waste ammonia is removed by diffusion through the tube feet and papulae. The body fluid contains phagocytic cells called coelomocytes, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to

7568-424: The whole canal system is lined with cilia . 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. Other chemicals and relaxation of the ampullae allow for release from

7656-411: 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. Brooding is especially common in polar and deep-sea species that live in environments unfavourable for larval development and in smaller species that produce just a few eggs. In

7744-517: Was in fact first used by Robert Paine in 1966 to describe a starfish, Pisaster ochraceus . 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. Similar results were found in

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