117-684: Pisaster ochraceus , generally known as the purple sea star , ochre sea star , or ochre starfish , is a common seastar found among the waters of the Pacific Ocean . Identified as a keystone species , P. ochraceus is considered an important indicator for the health of the intertidal zone . This sea star has five stout rays that range in length from 10 to 25 centimeters (4 to 10 in). The rays are arranged around an ill-defined central disk. While most individuals are purple, they can be orange, orange-ochre, yellow, reddish, or brown. The aboral surface contains many small spines (ossicles) that are arranged in
234-435: 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
351-609: 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
468-565: 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
585-484: 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
702-466: 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
819-401: A cassowary. A term used alongside keystone is ecosystem engineer . In North America , the prairie dog is an ecosystem engineer. Prairie dog burrows provide the nesting areas for mountain plovers and burrowing owls . Prairie dog tunnel systems also help channel rainwater into the water table to prevent runoff and erosion , and can also serve to change the composition of the soil in
936-411: 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
1053-411: 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
1170-399: 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
1287-497: 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
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#17328798548911404-539: A lack of predation caused channel incision because the beavers helped slow the water down, allowing soil to stay in place. Furthermore, predation keeps hydrological features such as creeks and streams in normal working order. When wolves were reintroduced, the beaver population and the whole riparian ecosystem recovered dramatically within a few years. As described by Paine in 1966, some sea stars (e.g., Pisaster ochraceus ) may prey on sea urchins , mussels , and other shellfish that have no other natural predators. If
1521-533: 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
1638-443: 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
1755-400: 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
1872-546: 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
1989-546: 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
2106-469: 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
2223-551: A netlike or pentagonal pattern on the central disk. The ossicles are no higher than 2 mm. In Pisaster the tube feet have suckers on their distal ends which allow them to attach to the rocky substrate and live in heavily wave-swept areas. P. ochraceus has a simple nervous system and does not have a brain. A nerve ring connects and relays impulses between the star's radial nerves. Two species that can be mistaken for P. ochraceus are P. giganteus , which has blue rings around white or purple spines, and P. brevispinus , which
2340-444: A pair of gonads branching into each ray off a circular genital strand which is along the oral inner surface of the central disc. The gonads look like a feathery collection of tubules. In females there are orange gonads and in males they are whitish. During maturation of the gametes, the gonads increase in size and can account for up to 40 percent of the sea star's weight. The gonopores are too small to be seen, and can only be found when
2457-432: 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
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#17328798548912574-433: A predator may allow other animals to increase to the point where they wipe out other species; removing a prey species may cause predator populations to crash, or may allow predators to drive other prey species to extinction; and removing a plant species may result in the loss of animals that depend on it, like pollinators and seed dispersers. Beavers too have been called keystone, not for eating other species but for modifying
2691-527: A profound influence on the balance of organisms in a particular ecosystem . Introduction or removal of a keystone predator, or changes in its population density, can have drastic cascading effects on the equilibrium of many other populations in the ecosystem. For example, grazers of a grassland may prevent a single dominant species from taking over. The elimination of the gray wolf from the Greater Yellowstone Ecosystem had profound impacts on
2808-411: 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
2925-410: A region by increasing aeration and reversing soil compaction that can be a result of cattle grazing. Prairie dogs also trim the vegetation around their colonies, perhaps to remove any cover for predators. Grazing species such as plains bison , which is another keystone species, the pronghorn , and the mule deer have shown a proclivity for grazing on the same land used by prairie dogs. The beaver
3042-578: 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
3159-456: 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,
3276-573: A species could be described as a keystone depended on the ecological context. Although the concept of the keystone species has a value in describing particularly strong inter-species interactions, and for allowing easier communication between ecologists and conservation policy-makers, it has been criticized by L. S. Mills and colleagues for oversimplifying complex ecological systems. The term has been applied widely in different ecosystems and to predators, prey, and plants (primary producers), inevitably with differing ecological meanings. For instance, removing
3393-504: 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
3510-541: A system in Makah Bay in Washington . In his 1969 paper, Paine proposed the keystone species concept, using Pisaster ochraceus , a species of starfish generally known as ochre starfish, and Mytilus californianus , a species of mussel, as a primary example. The ochre starfish is a generalist predator and feeds on chitons, limpets, snails, barnacles, echinoids, and even decapod crustacea. The favourite food for these starfish
3627-537: 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
Pisaster ochraceus - Misplaced Pages Continue
3744-454: 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:
3861-421: Is a well known ecosystem engineer and keystone species. It transforms its territory from a stream to a pond or swamp. Beavers affect the environment first altering the edges of riparian areas by cutting down older trees to use for their dams. This allows younger trees to take their place. Beaver dams alter the riparian area they are established in. Depending on topography, soils, and many factors, these dams change
3978-446: 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 ,
4095-440: 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
4212-444: Is from above the low-tide zone to 90 metres (300 ft). P. ochraceous is very durable and can tolerate a loss of thirty percent of its body weight in body fluids. A study found that P. ochraceus will not be affected by ocean acidification in the same way as most calcareous marine animals. This normally causes decreased growth due to the increased acidity dissolving calcium carbonate . Researchers found that when P. ochraceus
4329-431: Is large relative to its biomass dominance within a functional group." A classic keystone species is a predator that prevents a particular herbivorous species from eliminating dominant plant species. If prey numbers are low, keystone predators can be even less abundant and still be effective. Yet without the predators, the herbivorous prey would explode in numbers, wipe out the dominant plants, and dramatically alter
4446-455: Is no sexual dimorphism and sexes can be separated only by the presence of eggs or sperm in the gonads. They reproduce by broadcast spawning, which in the Puget Sound population occurs around May to July. There is no parental investment beyond spawning. Fertilization occurs in the water column and Pisaster ochraceous develops through several larval stages. The reproductive system consists of
4563-722: 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
4680-418: 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
4797-461: Is pink with small white spines. These two species have different aboral spines and coloration which allows one to distinguish between the species. Evasterias troschelii may be confused with P. ochraceus at times as well. It can be distinguished by its smaller disk size and longer, tapering rays which are often thickest a short distance out from their base rather than at the base as in P. ochraceus . Members of Pisaster are dioecious but there
Pisaster ochraceus - Misplaced Pages Continue
4914-401: 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
5031-501: Is the mussel which is a dominant competitor for the space on the rocks. The ochre starfish keeps the population numbers of the mussels in check along with the other preys allowing the other seaweeds, sponges, and anemones, that ochre starfish do not consume, to co-exist. When Paine removed the ochre starfish, the mussels quickly outgrew the other species crowding them out. At the start, the rock pools held 15 rock-clinging species. Three years later there were 8 such species; and ten years later
5148-445: Is the sole source of nectar for honeyeaters , which play an important role in pollination of numerous plant species. Therefore, the loss of this one species of tree would probably cause the honeyeater population to collapse, with profound implications for the entire ecosystem. Another example is frugivores , such as the cassowary , which spreads the seeds of many different trees. Some seeds will not grow unless they have been through
5265-415: 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
5382-598: 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
5499-515: The Serengeti , the presence of sufficient gnus in these grasslands reduces wildfire likelihood, which in turn promotes tree growth. The documentary The Serengeti Rules documents this in detail. The community ecologist Bruce Menge states that the keystone concept has been stretched far beyond Paine's original concept. That stretching can be quantified: the researcher Ishana Shukla has listed 230 species identified as keystones in some 157 studies in
5616-481: 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
5733-461: 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] Keystone species A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept
5850-451: 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
5967-591: 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
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#17328798548916084-495: 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
6201-424: The jaguar is a charismatic big cat which meets all of these definitions: The jaguar is an umbrella species, flagship species, and wilderness quality indicator. It promotes the goals of carnivore recovery, protecting and restoring connectivity through Madrean woodland and riparian areas, and protecting and restoring riparian areas. ... A reserve system that protects jaguars is an umbrella for many other species. ...
6318-493: The mammalian jungle ecosystem with its consumption of 87 different species of prey. The lion is another keystone species. Keystone mutualists are organisms that participate in mutually beneficial interaction, the loss of which would have a profound impact upon the ecosystem as a whole. For example, in the Avon Wheatbelt region of Western Australia , there is a period of each year when Banksia prionotes (acorn banksia)
6435-569: 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
6552-409: 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
6669-724: 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
6786-463: The trophic pyramid . Without predation, herbivores began to over-graze many woody browse species, affecting the area's plant populations. In addition, wolves often kept animals from grazing in riparian areas, which protected beavers from having their food sources encroached upon. The removal of wolves had a direct effect on beaver populations, as their habitat became grazing territory. Increased browsing on willows and conifers along Blacktail Creek due to
6903-414: The wolf and lion , are also apex predators . The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch . While the keystone is under the least pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of
7020-409: The 1960s demonstrated that a loss of only a few individual P. ochraceus seastars had a profound impact on mussel bed population, thereby reducing the health of the intertidal environment. With only a few natural predators (sea otters and seagulls) it is suggested that the principal threats to P. ochraceus are human collectors and casual tidepool visitors. Pisaster ochraceus has not been evaluated by
7137-464: The 50 years since Paine's paper. Menge's own work has shown that the purple Pisaster sea star that Paine had studied was a powerful keystone species in places exposed to strong wave action, but was far less important in sheltered places. Paine had indeed stated that in Alaska, without the relevant mussel species as prey, the predatory Pisaster was "just another sea star". In other words, the extent to which
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#17328798548917254-582: The International Union for Conservation of Nature (IUCN). P. ochraceus can be found from Prince William Sound in Alaska to Point Sal in Santa Barbara County, California. The subspecies found within the warmer waters from Santa Barbara County to Baja California is P. o. segnis . This sea star can be found in great numbers on mussel beds and on wave-washed rocky shores. The juveniles are often found in crevices and under rocks. Its depth range
7371-586: 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
7488-578: 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
7605-496: 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
7722-622: 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
7839-483: The character of the ecosystem. The exact scenario changes in each example, but the central idea remains that through a chain of interactions, a non-abundant species has an outsized impact on ecosystem functions. For example, the herbivorous weevil Euhrychiopsis lecontei is thought to have keystone effects on aquatic plant diversity by foraging on nuisance Eurasian watermilfoil in North American waters. Similarly,
7956-555: 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
8073-432: The ecosystem by measures of biomass or productivity . It became a popular concept in conservation biology , alongside flagship and umbrella species . Although the concept is valued as a descriptor for particularly strong inter-species interactions, and has allowed easier communication between ecologists and conservation policy-makers, it has been criticized for oversimplifying complex ecological systems. The concept of
8190-443: 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
8307-435: 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
8424-578: 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
8541-573: 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
8658-440: The jaguar [is] a keystone in subtropical and tropical America ... Sea otters protect kelp forests from damage by sea urchins. When the sea otters of the North American west coast were hunted commercially for their fur, their numbers fell to such low levels – fewer than 1000 in the north Pacific ocean – that they were unable to control the sea urchin population. The urchins, in turn, grazed the holdfasts of kelp so heavily that
8775-491: The kelp forests largely disappeared, along with all the species that depended on them. Reintroducing the sea otters has enabled the kelp ecosystem to be restored. For example, in Southeast Alaska some 400 sea otters were released, and they have bred to form a population approaching 25,000. Keystone predators may increase the biodiversity of communities by preventing a single species from becoming dominant. They can have
8892-474: The keystone species was introduced in 1969 by zoologist Robert T. Paine . Paine developed the concept to explain his observations and experiments on the relationships between marine invertebrates of the intertidal zone (between the high and low tide lines), including starfish and mussels . He removed the starfish from an area, and documented the effects on the ecosystem. In his 1966 paper, Food Web Complexity and Species Diversity , Paine had described such
9009-404: 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
9126-429: 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
9243-691: 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
9360-406: 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
9477-459: 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
9594-463: 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
9711-471: 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,
9828-404: 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
9945-595: The pools were largely occupied by a single species, mussels. The concept became popular in conservation, and was deployed in a range of contexts and mobilized to engender support for conservation, especially where human activities had damaged ecosystems, such as by removing keystone predators. A keystone species was defined by Paine as a species that has a disproportionately large effect on its environment relative to its abundance. It has been defined operationally by Davic in 2003 as "a strongly interacting species whose top-down effect on species diversity and competition
10062-425: The prey is too large to be swallowed whole, then it can use its tube feet to open shells. It can evert its stomach through its mouth and engulf its prey, liquify it with digestive enzymes and ingest the processed food. Mussels hold their valves together very securely but P. ochraceus can insert part of its everted stomach, or some digestive juices, through the narrow gap that exists where the byssal threads emerge from
10179-668: 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
10296-559: 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
10413-541: 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
10530-420: 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
10647-504: The riparian edges of streams and rivers into wetlands, meadows, or riverine forests. These dams have been shown to be beneficial to a myriad of species including amphibians, salmon, and song birds. In the African savanna , the larger herbivores, especially the elephants , shape their environment. The elephants destroy trees, making room for the grass species and creating habitat for various small animal species. Without these animals, much of
10764-570: The savanna would turn into woodland . In the Amazon river basin , peccaries produce and maintain wallows that are utilized by a wide variety of species. Australian studies have found that parrotfish on the Great Barrier Reef are the only reef fish that consistently scrape and clean the coral on the reef. Without these animals, the Great Barrier Reef would be under severe strain. In
10881-492: The sea star is removed from the ecosystem, the mussel population explodes uncontrollably, driving out most other species. These creatures need not be apex predators . Sea stars are prey for sharks , rays , and sea anemones . Sea otters are prey for orca . The jaguar , whose numbers in Central and South America have been classified as near threatened , acts as a keystone predator by its widely varied diet, helping to balance
10998-489: The sea stars are spawning. Many sea stars live to a minimal age of four years. P. ochraceus can live as long as twenty years. This species of seastar is often considered a keystone species in many intertidal regions. P. ochraceus is a predator of the California mussel, Mytilus californianus and reduces its abundance. This allows for other macroinvertebrates to persist. In an experimental removal of P. ochraceus , it
11115-452: 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
11232-437: The shell. The mussel needs to open its valves periodically to feed and breathe and the sea star can exert a powerful traction with its tube feet, pulling the two valves further open. Once the stomach is inside the mussel, digestion takes place. It is thought one sea star can consume eighty Californian mussels in a year. Pisaster ochraceus has been described as a keystone species . Experiments by zoologist Robert T. Paine in
11349-675: 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"
11466-417: 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
11583-467: 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
11700-412: 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
11817-452: 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
11934-537: 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
12051-408: 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 ,
12168-569: 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
12285-473: 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,
12402-415: 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
12519-412: 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
12636-559: The wasp species Agelaia vicina has been labeled a keystone species for its unparalleled nest size, colony size, and high rate of brood production. The diversity of its prey and the quantity necessary to sustain its high rate of growth have a direct impact on other species around it. The keystone concept is defined by its ecological effects, and these in turn make it important for conservation. In this it overlaps with several other species conservation concepts such as flagship species , indicator species , and umbrella species . For example,
12753-438: 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
12870-458: 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
12987-455: 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
13104-424: 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
13221-411: 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
13338-428: Was exposed to 21 °C (70 °F) and 770 ppm CO 2 (beyond rises expected in the next century) they survived. It is thought that this is because the animal's calcium is nodular and so it is able to compensate for the lack of carbonate by growing more fleshy tissue instead. Seastar † Calliasterellidae † Trichasteropsida Starfish or sea stars are star-shaped echinoderms belonging to
13455-517: 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
13572-429: Was introduced in 1969 by the zoologist Robert T. Paine . Keystone species play a critical role in maintaining the structure of an ecological community , affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. Some keystone species, such as
13689-549: Was shown that Mytilus californianus becomes almost completely dominant of the intertidal community. When P. ochraceus is present there is a diverse intertidal community. At the larval stage, Pisaster ochraceus are filter feeders and their diet consists of plankton . As an adult, P. ochraceus feeds on mussels such as Mytilus californianus and Mytilus trossulus . They also feed on chitons , limpets , snails , barnacles , echinoids , and even decapod crustacea. P. ochraceus uses its tube feet to handle its prey . If
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