148-396: Cnidaria ( / n ɪ ˈ d ɛər i ə , n aɪ -/ nih- DAIR -ee-ə, NY - ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in fresh water and marine environments (predominantly the latter), including jellyfish , hydroids , sea anemones , corals and some of the smallest marine parasites . Their distinguishing features are
296-445: A larva swims until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called strobilation . The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adult medusae have gonads in the gastroderm , and these release ova and sperm into
444-609: 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
592-570: 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
740-490: 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
888-471: 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
1036-411: 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
1184-556: A certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were. The most important objective measure in
1332-407: A character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan. A classification using this definition may be strongly affected by the chance survival of rare groups, which can make
1480-411: 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
1628-728: A decentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey . Their bodies consist of mesoglea , a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Cnidarians are also some of the only animals that can reproduce both sexually and asexually. Cnidarians mostly have two basic body forms: swimming medusae and sessile polyps , both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes, which are specialized stinging cells used to capture prey. Both forms have
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#17327810724341776-501: A diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as Hydra and colonial swimmers such as the Portuguese Man o' War . Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the parasitic Myxozoa and Polypodiozoa are now recognized as highly derived cnidarians rather than more closely related to
1924-399: 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
2072-496: A few are parasites . Many are preyed on by other animals including starfish , sea slugs , fish , turtles , and even other cnidarians. Many scleractinian corals—which form the structural foundation for coral reefs —possess polyps that are filled with symbiotic photo-synthetic zooxanthellae . While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in polar regions , and in freshwater. Cnidarians are
2220-497: 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
2368-402: A group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock] ( Phylon )." In plant taxonomy , August W. Eichler (1883) classified plants into five groups named divisions,
2516-602: A group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions. The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta, while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes), or for conifers alone as below. Since
2664-488: A hydra's nematocysts are lost from its tentacles when capturing a brine shrimp . Used cnidocytes have to be replaced, which takes about 48 hours. To minimise wasteful firing, two types of stimulus are generally required to trigger cnidocytes: nearby sensory cells detect chemicals in the water, and their cilia respond to contact. This combination prevents them from firing at distant or non-living objects. Groups of cnidocytes are usually connected by nerves and, if one fires,
2812-545: 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
2960-443: 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
3108-709: A larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding . They have complex life cycles and can reproduce both sexually and asexually . Most can regenerate damaged parts or lost arms and they can shed arms as
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#17327810724343256-400: 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
3404-546: 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
3552-449: A maximum depth of 46 m (151 ft), temperatures between 20 and 28 °C (68 and 82 °F), high salinity , and low carbon dioxide levels. Stauromedusae , although usually classified as jellyfish, are stalked, sessile animals that live in cool to Arctic waters. Cnidarians range in size from a mere handful of cells for the parasitic myxozoans through Hydra' s length of 5–20 mm ( 1 ⁄ 4 – 3 ⁄ 4 in), to
3700-546: 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
3848-475: 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
3996-423: A mix of calcified and uncalcified species). Cnidaria are diploblastic animals; in other words, they have two main cell layers, while more complex animals are triploblasts having three main layers. The two main cell layers of cnidarians form epithelia that are mostly one cell thick, and are attached to a fibrous basement membrane , which they secrete . They also secrete the jelly-like mesoglea that separates
4144-432: 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
4292-400: A phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by
4440-471: A phylum much more diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. The kingdom Plantae
4588-706: A phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta . These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete. Currently there are 40 bacterial phyla (not including " Cyanobacteria ") that have been validly published according to
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4736-411: 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
4884-592: 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
5032-401: A set of characters shared by all its living representatives. This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on
5180-722: A single orifice and body cavity that are used for digestion and respiration . Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids , or both (hence they are trimorphic ). Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors . Cnidarians also have rhopalia , which are involved in gravity sensing and sometimes chemoreception. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts , and some have simple eyes . Not all cnidarians reproduce sexually , but many species have complex life cycles of asexual polyp stages and sexual medusae stages. Some, however, omit either
5328-456: 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,
5476-504: 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
5624-401: A subjective decision about which groups of organisms should be considered as phyla. The approach is useful because it makes it easy to classify extinct organisms as " stem groups " to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display
5772-424: A term that remains in use today for groups of plants, algae and fungi. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier . Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan . At its most basic, a phylum can be defined in two ways: as a group of organisms with
5920-537: 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
6068-454: 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:
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6216-447: A type of muscle that, in more complex animals, arises from the middle cell layer . As a result, some recent text books classify ctenophores as triploblastic , and it has been suggested that cnidarians evolved from triploblastic ancestors. Most adult cnidarians appear as either free-swimming medusae or sessile polyps , and many hydrozoans species are known to alternate between the two forms. Both are radially symmetrical , like
6364-524: A very ancient phylum, with fossils having been found in rocks formed about 580 million years ago during the Ediacaran period , preceding the Cambrian Explosion . Other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. Molecular clock analysis of mitochondrial genes suggests an even older age for
6512-469: A wheel and a tube respectively. Since these animals have no heads, their ends are described as "oral" (nearest the mouth) and "aboral" (furthest from the mouth). Most have fringes of tentacles equipped with cnidocytes around their edges, and medusae generally have an inner ring of tentacles around the mouth. Some hydroids may consist of colonies of zooids that serve different purposes, such as defense, reproduction and catching prey. The mesoglea of polyps
6660-514: A worldwide range: some, such as Hydra , live in freshwater; Obelia appears in the coastal waters of all the oceans; and Liriope can form large shoals near the surface in mid-ocean. Among anthozoans , a few scleractinian corals , sea pens and sea fans live in deep, cold waters, and some sea anemones inhabit polar seabeds while others live near hydrothermal vents over 10 km (33,000 ft) below sea-level. Reef -building corals are limited to tropical seas between 30°N and 30°S with
6808-474: Is a paraphyletic taxon, which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system . Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by
6956-484: Is a level of classification or taxonomic rank below kingdom and above class . Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and
7104-446: 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 ,
7252-440: 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
7400-590: Is based, in part, on the presence in the hydra genome of genes homologous to genes in other genetically well studied species that have been demonstrated to play key roles in these DNA repair pathways. Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata , but increasing awareness of their differences caused them to be placed in separate phyla. Modern cnidarians are generally classified into four main classes : sessile Anthozoa ( sea anemones , corals , sea pens ); swimming Scyphozoa (jellyfish) and Cubozoa (box jellies); and Hydrozoa ,
7548-405: Is defined in various ways by different biologists (see Current definitions of Plantae ). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta , to form the clade Viridiplantae . The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida ,
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#17327810724347696-481: Is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista , below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish
7844-411: Is not known how long these can survive. In some species the ova release chemicals that attract sperm of the same species. The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere ( blastula ) and then a depression forms at one end ( gastrulation ) and eventually becomes the digestive cavity. However, in cnidarians the depression forms at the end further from
7992-752: 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
8140-418: 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
8288-401: 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
8436-405: Is still struggling. The mesoglea contains small numbers of amoeba -like cells, and muscle cells in some species. However, the number of middle-layer cells and types are much lower than in sponges. Polymorphism refers to the occurrence of structurally and functionally more than two different types of individuals within the same organism. It is a characteristic feature of Cnidarians, particularly
8584-415: 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
8732-408: Is usually thin and often soft, but that of medusae is usually thick and springy, so that it returns to its original shape after muscles around the edge have contracted to squeeze water out, enabling medusae to swim by a sort of jet propulsion . In medusae, the only supporting structure is the mesoglea . Hydra and most sea anemones close their mouths when they are not feeding, and the water in
8880-777: The Bacteriological Code Currently there are 2 phyla that have been validly published according to the Bacteriological Code Other phyla that have been proposed, but not validly named, include: Starfish † Calliasterellidae † Trichasteropsida Starfish or sea stars are star-shaped echinoderms belonging to 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
9028-565: The Catalogue of Life , and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith . Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP. The number of protist phyla varies greatly from one classification to
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#17327810724349176-693: The Lion's mane jellyfish , which may exceed 2 m (6 ft 7 in) in diameter and 75 m (246 ft) in length. Prey of cnidarians ranges from plankton to animals several times larger than themselves. Some cnidarians are parasites , mainly on jellyfish but a few are major pests of fish. Others obtain most of their nourishment from endosymbiotic algae or dissolved nutrients. Predators of cnidarians include: sea slugs , flatworms and comb jellies , which can incorporate nematocysts into their own bodies for self-defense (nematocysts used by cnidarian predators are referred to as kleptocnidae); starfish , notably
9324-616: 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
9472-481: 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
9620-466: The bilaterians . Stauromedusae, small sessile cnidarians with stalks and no medusa stage, have traditionally been classified as members of the Scyphozoa, but recent research suggests they should be regarded as a separate class, Staurozoa. The Myxozoa , microscopic parasites , were first classified as protozoans . Research then found that Polypodium hydriforme , a non-Myxozoan parasite within
9768-449: The corals Hetroxenia and Leptogorgia , depend almost completely on their endosymbionts and on absorbing dissolved nutrients. Cnidaria give their symbiotic algae carbon dioxide , some nutrients, and protection against predators. Predatory species use their cnidocytes to poison or entangle prey, and those with venomous nematocysts may start digestion by injecting digestive enzymes . The "smell" of fluids from wounded prey makes
9916-527: The crown group of cnidarians, estimated around 741 million years ago , almost 200 million years before the Cambrian period, as well as before any fossils. Recent phylogenetic analyses support monophyly of cnidarians, as well as the position of cnidarians as the sister group of bilaterians . The term cnidaria derives from the Ancient Greek word knídē ( κνίδη “nettle”), signifying
10064-447: The crown of thorns starfish , which can devastate corals; butterfly fish and parrot fish , which eat corals; and marine turtles , which eat jellyfish. Some sea anemones and jellyfish have a symbiotic relationship with some fish; for example clownfish live among the tentacles of sea anemones, and each partner protects the other against predators. Phylum In biology , a phylum ( / ˈ f aɪ l əm / ; pl. : phyla )
10212-554: 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
10360-427: The medusae . These "nettle cells" function as harpoons , since their payloads remain connected to the bodies of the cells by threads. Three types of cnidocytes are known: The main components of a cnidocyte are: It is difficult to study the firing mechanisms of cnidocytes as these structures are small but very complex. At least four hypotheses have been proposed: Cnidocytes can only fire once, and about 25% of
10508-576: 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
10656-409: 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
10804-519: The phase of the moon . Many species of Cnidaria may spawn simultaneously in the same location, so that there are too many ova and sperm for predators to eat more than a tiny percentage — one famous example is the Great Barrier Reef , where at least 110 corals and a few non-cnidarian invertebrates produce enough gametes to turn the water cloudy. These mass spawnings may produce hybrids , some of which can settle and form polyps, but it
10952-516: The polyp and medusa forms, or of zooids within colonial organisms like those in Hydrozoa . In Hydrozoans , colonial individuals arising from individual zooids will take on separate tasks. For example, in Obelia there are feeding individuals, the gastrozooids ; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals,
11100-724: 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
11248-599: 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
11396-497: The Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina , Kickxellomycotina , Mucoromycotina , and Zoopagomycotina . Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi. Protista
11544-458: The above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine
11692-585: 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
11840-496: 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
11988-622: 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
12136-401: The class Asteroidea. About 1,900 species of starfish live on the seabed in all the world's oceans , from warm, tropical zones to frigid, polar regions . They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface. Starfish are marine invertebrates . They typically have a central disc and usually five arms, though some species have
12284-847: The coiled thread reminiscent of cnidocytes. The word was first coined in 1766 by the Swedish naturalist Peter Simon Pallas . Cnidarians form a phylum of animals that are more complex than sponges , about as complex as ctenophores (comb jellies), and less complex than bilaterians , which include almost all other animals. Both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like basement membranes ; muscles ; nervous systems ; and some have sensory organs. Cnidarians are distinguished from all other animals by having cnidocytes that fire harpoon -like structures that are mainly used to capture prey. In some species, cnidocytes can also be used as anchors. Cnidarians are also distinguished by
12432-504: The connecting tunnels, so that gastroderm cells can absorb the nutrients. Absorption may take a few hours, and digestion within the cells may take a few days. The circulation of nutrients is driven by water currents produced by cilia in the gastroderm or by muscular movements or both, so that nutrients reach all parts of the digestive cavity. Nutrients reach the outer cell layer by diffusion or, for animals or zooids such as medusae which have thick mesogleas , are transported by mobile cells in
12580-555: 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
12728-419: The digestive cavity then acts as a hydrostatic skeleton , rather like a water-filled balloon. Other polyps such as Tubularia use columns of water-filled cells for support. Sea pens stiffen the mesoglea with calcium carbonate spicules and tough fibrous proteins , rather like sponges . In some colonial polyps, a chitinous epidermis gives support and some protection to the connecting sections and to
12876-439: The digestive cavity without opening the mouth. This improves respiration after feeding and allows these animals, which use the cavity as a hydrostatic skeleton , to control the water pressure in the cavity without expelling undigested food. Cnidaria that carry photosynthetic symbionts may have the opposite problem, an excess of oxygen, which may prove toxic . The animals produce large quantities of antioxidants to neutralize
13024-625: The egg cells of sturgeon , is closely related to the Myxozoa and suggested that both Polypodium and the Myxozoa were intermediate between cnidarians and bilaterian animals. More recent research demonstrates that the previous identification of bilaterian genes reflected contamination of the Myxozoan samples by material from their host organism, and they are now firmly identified as heavily derived cnidarians, and more closely related to Hydrozoa and Scyphozoa than to Anthozoa. Some researchers classify
13172-559: The excess oxygen. All cnidarians can regenerate , allowing them to recover from injury and to reproduce asexually . Medusae have limited ability to regenerate, but polyps can do so from small pieces or even collections of separated cells. This enables corals to recover even after apparently being destroyed by predators. Cnidarian sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example, in Scyphozoa (jellyfish) and Cubozoa (box jellies)
13320-443: 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
13468-586: The extinct conulariids as cnidarians, while others propose that they form a completely separate phylum . Current classification according to the World Register of Marine Species : Many cnidarians are limited to shallow waters because they depend on endosymbiotic algae for much of their nutrients. The life cycles of most have polyp stages, which are limited to locations that offer stable substrates. Nevertheless, major cnidarian groups contain species that have escaped these limitations. Hydrozoans have
13616-452: The eyes probably do not form images, Cubozoa can clearly distinguish the direction from which light is coming as well as negotiate around solid-colored objects. Cnidarians feed in several ways: predation , absorbing dissolved organic chemicals, filtering food particles out of the water, obtaining nutrients from symbiotic algae within their cells, and parasitism. Most obtain the majority of their food from predation but some, including
13764-541: The fact that they have only one opening in their body for ingestion and excretion i.e. they do not have a separate mouth and anus. Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians; more complex animals have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelled diploblastic , along with sponges. However, both cnidarians and ctenophores have
13912-435: 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
14060-507: The first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders , many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses . Wolf plants Hepatophyta Liver plants Coniferophyta Cone-bearing plant Phylum Microsporidia
14208-652: The freshwater cnidarians as well as many marine forms, and which has both sessile members, such as Hydra , and colonial swimmers (such as the Portuguese man o' war )). Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the highly derived parasitic Myxozoa and Polypodiozoa were firmly recognized as cnidarians only in 2007. Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from symbiotic dinoflagellates , and
14356-583: The fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta . The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon ( φῦλον , "race, stock"), related to phyle ( φυλή , "tribe, clan"). Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as
14504-578: 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
14652-573: 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
14800-431: The involvement of sexual reproduction process, was observed in both Hydrozoa ( Turritopsis dohrnii and Laodicea undulata ) and Scyphozoa ( Aurelia sp.1). Anthozoa have no medusa stage at all and the polyps are responsible for sexual reproduction. Spawning is generally driven by environmental factors such as changes in the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or
14948-404: 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
15096-429: 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
15244-651: 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
15392-406: The layers. The layer that faces outwards, known as the ectoderm ("outside skin"), generally contains the following types of cells: In addition to epitheliomuscular, nerve and interstitial cells, the inward-facing gastroderm ("stomach skin") contains gland cells that secrete digestive enzymes . In some species it also contains low concentrations of cnidocytes, which are used to subdue prey that
15540-406: 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
15688-554: The lower parts of individual polyps. A few polyps collect materials such as sand grains and shell fragments, which they attach to their outsides. Some colonial sea anemones stiffen the mesoglea with sediment particles. A mineralized exoskeleton made of calcium carbonate is found in subphylum Anthozoa in the order Scleractinia (stony corals; class Hexacorallia) and the class Octocorallia , and in subphylum Medusozoa in three hydrozoan families in order Anthoathecata ; Milleporidae , Stylasteridae and Hydractiniidae (the latter with
15836-630: The margin of the bell that work together to control the motor nerve net, that directly innervates the swimming muscles. Most cnidarians also have a parallel system. In scyphozoans, this takes the form of a diffuse nerve net, which has modulatory effects on the nervous system. As well as forming the "signal cables" between sensory neurons and motoneurons, intermediate neurons in the nerve net can also form ganglia that act as local coordination centers. Communication between nerve cells can occur by chemical synapses or gap junctions in hydrozoans, though gap junctions are not present in all groups. Cnidarians have many of
15984-642: The medusae of some hydrozoans can divide down the middle. Scyphozoan polyps can both bud and split down the middle. In addition to both of these methods, Anthozoa can split horizontally just above the base. Asexual reproduction makes the daughter cnidarian a clone of the adult. The ability of Cnidarians to asexually reproduce ensures a greater number of mature medusa that can mature to reproduce sexually. Two classical DNA repair pathways, nucleotide excision repair and base excision repair , are present in hydra , and these repair pathways facilitate unhindered reproduction. The identification of these pathways in hydra
16132-578: The mesoglea. Indigestible remains of prey are expelled through the mouth. The main waste product of cells' internal processes is ammonia , which is removed by the external and internal water currents. There are no respiratory organs, and both cell layers absorb oxygen from and expel carbon dioxide into the surrounding water. When the water in the digestive cavity becomes stale it must be replaced, and nutrients that have not been absorbed will be expelled with it. Some Anthozoa have ciliated grooves on their tentacles, allowing them to pump water out of and into
16280-459: 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
16428-463: 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
16576-446: The movements of internal mineral grains called statoliths. If the body tilts in the wrong direction, the animal rights itself by increasing the strength of the swimming movements on the side that is too low. Most species have ocelli ("simple eyes"), which can detect sources of light. However, the agile box jellyfish are unique among Medusae because they possess four kinds of true eyes that have retinas , corneas and lenses . Although
16724-411: The next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista , and all alveolates except ciliates in one phylum Myzozoa , later lowered in rank and included in a paraphyletic phylum Miozoa . Even within
16872-471: 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,
17020-404: 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
17168-539: The other hand, the highly parasitic phylum Mesozoa was divided into two phyla ( Orthonectida and Rhombozoa ) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes . This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size. A definition of
17316-485: The parents are immobile, these feeding capabilities extend the larvae's range and avoid overcrowding of sites. Scyphozoan and hydrozoan larvae have little yolk and most lack endosymbiotic algae, and therefore have to settle quickly and metamorphose into polyps. Instead, these species rely on their medusae to extend their ranges. All known cnidaria can reproduce asexually by various means, in addition to regenerating after being fragmented. Hydrozoan polyps only bud, while
17464-528: The polyp or the medusa stage, and the parasitic classes evolved to have neither form. Cnidarians were formerly grouped with ctenophores , also known as comb jellies, in the phylum Coelenterata , but increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians are classified into four main groups: the almost wholly sessile Anthozoa ( sea anemones , corals , sea pens ); swimming Scyphozoa ( jellyfish ); Cubozoa (box jellies); and Hydrozoa (a diverse group that includes all
17612-450: The polyp stage completely, and cubozoan polyps produce only one medusa. Hydrozoa have a variety of life cycles. Some have no polyp stages and some (e.g. hydra ) have no medusae. In some species, the medusae remain attached to the polyp and are responsible for sexual reproduction; in extreme cases these reproductive zooids may not look much like medusae. Meanwhile, life cycle reversal, in which polyps are formed directly from medusae without
17760-668: 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
17908-559: 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
18056-541: 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
18204-475: The relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids , so the phyla were merged (the bearded worms are now an annelid family ). On
18352-420: 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
18500-1268: The rest of the group requires a weaker minimum stimulus than the cells that fire first. Medusae swim by a form of jet propulsion: muscles, especially inside the rim of the bell, squeeze water out of the cavity inside the bell, and the springiness of the mesoglea powers the recovery stroke. Since the tissue layers are very thin, they provide too little power to swim against currents and just enough to control movement within currents. Hydras and some sea anemones can move slowly over rocks and sea or stream beds by various means: creeping like snails, crawling like inchworms , or by somersaulting . A few can swim clumsily by waggling their bases. Cnidarians are generally thought to have no brains or even central nervous systems. However, they do have integrative areas of neural tissue that could be considered some form of centralization. Most of their bodies are innervated by decentralized nerve nets that control their swimming musculature and connect with sensory structures, though each clade has slightly different structures. These sensory structures, usually called rhopalia, can generate signals in response to various types of stimuli such as light, pressure, chemical changes, and much more. Medusa usually have several of them around
18648-582: The same neurotransmitters as bilaterians, including chemicals such as glutamate, GABA, and glycine. Serotonin, dopamine, noradrenaline, octopamine, histamine, and acetylcholine, on the other hand, are absent. This structure ensures that the musculature is excited rapidly and simultaneously, and can be directly stimulated from any point on the body, and it also is better able to recover after injury. Medusae and complex swimming colonies such as siphonophores and chondrophores sense tilt and acceleration by means of statocysts , chambers lined with hairs which detect
18796-452: 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
18944-675: 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"
19092-417: 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
19240-467: 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
19388-412: 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
19536-452: 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
19684-551: 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
19832-451: The tentacles and oral arms often have rows of cilia whose beating creates currents that flow towards the mouth, and some produce nets of mucus to trap particles. Their digestion is both intra and extracellular. Once the food is in the digestive cavity, gland cells in the gastroderm release enzymes that reduce the prey to slurry, usually within a few hours. This circulates through the digestive cavity and, in colonial cnidarians, through
19980-679: The tentacles fold inwards and wipe the prey off into the mouth. In medusae, the tentacles around the edge of the bell are often short and most of the prey capture is done by "oral arms", which are extensions of the edge of the mouth and are often frilled and sometimes branched to increase their surface area. These "oral arms" aid in Cnidarians ability to move prey towards their mouth once it has been poisoned and entangled. Medusae often trap prey or suspended food particles by swimming upwards, spreading their tentacles and oral arms and then sinking. In species for which suspended food particles are important,
20128-408: 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 ,
20276-569: 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
20424-473: 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,
20572-415: 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
20720-412: 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
20868-438: 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
21016-464: 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
21164-455: 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
21312-484: The water in the breeding season. This phenomenon of succession of differently organized generations (one asexually reproducing, sessile polyp, followed by a free-swimming medusa or a sessile polyp that reproduces sexually) is sometimes called "alternation of asexual and sexual phases" or "metagenesis", but should not be confused with the alternation of generations as found in plants. Shortened forms of this life cycle are common, for example some oceanic scyphozoans omit
21460-424: 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
21608-501: The yolk (at the animal pole ), while in bilaterians it forms at the other end ( vegetal pole ). The larvae, called planulae , swim or crawl by means of cilia . They are cigar-shaped but slightly broader at the "front" end, which is the aboral, vegetal-pole end and eventually attaches to a substrate if the species has a polyp stage. Anthozoan larvae either have large yolks or are capable of feeding on plankton , and some already have endosymbiotic algae that help to feed them. Since
21756-411: 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
21904-517: 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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