Starfish

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Starfish
Temporal range: Ordovician–Recent
Scientific classification
Kingdom:Animalia
Phylum:Echinodermata
Subphylum:Asterozoa
Class:Asteroidea
De Blainville, 1830
Orders

Brisingida
Forcipulatida
Notomyotida
Paxillosida
Spinulosida
Valvatida
Velatida [1]
Calliasterellidae
Trichasteropsida[2]

 
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Starfish
Temporal range: Ordovician–Recent
Scientific classification
Kingdom:Animalia
Phylum:Echinodermata
Subphylum:Asterozoa
Class:Asteroidea
De Blainville, 1830
Orders

Brisingida
Forcipulatida
Notomyotida
Paxillosida
Spinulosida
Valvatida
Velatida [1]
† Calliasterellidae
† Trichasteropsida[2]

Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of this class. However, common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as "brittle stars" or "basket stars". About 1,500 living species of starfish occur on the seabed in all the world's oceans, from the tropics to subzero polar waters. They are found from the intertidal zone down to abyssal depths, 6,000 m (20,000 ft) below the surface.

Starfish are among the most familiar of marine invertebrates. They typically have a central disc and five arms, though some species have more than this. 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 having specialized feeding behaviours including eversion of their stomachs and suspension feeding. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of defence. The Asteroidea occupy several significant ecological roles. Starfish, such as the ochre sea star (Pisaster ochraceus) and the reef sea star (Stichaster australis), have become widely known as examples of the keystone species concept in ecology. The tropical crown-of-thorns starfish (Acanthaster planci) is a voracious predator of coral throughout the Indo-Pacific region, and the northern Pacific sea star is considered to be one of the world's 100 worst invasive species.

The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death. Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture. They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.

Contents

Taxonomy

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 this is lost during metamorphosis after which they develop radial symmetry, typically pentamerism.[3] Adult echinoderms are characterized by the possession of a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres.[4] The starfish are included in the subphylum Asterozoa, the characteristics of which include a flattened, star-shaped body as adults consisting of a central disc and multiple radiating arms. The subphylum includes the two classes Asteroidea, the starfish, and Ophiuroidea, the brittle stars and basket stars. The asteroids have broad-based arms with skeletal support provided by calcareous plates in the body wall[5] while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed "vertebrae".[6]

The starfish are a large and diverse class with about 1,500 living species. There are seven extant orders, Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida and Velatida[1] and two extinct ones, Calliasterellidae and Trichasteropsida.[2]

Anatomy

Most starfish have five arms that radiate from a central disc, but the number varies with group. Luidia ciliaris has seven arms, members of the Solasteridae have ten to fifteen while the Antarctic Labidiaster annulatus can have up to fifty. It is not unusual in species that typically have five arms for individuals to possess six or more through abnormal development.[7]

Body wall

Pedicellariae and retracted papulae among the spines of Acanthaster planci

The body wall consists of 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.[8] The dermis contains an endoskeleton of plate-like calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice. They vary in form, some bearing external granules, tubercles and spines, sometimes organised into definite patterns. Some are specialised structures such as the madreporite (the entrance to the water vascular system), pedicellariae and paxillae.[8] 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.[9] Paxillae are umbrella-like structures found on starfish that live buried in sediment. The edges of adjacent paxillae meet to form a false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected. All the ossicles, including those projecting externally, are covered by the epidermal layer.[8]

Several groups of starfish, including Valvatida and Forcipulatida, possess pedicellariae.[9] In Forcipulatida, such as Asterias and Pisaster, they occur in pompom-like tufts at the base of each spine, whereas in the Goniasteridae, such as Hippasteria phrygiana, the pedicellariae are scattered over the body surface. Some are thought to assist in defence, while others aid in feeding or in the removal of organisms attempting to settle on the starfish's surface.[10] The Antarctic Labidiaster annulatus uses its large pedicellariae to capture krill, while the North Pacific Stylasterias forreri uses its pedicellariae to snare small fish.[11]

There may also be papulae, thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water. These serve a respiratory function.[3] 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.[12]

Water vascular system

Arm tip with tube feet
Arm tip of Leptasterias polaris showing tube feet and eyespot

The water vascular system of the starfish is a hydraulic system made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and gas exchange. Water enters the system through the madreporite, a porous, often conspicuous, sieve-like ossicle on the aboral surface. It is linked through a stone canal, often lined with calcareous material, to a ring canal around the mouth opening. A radial canal leads off this and runs along the ambulacral groove in each arm. There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla. These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove. There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows. The interior of the whole canal system is lined with cilia.[13]

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 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.[14][15] In some circumstances, the tube feet seem to work as levers, but when moving on vertical surfaces, they form a traction system.[16]

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.[17] Some burrowing species from the genera Astropecten and Luidia have points rather than suckers on their long tube feet and are capable of much more rapid motion, "gliding" across the ocean floor. The sand star (Luidia foliolata) can travel at a speed of 2.8 m (9 ft 2 in) per minute.[18]

Apart from their function in locomotion, the tube feet act as accessory gills. The water vascular system serves to transport oxygen from, and carbon dioxide to, the tube feet and also nutrients from the gut to the muscles involved in locomotion. Fluid movement is bidirectional and initiated by cilia.[13] 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 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.[19]

Digestive system and excretion

Diagram of starfish anatomy
Aboral view of partially dissected starfish
1. Pyloric stomach, 2. Intestine and anus,
3. Rectal sac, 4. Stone canal,
5. Madreporite, 6. Pyloric caecum,
7. Digestive glands, 8. Cardiac stomach,
9. Gonad, 10. Radial canal,
11. Ambulacral ridge

The gut of a starfish occupies most of the disc and extends into the arms. The mouth is located in the centre of the oral surface, where it is surrounded by a 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: 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.[20]

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 is passed into their pyloric stomachs and caeca where digestion continues and absorption occurs.[20] 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, the starfish pulls with its tube feet to separate the two valves slightly, and inserts a small section of its stomach, which releases enzymes to digest the prey. The stomach and the partially digested prey are later retracted into the disc. Here the food is passed on to the pyloric stomach, which always remains inside the disc.[21]

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 the water in sticky mucus strands that are swept towards the mouth along ciliated grooves.[20]

The main nitrogenous waste product is ammonia. With no distinct excretory organs, this is removed by diffusion through the tube feet and papulae.[19] The body fluid contains phagocytic cells, coelomocytes, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to the tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water. Some waste may also be excreted by the pyloric glands and voided with the faeces.[19]

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, the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in estuarine environments.[19]

Sensory and nervous systems

Although starfish do not have many well-defined sense organs, they are sensitive to touch, light, temperature, orientation and the status of the water around them. The tube feet, spines and pedicellariae are sensitive to touch. The tube feet, especially those at the tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food.[22] There are eyespots at the ends of the arms, each one composed of eighty to two hundred simple ocelli. These are composed of pigmented epithelial cells that respond to light, with narrow sensory cells lying between them. They 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 species.[23]

While starfish lack a centralized brain, their bodies have complex nervous systems which are coordinated by what might be termed a distributed brain. There is a circumoral nerve ring that surrounds 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.[24] The ring nerves and radial nerves have sensory and motor components and coordinate the starfish's balance and directional systems.[3] The sensory component receive input from the sensory organs while the motor nerves control the tube feet and musculature. The starfish does not have the capacity to plan its actions. If one arm detects an attractive odour, it becomes dominant and temporarily over-rides the other arms to initiate movement towards the prey. The mechanism for this is not fully understood.[24]

Circulatory system

The body cavity contains the circulatory or haemal system. The blood vessels form several rings; one around the mouth (the hyponeural hemal ring), another around the digestive system (the gastric ring) and a third near the aboral surface (the genital ring). A vertical channel (the axial vessel) connects the three rings and the heart, which beats about six times a minute, is at its apex, near the madreporite. The genital ring has further channels running along the arms next to the gonads. These have blind ends and there is no continuous circulation of the blood which does not contain a pigment and has no respiratory function, but is probably used to transport nutrients around the body.[25]

Secondary metabolites

Starfish produce a large number of secondary metabolites in the form of lipids, including steroidal derivatives of cholesterol, and fatty acids, mostly 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 a small number of alkaloids have also been identified. The ecological role in starfish of these chemicals has 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 in the prevention of other organisms from settling on the starfish's aboral surface. Some are alarm pheromones and escape-eliciting chemicals, the release of which triggers responses in conspecific starfish but often produces an escape response in potential prey.[26] Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.[27]

Life cycle

Sexual reproduction

Most species of starfish are dioecious, there being separate male and female individuals. These are usually not distinguishable externally, as the gonads cannot be seen, but their sex is apparent when they are spawning. Some species are simultaneous hermaphrodites (producing eggs and sperm at the same time). In a few of these, the same gonad, called an ovotestis,[28] produces both eggs and sperm. Yet other starfish are sequential hermaphrodites, with some species being protandrous. In these, young individuals are males that change sex into females as they grow older, Asterina gibbosa being an example of these. Others are protogynous and change sex during their lives from female to male. In some species, when a large female divides, the smaller individuals produced become males. When they grow big enough, they change back into females.[29]

Each arm contains two gonads, which release gametes through openings called gonoducts, located on the central disc between the arms. Fertilization is external in most species, though a few show internal fertilization. In most species, the buoyant eggs and sperm are simply released into the water (free spawning) and the resulting embryos and larvae live as part of the plankton. In others, the eggs may be stuck to the undersides of rocks to develop.[30] In certain species of starfish, the females brood their eggs – either by simply enveloping them [30] or by holding them in specialised structures. These structures include chambers on their aboral surfaces,[31] the pyloric stomach (Leptasterias tenera)[32] or even the gonads themselves.[28] Those starfish that brood their eggs by covering them usually raise their disc and assume a humped posture.[33] Pteraster militaris broods a few of its young and broadcasts the remaining eggs which are too voluminous to fit into its pouch.[31] In these brooding species, the eggs are relatively large, and supplied with yolk, and they generally, but not always,[28] develop directly into miniature starfish without an intervening larval stage. The developing young are called "lecithotrophic" because they obtain their nutrition from the yolk, as opposed to planktotrophic larvae which feed on plankton. In Parvulastra parvivipara, an intragonadal brooder, the young starfish obtain their nutrition by eating other eggs and embryos in the brood pouch.[34] Brooding is especially common in polar and deep-sea species that live in environments unfavourable for larval development [32][35] and in smaller species that produce few eggs.[36]

Reproduction occurs at different times of year according to species. To increase the chances of their eggs being fertilized, starfish may synchronize their spawning, aggregating in groups [30] or forming pairs.[37] This latter behaviour is called pseudo-copulation [38] and the male climbs on top of the female, placing his arms between hers, and releases sperm into the water. This stimulates her to release her eggs. 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 to each other.[39] In some species, mature females produce chemicals to attract sperm in the sea water.[39]

Larval development

Starfish larvae
Three kinds of bilaterally symmetric starfish larvae (from left to right) scaphularia larva, bipinnaria larva, brachiolaria larva, all of Asterias sp. Painted by Ernst Haeckel

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. 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 the mouth.[40]

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 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 are moved to new positions. Some of the body cavities degenerate but others become the water vascular system and the visceral coelom. The starfish is now pentaradially symmetrical. It casts off its stalk and becomes a free-living juvenile starfish about 1 mm (0.04 in) in diameter.[40] Starfish of the order Paxillosida have no brachiolaria stage, with the bipinnaria larva settling on the seabed and developing directly into a juvenile.[40]

Asexual reproduction

Regeneration from an arm
"Comet" of Linckia guildingi, showing starfish body regrowing from a single arm

Some species of starfish are able to reproduce asexually as adults either by fission[41] of their central discs or by the autotomy of one or more of their arms. Which of these processes occurs depends on the genus. Among starfish that are able to regenerate their whole body from a single arm, some can do so even from fragments just 1 cm (0.4 in) long.[42] Single arms that are regenerating 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.[43]

The larvae of several species of starfish can reproduce asexually before they reach maturity.[44] They do this by autotomising some parts of their bodies or by budding.[45] When such a larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development.[46] 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.[47]

Regeneration

Arms being regenerated
Sunflower seastar regenerating missing arms

Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.[42] A few can regrow a complete new disc from a single arm,[48] while others need at least part of the central disc to be attached to the detached part.[48] Regrowth can take several months or years,[42][48] and starfish are vulnerable to infections during the early stages after the loss of an arm.[42] 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 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, a process known as autotomy. 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.[49][50]

Lifespan

The lifespan of a starfish varies considerably between species, generally being longer in larger forms and in those with planktonic larvae. For example, Leptasterias hexactis broods a small number of large-yolked eggs. It has an adult weight of 20 g (0.7 oz), reaches sexual maturity in two years and lives for about 10 years in total.[40] Pisaster ochraceus releases a large number of eggs into the sea each year and has an adult weight of 80 g (2.8 oz). It reaches maturity in five years and may live to the age of 34.[40]

Ecology

Distribution and habitat

Royal starfish
Royal starfish (Astropecten articulatus) on the beach

Echinoderms, including starfish, maintain a delicate internal electrolyte balance which is in equilibrium with sea water. This means that it is only possible for them to live in a marine environment and they are not found in any freshwater habitats.[14] Starfish species are found in greatest variety in the tropical Indo-Pacific. Other areas known for their great diversity include the tropical-temperate regions around Australia, the tropical East Pacific and the cold-temperate water of the North Pacific (California to Alaska). All starfish live on the sea bed, but their larvae are planktonic, which allows them to disperse to new locations. Habitats range from tropical coral reefs, rocks, shell brash, gravel, mud, and sand to kelp forests, seagrass meadows[51] and the deep-sea floor down to at least 6,000 metres (20,000 ft).[52]

Diet

Starfish devouring mussel
Pisaster ochraceus consuming a mussel in central California

Most species are generalist predators, eating molluscs such as clams, oysters, some snails, or any other animal too slow to evade their attack (e.g. other echinoderms or dying fish). Some species are detritivores, eating decomposing animal and plant material or organic films attached to substrates. Others, such as members of the order Brisingida, feed on sponges or plankton and suspended organic particles. The crown-of-thorns starfish consumes coral polyps.[53]

The processes of feeding and capture may be aided by special parts; Pisaster brevispinus, the short-spined pisaster from the West Coast of America, may use a set of specialized tube feet to dig itself deep into the soft substrata to extract prey (usually clams).[54] 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 an opening to devour the soft tissues. The gap between the valves need only be a fraction of a millimetre wide for the stomach to gain entry.[14]

Ecological impact

Starfish are keystone species in their respective marine communities. Their relatively large sizes, diverse diets, indeterminate growth and ability to adapt to different environments gives them great ecological importance.[55] The term "keystone species" was in fact first used by Robert Paine in 1966 to describe a starfish, Pisaster ochraceus.[56] 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.[57] Similar results were found in a 1971 study of Stichaster australis on the intertidal coast of the South Island of New Zealand. S. australis was found to have removed most of a batch of transplanted mussels within two or three months of their placement, while in an area from which S. australis had been removed, the mussels increased in number dramatically, overwhelming the area and threatening biodiversity.[58]

The feeding activity of the omnivorous starfish Oreaster reticulatus on sandy and seagrass bottoms in the Virgin Islands appears to regulate the diversity, distribution and abundance of micro-organisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles.[59] Organisms that dislike this disturbance are replaced by others better able to rapidly recolonise "clean" sediment. In addition, foraging by these migratory starfish creates diverse patches of organic matter which may play a role in the distribution and abundance of macro-organisms such as fish, crabs and sea urchins that feed on the sediment.[60]

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.[61][62] A study in Polynesia found that coral cover declined drastically with the arrival of migratory starfish in 2006, dropping from 50% to under 5% in three years. This had a knock-on effect on reef-feeding fish and the whole benthic community.[62] Asterias amurensis is one of a very 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,[63] and are on the Invasive Species Specialist Group list of the world's 100 worst invasive species.[64]

Threats

Gull feeding on starfish
American herring gull feeding on a starfish

Starfish may be preyed on by conspecifics, other starfish species, tritons, crabs, fish, gulls and sea otters.[63][65][66][67] Their first line of defence is the saponins present in their body wall which have unpleasant flavours.[68] 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.[69] The crown-of-thorns starfish is particularly unattractive to potential predators, being defended by sharp spines, toxins and bright warning colouration.[70] Other species concentrate on protecting their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities.[69]

Several species may suffer a wasting disease caused by the bacteria in the genus Vibrio. The protozoan Orchitophrya stellarum is known to infect the gonads of starfish, damaging tissue and reducing testis size.[66] Starfish are vulnerable to high temperatures and experiments have shown that 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).[71][72] This same species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide.[73] It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs like the stomach.[74]

Starfish and other echinoderms pump water directly into their bodies through their water vascular system. This makes them vulnerable to water pollution, as they have little ability to filter out toxins and contaminants. Oil spills and similar events often take a toll on echinoderm populations, with far-reaching consequences for marine ecosystems.[75] A 2009 study found that P. ochraceus is unlikely to be affected by ocean acidification as severely as other marine animals with calcareous skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when the pH is lowered. Researchers found that when P. ochraceus was exposed to 21 °C (70 °F) and 770 ppm CO2 (beyond rises expected in the next century), they were relatively unaffected. It is thought that their survivability is due to the nodular nature of their skeletons, which are able to compensate for a shortage of carbonate by growing more fleshy tissue.[76]

Evolutionary history

Starfish fossil
Starfish fossil, Riedaster reicheli, from the Plattenkalk Upper Jurassic limestone, Solnhofen

Echinoderms first appeared in the fossil record in the Cambrian. The majority of the early fossils were sea urchins, probably because their hard tests are easily preserved. The first known asterozoans, both starfish and brittle stars, date back to the Ordovician. They were the Somasteroidea which exhibit characteristics of both groups. Modern starfish and brittle stars probably had a common somasteroid ancestor.[77] Starfish are infrequently found as fossils but this may be because their hard skeletal components separate as the animal decays and the soft tissues collapse into distorted, unrecognisable remains. Another reason may be that most starfish live on hard substrates where conditions are not favourable for fossilisation. However, although starfish fossils are uncommon, there are a few places where very rich accumulations of a small number of species occur in "starfish beds".[78]

By the late Paleozoic, the crinoids and blastoids were the predominant echinoderms, and some limestones from this period are composed 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 crinoid survived.[77] Many starfish species also became extinct in these events, but afterwards the surviving few species diversified rapidly within about sixty million years during the Early Jurassic and the beginning of the Middle Jurassic.[5] A 2012 study found that speciation in starfish can occur with great rapidity. During the last 6,000 years, divergence in the larval development of Cryptasterina hystera and Cryptasterina pentagona has taken place, the former adopting internal fertilization and brooding and the latter remaining a broadcast spawner.[79]

Red-knobbed starfish
Red-knobbed starfish (Protoreaster linckii) from the Indian Ocean

Diversity

18-arm starfish
A large eighteen-armed member of Brisingida

Living groups

Species in this order have a small, inflexible disc and between six and 20 long, thin arms which they use for suspension feeding. They have a single series of marginal plates, a fused ring of disc plates, no actinal plates, a spool-like ambulacral column, reduced abactinal plates, crossed pedicellariae, and several series of long spines on the arms. They live almost exclusively in deep-sea habitats, although a few live in shallow waters in the Antarctic.[81][82] In some species, the tube feet have rounded tips and lack suckers.[83]
Species in this order have distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. They tend to have robust bodies[85] and have tube feet with flat-tipped suckers usually arranged in four rows.[83] The order includes well-known species from temperate regions, including the common starfish of North Atlantic coasts and rock pools, as well as cold-water and abyssal species.[86]
These starfish are deep-sea dwelling and have flexible arms. The inner dorso-lateral surface of the arms contain characteristic longitudinal muscle bands.[88] In some species, the tube feet lack suckers.[83]
This is a primitive order and members do not extrude their stomach when feeding, lack an anus and have no suckers on their tube feet. Papulae are plentiful on their aboral surface, they possess marginal plates and have sessile pedicellariae. They mostly inhabit soft-bottomed areas of sand or mud.[85] There is no brachiolaria stage in their larval development.[90] The comb starfish (Astropecten polyacanthus), one of the commonest Indo-Pacific species, is a member of this order.
Cushion star
Cushion star, a member of Valvatida
Most species in this order lack pedicellariae and all have a delicate skeletal arrangement with small or no marginal plates on the disc and arms. They have numerous groups of low spines on the aboral surface.[92][93] This group includes relatively slender five-armed species such as the red starfish Echinaster sepositus and the plump leather star.
Most species in this order have five arms and two rows of tube feet with suckers. There are conspicuous marginal plates on the arms and disc. Some species have paxillae and in some, the main pedicellariae are clamp-like and recessed into the skeletal plates.[93] This group includes the rigid-bodied cushion stars of warm tropical waters and the deep water sea daisies.
This order of starfish consists mostly of deep-sea and other cold-water starfish often with a global distribution. The shape is pentagonal or star-shaped with five to fifteen arms. They mostly have poorly developed skeletons with papulae widely distributed on the aboral surface and spiny or sessile pedicellariae.[96] This group includes the multi-armed morning sun star and common sunstar.

Extinct groups

Extinct groups within the Asteroidea include:[2]

Phylogeny

The phylogeny of the Asteroidea has been difficult to resolve, with visible (morphological) features proving inadequate, and the question of whether traditional taxa are clades in doubt.[2] The phylogeny proposed by Gale in 1987 is:[2][99]



† Palaeozoic Asteroids





Paxillosida




Valvatida, including Velatida, Spinulosida (not a clade)[2]



Forcipulatida, including Brisingida






The phylogeny proposed by Blake in 1987 is:[2][100]



† Palaeozoic Asteroids





† Calliasterellidae




† Compasteridae




† Trichasteropsida





Brisingida



Forcipulatida






Spinulosida



Velatida






Notomyotida



Valvatida




Paxillosida










Later work making use of molecular evidence, with or without the use of morphological evidence, had by 2000 failed to resolve the argument.[2] In 2011, on further molecular evidence, Janies and colleagues noted that the phylogeny of the echinoderms "has proven difficult",[101] and that "the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods".[101] They presented a phylogenetic tree for the living Asteroidea only; using the traditional names of starfish orders where possible, and indicating "part of" otherwise, the phylogeny is shown below. The Solasteridae are split from the Velatida, and the old Spinulosida is broken up.[101]








Solasteridae and part of Spinulosida, e.g. Stegnaster and part of Valvatida, e.g. Asterina



Odontasteridae, which was a part of Valvatida




Paxillosida




part of Spinulosida, e.g. Echinaster, part of Valvatida, e.g. Archaster





Forcipulatida



Brisingida with part of Velatida, e.g. Caymanostella and part of Forcipulatida, e.g. Stichaster






Velatida except for Solasteridae




Notomyotida (not analysed)


Human relations

In legend and literature

An aboriginal Australian fable retold by the Welsh school headmaster William Jenkyn Thomas (1870–1959)[102] tells how some animals needed a canoe to cross the ocean. Whale had one but refused to lend it, so Starfish kept him busy, telling him stories and grooming him to remove parasites, while the others stole the canoe. When Whale realized the trick he beat Starfish ragged, which is how Starfish still is today.[103]

A traditional tale relates how a man walking along a beach saw a boy throwing stranded starfish into the waves. When asked why he was doing this, the boy replied that with the ebbing tide, the starfish would die from exposure. The man pointed out that there were miles and miles of beach and thousands of stranded starfish. What difference could the boy make? The boy bent down and returned another starfish to the ocean. "It makes a difference to this one" he said.[104]

In 1900, the New Zealand scholar Edward Tregear documented The Creation Song, which he describes as "an ancient prayer for the dedication of a high chief"[105] of Hawaii. Among the "uncreated gods" described early in the song are the male Kumilipo ("Creation") and the female Poele, both born in the night; a coral insect, the earthworm, and "The starfish was born, whose children were starry".[105] The song then names the various types of shellfish.[105]

Georg Eberhard Rumpf's 1705 The Ambonese Curiosity Cabinet describes the tropical varieties of Stella Marina or Bintang Laut, "Sea Star", in Latin and Malay respectively, known in the waters around Ambon. He writes that the Histoire des Antilles reports that when the sea stars "see thunder storms approaching, [they] grab hold of many small stones with their little legs, looking to ... hold themselves down as if with anchors".[106]

Starfish is the title of a number of books. Peter Watts wrote a science fiction book called Starfish in 2008, part of his "Rifters" trilogy.[107] Jennie Orbell wrote a novel of this name in 2012.[108] Alice Addison wrote a non-fiction book subtitled "A year in the life of bereavement and depression".[109] The Starfish and the Spider is a 2006 business management book by Ori Brafman and Rod Beckstrom; its title alludes to the ability of the starfish to regenerate itself because of its decentralized nervous system, and the book suggests ways that a decentralized organization may flourish.[110]

In popular culture

Fried starfish on sale
Deep-fried starfish on sale in a food stall in Beijing, 2008

The 1988 album Starfish by the Australian alternative rock band The Church made the band's name, selling 600,000 copies in the United States.[111] The song Starfish-On-The-Toast was composed by the Scottish singer and songwriter Donovan. It formed one of the tracks of his 1967 double album A Gift from a Flower to a Garden. Like the other songs on the second record of the album, For Little Ones, it was played with acoustic instruments, ostensibly for children, according to the album's liner notes.[112] The song includes the lines:

Fine rock pooling coast
this starfish on the toast
the men in the crabbing boats they cry...[113]

In the Nickelodeon animated television series SpongeBob SquarePants, the eponymous character's best friend is a dim-witted starfish, Patrick Star.[114] In the "silly and lame" 2006 Australian-American teen fantasy comedy film Aquamarine,[115] the eponymous mermaid gives each of the two protagonists Hailey and Claire a live starfish earring. The starfish are voiced by Emma Roberts (Claire's), Joanna Levesque (Hailey's), and Sara Paxton (Aquamarine's).[116]

As food

Starfish are widespread in the oceans, but are only occasionally used as food. There may be good reason for this: many species are toxic, as the body wall contains saponins[68] and tetrodotoxins.[117] Some species that prey on bivalve molluscs can transmit paralytic shellfish poisoning.[118] Georg Eberhard Rumpf found few starfish being used for food in the Indonesian archipelago, other than as bait in fish traps, but on "Huamobel" the people cut them up, squeeze out the black blood and cook them with sour tamarind leaves; after resting the pieces for a day or two, they remove the outer skin and cook them in coconut milk.[106] The Amakusa TV company markets an ebook called "Cooking Starfish in Japan", available in English and Japanese, in their guidebook series.[119] Packets of dried starfish, "ヒトデ乾燥品 小袋タイプ 150g" are sold in Japan.[120]

As collectables

Starfish on sale as souvenirs in Cyprus

Starfish are in some cases taken from their habitat and sold to tourists as souvenirs, ornaments, curios or for display in aquariums. In particular, Oreaster reticulatus, with its easily accessed habitat and conspicuous coloration, is widely collected in the Caribbean. In the early to mid 20th century, this species was common along the coasts of the West Indies, but collection and trade have severely reduced its numbers. In the State of Florida, O. reticulatus is listed as endangered and its collection is illegal. Nevertheless, it is still sold throughout its range and beyond.[67] A similar phenomenon exists in the Indo-Pacific for species like Protoreaster nodosus.[121]

In industry and military history

With its multiple arms, the starfish provides a popular metaphor for computer networks,[122] companies[123][124] and software tools.[125] It is also the name of a seabed imaging system and company.[126]

Starfish has repeatedly been chosen as a name in military history. Three ships of the Royal Navy have borne the name HMS Starfish: an A class destroyer launched in 1894;[127] an R class destroyer launched in 1916;[128] and an S class submarine launched in 1933 and lost in 1940.[129] In the Second World War, Starfish sites were large scale night-time decoys created during The Blitz to simulate burning British cities.[130] Starfish Prime was a high-altitude nuclear test conducted by the United States of America on 9 July 1962; the device exploded 250 miles (400 km) above the Pacific Ocean with a yield equivalent to 1.4 megatons of TNT.[131]

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Bibliography

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