Starfish

Starfish

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

Calliasterellidae
Trichasteropsida[2]

Starfish or sea stars are star-shaped echinoderms belonging to the class (biology) Asteroidea. 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 species of starfish occur on the seabed in all the world's oceans, from the tropics to frigid polar waters. They are found from the intertidal zone down to abyssal depths, 6,000 m (20,000 ft) below the surface.

Starfish are marine invertebrates. They typically have a central disc and five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of 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 1
  • Anatomy 2
    • Body wall 2.1
    • Water vascular system 2.2
    • Digestive system and excretion 2.3
    • Sensory and nervous systems 2.4
    • Circulatory system 2.5
    • Secondary metabolites 2.6
  • Life cycle 3
    • Sexual reproduction 3.1
    • Larval development 3.2
    • Asexual reproduction 3.3
    • Regeneration 3.4
    • Lifespan 3.5
  • Ecology 4
    • Distribution and habitat 4.1
    • Diet 4.2
    • Ecological impact 4.3
    • Threats 4.4
  • Evolutionary history 5
    • Diversity 5.1
      • Living groups 5.1.1
      • Extinct groups 5.1.2
    • Phylogeny 5.2
  • Human relations 6
    • In research 6.1
    • In legend and culture 6.2
    • As food 6.3
    • As collectables 6.4
    • In industry and military history 6.5
  • References 7
  • Bibliography 8
  • External links 9

Taxonomy

Tube feet movement of a starfish.

The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830.[3] It is derived from the Greek aster, ἀστήρ (a star) and the Greek eidos, εἶδος (form, likeness, appearance).[4] The class Asteroidea belongs to the phylum Echinodermata. As well as the starfish, the echinoderms include sea urchins, sand dollars, brittle and basket stars, sea cucumbers and crinoids. The larvae of echinoderms have bilateral symmetry, but during metamorphosis this is replaced with radial symmetry, typically pentameric.[5] 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.[6] Starfish are included in the subphylum Asterozoa, the characteristics of which include a flattened, star-shaped body as adults consisting of a central disc and multiple radiating arms. The subphylum includes the two classes of Asteroidea, the starfish, and Ophiuroidea, the brittle stars and basket stars. Asteroids have broad-based arms with skeletal support provided by calcareous plates in the body wall[7] while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed "vertebrae".[8]

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

Luidia maculata, a seven armed starfish

Most starfish have five arms that radiate from a central disc, but the number varies with the 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 some individuals to possess six or more through abnormal development.[9]

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. The dermis contains an endoskeleton of calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice.[10] They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in a tessellated manner and form the main covering of the aboral surface.[11] Some are specialised structures such as the madreporite (the entrance to the water vascular system), pedicellariae and paxillae.[10] 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.[12][13] 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.[10]

Several groups of starfish, including

  • Mah, Christopher L. "The Echinoblog". , a blog about sea stars by a passionate and professional specialist.

External links

  • Lawrence, J. M., ed. (2013). Starfish: Biology and Ecology of the Asteroidea. Johns Hopkins University Press.  
  • Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition. Cengage Learning.  

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  131. ^ Asakawa, M.; Nishimura, F.; Miyazawa, K.; Noguchi, T. (1997). "Occurrence of paralytic shellfish poison in the starfish Asterias amurensis in Kure Bay, Hiroshima Prefecture, Japan". Toxicon 35 (7): 1081–1087.  
  132. ^ "Indulging in Exotic Cuisine in Beijing". The China Guide. 2011. Retrieved 28 February 2014. 
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  134. ^ "Pouch A" (in Japanese). Kenko.com. Retrieved 2013-05-18. 
  135. ^ Johannes, Robert Earle (1981). Words of the Lagoon: Fishing and Marine Lore in the Palau District of Micronesia. University of California Press. p. 87. 
  136. ^ Bos, A. R.; Gumanao, G. S.; Alipoyo, J. C. E.; Cardona, L. T. (2008). (Echinodermata; Asteroidea)"Protoreaster nodosus"Population dynamics, reproduction and growth of the Indo-Pacific horned sea star, . Marine Biology 156 (1): 55–63. 
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  139. ^ "Motorola to Acquire Starfish". Motorola. 1998-07-14. Retrieved 2013-05-11.  (See also Starfish Software.)
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  145. ^ Crowdy, Terry (2008). Deceiving Hitler: double cross and deception in World War II. Osprey Publishing. p. 61.  
  146. ^ Dyal, P. (1965-12-10). "Operation Dominic. Fish Bowl Series. Debris Expansion Experiment" (PDF). Report ADA995428. Air Force Weapons Laboratory. Retrieved 2013-05-11. 

References

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;[142] an R-class destroyer launched in 1916;[143] and an S-class submarine launched in 1933 and lost in 1940.[144] In the World War II, Starfish sites were large-scale night-time decoys created during The Blitz to simulate burning British cities.[145] Starfish Prime was a high-altitude nuclear test conducted by the United States on 9 July 1962.[146]

With its multiple arms, the starfish provides a popular metaphor for computer networks,[137] companies[138][139] and software tools.[140] It is also the name of a seabed imaging system and company.[141]

In industry and military history

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.[72] A similar phenomenon exists in the Indo-Pacific for species such as Protoreaster nodosus.[136]

Starfish on sale as souvenirs in Cyprus

As collectables

Starfish are widespread in the oceans, but are only occasionally used as food. There may be good reason for this: the bodies of numerous species are dominated by bony ossicles, and the body wall of many species contains Indonesian archipelago, other than as bait in fish traps, but on the island of "Huamobel" [sic] the people cut them up, squeeze out the "black blood" and cook them with sour tamarind leaves; after resting the pieces for a day or two, they remove the outer skin and cook them in coconut milk.[123] Starfish are sometimes eaten in China,[132] Japan[133][134] and in Micronesia.[135]

Fried starfish skewers in China

As food

In the Nickelodeon animated television series SpongeBob SquarePants, the eponymous character's best friend is a dim-witted starfish, Patrick Star.[129]

[128] is the title of novels by Starfish

in his 2002 book The Divine Mystery Fort, Indian saint Sri Sai Kaleshwar Swami writes: "Sometimes at the full moon time, when the moon is really dazzling and hitting on the ocean, a starfish jumps out of the water and falls down. If you can get that (the starfish) you can suck unbelievable cosmic energy. You can use it as your own power object."[124]

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".[123]

In 1900, the scholar Edward Tregear documented The Creation Song, which he describes as "an ancient prayer for the dedication of a high chief" of Hawaii. Among the "uncreated gods" described early in the song are the male Kumilipo ("Creation") and the female Poele, both born in the night, a coral insect, the earthworm, and the starfish.[122]

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. The boy replied: "It makes a difference to this one."[121]

An aboriginal Australian fable retold by the Welsh school headmaster William Jenkyn Thomas (1870–1959)[119] 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.[120]

A starfish with 5 legs. Used as an illustration of "Hope in God", a poem by Lydia Sigourney which appeared in Poems for the Sea

In legend and culture

Starfish also have an unusual ability to expel foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.[118]

Another area of research is the ability of starfish to regenerate lost body parts. The stem cells of adult humans are incapable of much differentiation and understanding the regrowth, repair and cloning processes in starfish may have implications for human medicine.[117]

[116] Starfish are

In research

Human relations


Notomyotida (not analysed)








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


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", and that "the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods". They presented a phylogenetic tree for the living Asteroidea only; using the traditional names of starfish orders where possible, and indicating "part of" otherwise, the phylogeny is shown below. The Solasteridae are split from the Velatida, and the old Spinulosida is broken up.[113]



† Palaeozoic Asteroids




† Calliasterellidae



† Compasteridae



† Trichasteropsida




Brisingida


Forcipulatida





Spinulosida


Velatida





Notomyotida


Valvatida



Paxillosida









The phylogeny proposed by Blake in 1987 is:[2][112]



† Palaeozoic Asteroids




Paxillosida



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


Forcipulatida, including Brisingida





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][111]

Phylogeny

Extinct groups within the Asteroidea include:[2]

Extinct groups

This order of starfish consists mostly of deep-sea and other cold-water starfish often with a global distribution. The shape is pentagonal or star-shaped with five to fifteen arms. They mostly have poorly developed skeletons with papulae widely distributed on the aboral surface and often spiny pedicellariae.[108] This group includes the slime star.[109]
  • Velatida (4 families, 16 genera, 138 species) [107]
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.[101] This group includes the cushion stars,[104] the leather star[105] and the sea daisies.[106]
  • Valvatida (16 families, 172 genera, 695 species) [103]
Most species in this order lack pedicellariae and all have a delicate skeletal arrangement with small or no marginal plates on the disc and arms. They have numerous groups of short spines on the aboral surface.[100][101] This group includes the red starfish Echinaster sepositus.[102]
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 and they possess marginal plates and paxillae. They mostly inhabit soft-bottomed areas of sand or mud.[12] There is no brachiolaria stage in their larval development.[97] The comb starfish (Astropecten polyacanthus) is a member of this order.[98]
  • Paxillosida (7 families, 48 genera, 372 species) [96]
These starfish are deep-sea dwelling and have particularly flexible arms. The inner dorso-lateral surfaces of the arms contain characteristic longitudinal muscle bands.[1] In some species, the tube feet lack suckers.[91]
Species in this order have distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. They tend to have robust bodies[93] and have tube feet with flat-tipped suckers usually arranged in four rows.[91] 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.[94]
Species in this order have a small, inflexible disc and 6–20 long, thin arms, which they use for suspension feeding. They have a single series of marginal plates, a fused ring of disc plates, a reduced number of aboral plates, crossed pedicellariae, and several series of long spines on the arms. They live almost exclusively in deep-sea habitats, although a few live in shallow waters in the Antarctic.[89][90] In some species, the tube feet have rounded tips and lack suckers.[91]
  • Brisingida (2 families, 17 genera, 111 species) [88]

Living groups

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

Living asteroids, the Neoasteroidea, are morphologically distinct from their forerunners in the Paleozoic. The taxonomy of the group is relatively stable but there is ongoing debate about the status of the Paxillosida, and the deep-water sea daisies, though clearly Asteroidea and currently included in Velatida, do not fit easily in any accepted lineage. Phylogenetic data suggests that they may be a sister group, the Concentricycloidea, to the Neoasteroidea, or that the Velatida themselves may be a sister group.[86]

Diversity

By the late Paleozoic, the crinoids and blastoids were the predominant echinoderms, and some limestones from this period are made almost entirely from fragments from these groups. In the two major extinction events that occurred during the late Devonian and late Permian, the blastoids were wiped out and only a few species of crinoids survived.[84] 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.[7][86] A 2012 study found that speciation in starfish can occur rapidly. During the last 6,000 years, divergence in the larval development of Cryptasterina hystera and Cryptasterina pentagona has taken place, the former adopting internal fertilization and brooding and the latter remaining a broadcast spawner.[87]

Echinoderms first appeared in the fossil record in the Cambrian. The first known asterozoans were the Somasteroidea, which exhibit characteristics of both groups. Modern starfish and brittle stars probably had a common somasteroid ancestor.[84] Starfish are infrequently found as fossils, possibly because their hard skeletal components separate as the animal decays. However, although starfish fossils are uncommon, there are a few places where accumulations of complete skeletal structures occur, fossilized in place in Lagerstätten — so-called "starfish beds".[85]

Ray fragment (oral surface; ambulacrum) of goniasterid asteroid; Zichor Formation (Coniacian, Upper Cretaceous), southern Israel.
Starfish fossil
Starfish fossil, Riedaster reicheli, from the Plattenkalk Upper Jurassic limestone, Solnhofen

Evolutionary history

Starfish and other echinoderms are sensitive to marine pollution.[81] The common starfish is considered to be a bioindicator for marine ecosystems.[82] A 2009 study found that P. ochraceus is unlikely to be affected by ocean acidification as severely as other marine animals with calcareous skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when the pH is lowered. Researchers found that when P. ochraceus were exposed to 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.[83]

[80] Several species sometimes suffer from a

Warning coloration in the crown-of-thorns starfish

Starfish may be preyed on by conspecifics, other starfish species, tritons, crabs, fish, gulls and sea otters.[40][69][71][72] Their first lines of defence are the saponins present in their body walls, which have unpleasant flavours.[73] 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.[74] 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.[75] Other species protect their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities.[74]

Gull feeding on starfish
American herring gull feeding on a starfish

Threats

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.[60][68] 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 an unintended effect on reef-feeding fish and the whole benthic community.[60] 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,[69] and are on the Invasive Species Specialist Group's list of the world's 100 worst invasive species.[70]

[67] The feeding activity of the

[65].biodiversity had been removed, the mussels increased in number dramatically, overwhelming the area and threatening 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. New Zealand of South Island on the intertidal coast of the Stichaster australis Similar results were found in a 1971 study of [64] Starfish are

Starfish devouring mussel
Pisaster ochraceus consuming a mussel in central California

Ecological impact

[19] 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.adductor Grasping the shellfish, the starfish slowly pries open the prey's shell by wearing out its [61]).clams of America, can use a set of specialized tube feet to dig itself deep into the soft substrate to extract prey (usually West Coast, the short-spined pisaster from the Pisaster brevispinus The processes of feeding and capture may be aided by special parts; [60] polyps.coral consumes crown-of-thorns starfish The [59][58] Most species are generalist predators, eating

Diet

Echinoderms, including starfish, maintain a delicate internal electrolyte balance that 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.[19] 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[56] and the deep-sea floor down to at least 6,000 m (20,000 ft).[57] The greatest diversity of species occurs in coastal areas.[56]

Royal starfish
Royal starfish (Astropecten articulatus) on the beach

Distribution and habitat

Ecology

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 ten years.[12] 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 has a maximum recorded lifespan of 34 years.[12]

Lifespan

Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.[49] A few can regrow a complete new disc from a single arm, while others need at least part of the central disc to be attached to the detached part.[24] Regrowth can take several months or years,[49] 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.[49] 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.[24] 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.[54] An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms.[55]

Arms being regenerated
Sunflower seastar regenerating missing arms

Regeneration

The larvae of several species of starfish can reproduce asexually before they reach maturity.[51] They do this by autotomising some parts of their bodies or by budding.[52] When such a larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development.[53] 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.[52]

Some species of starfish are able to reproduce asexually as adults either by fission of their central discs[48] or by 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.[49] 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.[50]

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

Asexual reproduction

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 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 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.[12]

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.[12]

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

Larval development

In the tropics, a plentiful supply of phytoplankton is continuously available for starfish larvae to feed on. Spawning takes place at any time of year, each species having its own characteristic breeding season.[42] 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.[43] In other species, a male and female may come together and form a pair.[44][45] This behaviour is called pseudocopulation[46] 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.[43] Starfish may use environmental signals to coordinate the time of spawning (day length to indicate the correct time of the year,[44] dawn or dusk to indicate the correct time of day), and chemical signals to indicate their readiness to breed. In some species, mature females produce chemicals to attract sperm in the sea water.[47]

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 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.[34] In certain species of starfish, the females brood their eggs – either by simply enveloping them[34] or by holding them in specialised structures. Brooding may be done in pockets on the starfish's aboral surface,[35][36] inside the pyloric stomach (Leptasterias tenera)[37] or even in the interior of the gonads themselves.[32] Those starfish that brood their eggs by "sitting" on them usually assume a humped posture with their discs raised off the substrate.[38] Pteraster militaris broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch.[35] 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.[32] The developing young are called lecithotrophic because they obtain their nutrition from the yolk as opposed to "planktotrophic" larvae that feed in the water column. In Parvulastra parvivipara, an intragonadal brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch.[39] Brooding is especially common in polar and deep-sea species that live in environments unfavourable for larval development[36] and in smaller species that produce just a few eggs.[40][41]

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 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.[32] Other starfish are sequential hermaphrodites. Protandrous individuals of species like Asterina gibbosa start life as males before changing sex into females as they grow older. In some species such as Nepanthia belcheri, a large female can split in half and the resulting offspring are males. When these grow large enough they change back into females.[33]

Sexual reproduction

Life cycle

Starfish produce a large number of pheromones and escape-eliciting chemicals, the release of which trigger responses in conspecific starfish but often produce escape responses in potential prey.[30] Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.[31]

Secondary metabolites

The body cavity contains the circulatory or haemal system. The vessels form three rings: one around the mouth (the hyponeural haemal ring), another around the digestive system (the gastric ring) and the third near the aboral surface (the genital ring). The heart beats about six times a minute and is at the apex of a vertical channel (the axial vessel) that connects the three rings. 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 the body.[29]

Circulatory system

[28] While a starfish lacks a

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.[26] 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 the species.[28]

Sensory and nervous systems

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 many estuarine environments.[24]

[24].faeces 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 phagocytic The body fluid contains [24] The main nitrogenous waste product is

Because of this ability to digest food outside the body, starfish can hunt prey much larger than their mouths. Their diets include clams and grazers, but others trap food particles from the water in sticky mucus strands that are swept towards the mouth along ciliated grooves.[25]

Primitive starfish, such as Astropecten and Luidia, swallow their bivalve, the starfish pulls with its tube feet to separate the two valves slightly, and inserts a small section of its stomach, which releases enzymes to digest the prey. The stomach and the partially digested prey are later retracted into the disc. Here the food is passed on to the pyloric stomach, which always remains inside the disc.[26] The retraction and contraction of the cardiac stomach is activated by a neuropeptide known as NGFFYamide.[27]

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.[25]

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

Digestive system and excretion

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.[17] 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.[24]

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.[22] 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.[23]

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.[18] 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.[19][20] Some multi-armed, fast-moving starfish such as the sunflower seastar (Pycnopodia helianthoides) pull themselves along with some of their arms while letting others trail behind. Other 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.[21]

The water vascular system of the starfish is a cilia.[17]

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

Water vascular system

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.[5] 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.[16]

[15] use their large pedicellariae to capture small fish and crustaceans.Novodinia antillensis and Rathbunaster californicus, Labidiaster annulatus Some species like [14]