Starfish: Difference between revisions

Starfish Temporal range: Ordovician–Recent PreꞒ Ꞓ O S D C P T J K Pg N
Fromia monilis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	Asterozoa
Class:	Asteroidea De Blainville, 1830
Child taxa and orders
Infraclass Concentricycloidea Peripodida Superorder Forcipulatacea Brisingida Forcipulatida Superorder Spinulosacea Spinulosida Superorder Valvatacea Notomyotida Paxillosida Valvatida Velatida[1] † Calliasterellidae † Trichasteropsida[2]

Starfish or sea stars are star-shaped

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.

Taxonomy

The scientific name Asteroidea was given to starfish by the French zoologist

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

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

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]

Water vascular system

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 set of radial canals leads off this; one radial canal 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.^[17]

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 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]

In some situations, particularly when hunting or in danger, starfish may move in a bilateral fashion. When crawling, some arms act as the leading arms, while others trail behind.^[22][13] 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.^[23] 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.^[24] 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.^[22]

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.^[25]

Digestive system and excretion

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

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 ensues.^[26] 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 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.^[27] The retraction and contraction of the cardiac stomach is activated by a neuropeptide known as NGFFYamide.^[28]

Because of this ability to digest food outside the body, starfish can hunt prey much larger than their mouths. Their diets include clams and

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 the two.^[29] The ring nerves and radial nerves have sensory and motor components and coordinate the starfish's balance and directional systems.^[5] 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 the other arms to initiate movement towards the prey. The mechanism for this is not fully understood.^[29]

Circulatory system

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.^[30]

Secondary metabolites

Starfish produce a large number of

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.[43] 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.^[44] In other species, a male and female may come together and form a pair.^[45][46] This behaviour is called pseudocopulation^[47] 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.^[44] Starfish may use environmental signals to coordinate the time of spawning (day length to indicate the correct time of the year,^[45] 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.^[48]

Larval development

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]

Asexual reproduction

Some species of starfish are able to reproduce asexually as adults either by fission of their central discs^[49] 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.^[50] 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.^[51]

The larvae of several species of starfish can reproduce asexually before they reach maturity.^[52] They do this by autotomising some parts of their bodies or by budding.^[53] When such a larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development.^[54] 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.^[53]

Regeneration

Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.^[50] 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.^[25] Regrowth can take several months or years,^[50] 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.^[50] 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.^[25] 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.^[55] An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms.^[56]

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 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]

Ecology

Distribution and habitat

Echinoderms, including starfish, maintain a delicate internal

Most species are generalist predators, eating

Ecological impact

Starfish are

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 microorganisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles.^[68] 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 organisms such as fish, crabs and sea urchins that feed on the sediment.^[69]

Starfish sometimes have negative effects on ecosystems. Outbreaks of crown-of-thorns starfish have caused damage to coral reefs in Northeast Australia and

Starfish may be preyed on by conspecifics, other starfish species,

Starfish and other echinoderms are sensitive to

Echinoderms first appeared in the

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.^[88]

Living groups

• Brisingida (2 families, 17 genera, 111 species) ^[90]

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.^[91][92] In some species, the tube feet have rounded tips and lack suckers.^[93]

• Forcipulatida (6 families, 63 genera, 269 species) ^[94]

Species in this order have distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. They tend to have robust bodies^[95] and have tube feet with flat-tipped suckers usually arranged in four rows.^[93] 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.^[96]

• Notomyotida (1 family, 8 genera, 75 species) ^[97]

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.^[93]

• Paxillosida (7 families, 48 genera, 372 species) ^[98]

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.^[99] The comb starfish (Astropecten polyacanthus) is a member of this order.^[100]

• Spinulosida (1 family, 8 genera, 121 species) ^[101]

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.^[102][103] This group includes the red starfish Echinaster sepositus.^[104]

• Valvatida (16 families, 172 genera, 695 species) ^[105]

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.^[103] This group includes the cushion stars,^[106] the leather star^[107] and the sea daisies.^[108]

• Velatida (4 families, 16 genera, 138 species) ^[109]

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.^[110] This group includes the slime star.^[111]

Extinct groups

Extinct groups within the Asteroidea include:^[2]

• † Calliasterellidae, with the type genus Calliasterella from the Devonian and Carboniferous.^[112]
• †
  Palastericus^[113]
• †
  Paleocoma^[114]
• † Trichasteropsida, with the Triassic genus Trichasteropsis (at least 2 species).^[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][115]

The phylogeny proposed by Blake in 1987 is:^[2][116]

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.^[117]

Human relations

In research

Starfish are

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.^[121]

Starfish also have an unusual ability to expel foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.^[122]

In legend and culture

An

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.^[125]

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

Starfish are in some cases taken from their habitat and sold to tourists as

With its multiple arms, the starfish provides a popular metaphor for computer networks,^[139] companies^[140][141] and software tools.^[142] It is also the name of a seabed imaging system and company.^[143]

Starfish has repeatedly been chosen as a name in

Bibliography

• Lawrence, J. M., ed. (2013). Starfish: Biology and Ecology of the Asteroidea. Johns Hopkins University Press.
  ISBN 978-1-4214-0787-6
  .
• Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition. Cengage Learning.
  ISBN 81-315-0104-3.{{cite book}}: CS1 maint: multiple names: authors list (link
  )

External links

Wikimedia Commons has media related to starfish.

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

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