"Asteroidea" redirects here. For the Asteraceae subfamily, see
Asteroideae.
Starfish or sea stars are echinoderms belonging to the class Asteroidea.[2] The names "starfish" and "sea star" essentially refer to members of the class Asteroidea. However, common usage frequently finds "starfish" and "sea star" also applied to ophiuroids which are correctly referred to as "brittle stars" or "basket stars".
About 1,800 living species of starfish occur in all the world's oceans, including the Atlantic, Pacific, Indian, Arctic and Southern Oceans regions. Starfish occur across a broad depth range from the intertidal to abyssal depths (>6000 m).
Starfish are among the most familiar of marine animals and possess a number of widely known characteristics, such as regeneration and feeding on mussels. They possess a wide diversity of body forms and feeding methods. The extent to which Asteroidea can regenerate varies with individual species. Broadly speaking, starfish are opportunistic feeders, with several species having specialized feeding behaviors, including suspension feeding and specialized predation on specific prey.
The Asteroidea occupy several important roles throughout ecology and biology. Starfish, such as the ochre sea star (Pisaster ochraceus) have become widely known as the example 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. Other starfish, such as members of the Asterinidae, are frequently used in developmental biology.
Closeup of the top surface of a starfish
Starfish are radially symmetric and typically express pentamerism or pentaradial symmetry as adults. However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Modern starfish, as well as other echinoderms, exhibit bilateral symmetry in their larval forms.[3]
Most starfish typically have five rays or arms, which radiate from a central disc. However, several species frequently have six or more arms. Several asteroid groups, such as the Solasteridae, have 10–15 arms, whereas some species, such as the Antarctic Labidiaster annulatus can have up to 50. It is not unusual for species that typically have five-rays to exceptionally possess six or more rays due to developmental abnormalities.[4]
The surface of starfish bear plate-like calcium carbonate components known as ossicles. These form the endoskeleton, which takes on a number of forms that are externally expressed as a variety of structures, such as spines and granules. These may be arranged in patterns or series and their architecture, individual shapes and locations are used to classify the different groups within the Asteroidea. Terminology referring to body location in starfish is usually based in reference to the mouth to avoid incorrect assumptions of homology with the dorsal and ventral surfaces in other bilateral animals. The bottom surface is often referred to as the oral or actinal surface, whereas the top surface is referred to as the aboral or abactinal side.[5]
The body surfaces of starfish have several structures that comprise the basic anatomy of the animal and can sometimes assist in its identification. The madreporite can be easily identified as the light-colored circle, located slightly off center on the central disc. This is a porous plate which is connected via a calcified channel to the animal's water vascular system in the disc. Its function is, at least in part, to provide additional water for the animal's needs, including replenishing water to the water vascular system.[5] Near the madreporite, also off center, is the anus. On the oral surface there is an ambulacral groove running down each arm. On either side of this there is a double row of unfused ossicles. The tube feet extend through notches in these and are connected internally to the water vascular system.[5]
Several groups of asteroids, including the Valvatacea but especially the Forcipulatacea, possess small bear-trap or valve-like structures known as pedicellariae. These can occur widely over the body surfaces. In forcipulate asteroids, such as Asterias or Pisaster, pedicellariae occur in pom-pon-like tufts at the base of each spine, whereas in goniasterids, such as Hippasteria, pedicellariae are scattered over the body surface. Although the full range of function for these structures is unknown, some are thought to act as defense, where others have been observed to aid in feeding or in the removal of other organisms attempting to settle on the surface of the starfish.[5] The Antarctic Labidiaster annulatus uses its large pedicellariae to capture active krill prey. A North Pacific Stylasterias species has been observed to capture small fish with its pedicellariae.
Other types of structures vary by taxon. For example, Porcellanasteridae employ additional cribriform organs which occur among their lateral plate series, and which are thought to generate currents in the burrows made by these infaunal starfish.[6]
Dissection of
Asterias rubens
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 – Tube feet
As echinoderms, starfish possess a hydraulic water vascular system that aids in locomotion.[7] This system has many projections called tube feet on the ventral faces of the starfish's arms which function in locomotion and aid with feeding. Tube feet emerge through openings in the endoskeleton and are externally expressed through the open grooves present along the bottom of each arm.
The body cavity not only contains the water vascular system that operates the tube feet, but also the circulatory system, called the hemal system. Hemal channels form rings around the mouth (the oral hemal ring), nearer to the aboral surface and around the digestive system (the gastric hemal ring).[8] A portion of the body cavity called the axial sinus connects the three rings. Each ray also has hemal channels running next to the gonads. These have blind ends and there is no continuous circulation of the blood.[9]
On the end of each arm or ray is a tiny simple eye which allows the starfish to perceive the difference between light and darkness. This is useful in the detection of moving objects.[10] Only part of the cells are pigmented (thus a red or black color) and there is no cornea or iris. This eye is known as a pigment spot ocellus.[11]
Several types of toxins and secondary metabolites have been extracted from several species of starfish. Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.
The body wall consists of a thin, outer epidermis, a thick dermis formed of connective tissue and a thin, inner peritoneum. The dermis contains the ossicles (bony plates) which are rather loosely organised. Some bear external granules, tubercles and spines, sometimes organised in definite patterns and some specialised as pedicellariae.[5] There may also be papulae, hollow protusions of the peritoneum that extend through the body wall.[9]
The mouth of a starfish is located on the underside of the body, and opens through a short esophagus into firstly a cardiac stomach, and then, a second, pyloric stomach. Each arm also contains two pyloric caeca, long hollow tubes branching outwards from the pyloric stomach. Each pyloric caecum is lined by a series of digestive glands, which secrete digestive enzymes and absorb nutrients from the food. A short intestine runs from the upper surface of the pyloric stomach to open at an anus in the center of the upper body.[12]
Many starfish, such as Astropecten and Luidia, swallow their prey whole, and start to digest it in the stomachs before passing it into the pyloric caeca.[12] However, in a great many species, the cardiac stomach can be everted from the organism's body to engulf and digest food. In these species, the cardiac stomach fetches the prey, and then passes it to the pyloric stomach, which always remains internal.[13]
Some species are able to use their water vascular systems to force open the shells of bivalve molluscs, such as clams and mussels, by injecting their stomachs into the shells. With the stomach inserted inside the shell, the starfish is able to digest the mollusc in place. The cardiac stomach is then brought back inside the body, and the partially digested food is moved to the pyloric stomach.[14] Further digestion occurs in the intestine. Waste is excreted through the anus on the aboral side of the body.[15]
Because of this ability to digest food outside of its body, the starfish is able to hunt prey much larger than its mouth would otherwise allow, such as clams and oysters, arthropods, small fish, and molluscs. However, some species are not pure carnivores, and may supplement 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 can be swept towards the mouth along ciliated grooves.[12]
Some echinoderms can live for several weeks without food under artificial conditions. Scientists believe they may receive some nutrients from organic material dissolved in seawater.[citation needed]
Starfish have rather complex nervous systems, with a distributed brain. They have a network of interlacing nerves called a nerve plexus which lies within, as well as below, the skin.[6] The esophagus is also surrounded by a central nerve ring, which sends radial nerves into each of the arms, often parallel with the branches of the water vascular system. These all connect to form a brain. The ring nerves and radial nerves coordinate the starfish's balance and directional systems.[9]
Although starfish do not have many well-defined sensory inputs, they are sensitive to touch, light, temperature, orientation, and the status of water around them.[16] The tube feet, spines, and pedicellariae found on starfish are sensitive to touch, while eyespots on the ends of the rays are light-sensitive.[17] The tube feet, especially those at the tips of the rays, are also sensitive to chemicals and this sensitivity is used in locating odor sources, such as food.[18]
The eyespots each consist of a mass of ocelli, consisting of pigmented epithelial cells that respond to light and narrow sensory cells lying between them. Each ocellus is covered by a thick, transparent cuticle that both protects them and acts as a lens. Many starfish also possess individual photoreceptor cells across their bodies and are able to respond to light even when their eyespots are covered.[12]
The underside of a starfish: The inset shows a magnified view of the tube feet.
Starfish move using a water vascular system. Water comes into the system via the madreporite. It is then circulated from the stone canal to the ring canal and into the radial canals. The radial canals carry water to the ampulla portion of tube feet.
Each tube foot consists of an internal ampulla and an external podium, or "foot". When the ampulla is squeezed, it forces water into the podium, which expands to contact the substrate. Although the podium resembles a suction cup in appearance, the gripping action is a function of adhesive chemicals rather than suction. Unadhesive chemicals and podial contraction allow for the release off of substrate.[19][20]
The tube feet latch on to surfaces and move in a wave, with one body section attaching to the surfaces as another releases. Most starfish cannot move quickly. However, some burrowing species from the genera Astropecten and Luidia are capable of rapid, creeping motion, "gliding" across the ocean floor[citation needed], which results from their pointed tubefeet adapted specially for excavating patches of sand.[citation needed]
Respiration occurs mainly through the tube feet and through the papulae that dot the body surface. These are thin-walled projections of the body cavity, reaching through the muscular body wall and into the surrounding water. Oxygen from the water is distributed through the body mainly by the fluid in the main body cavity; the hemal system may also play a minor role.[12]
There are no distinct excretory organs and excretion of nitrogenous waste is performed through the tube feet and papulae. The body fluid contains phagocytic cells called coelomocytes, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to the tips of the papulae, where they are ejected into the surrounding water. Some waste may also be excreted by the pyloric glands and voided with the faeces.[12]
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.[12]
Starfish are capable of both sexual and asexual reproduction.
Most species are dioecious, with separate male and female individuals (also referred to as gonochoric). Male and female starfish are usually not distinguishable from the outside; one needs to see the gonads or observe them 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[21]) produces eggs and sperm. Yet other starfish are sequential hermaphrodites, with some species being protandrous (i.e. young individuals are males that change into females as they grow larger, Asterina gibbosa, for example) and others are protogynous. In some species, when a large female divides, the smaller individuals produced become males. When they grow big enough, they change back into females.[22]
Each arm contains two gonads, which release gametes through openings called gonoducts, located on the central body 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 onto the undersides of rocks to develop.[23] Certain species of starfish brood their eggs – either by simply sitting on them[23] or by holding them in specialised structures. These structures include chambers on their aboral surfaces,[24] the pyloric stomach(Leptasterias tenera)[25] or even the gonads themselves.[21] Those starfish that brood their eggs by sitting on them usually raise their disc and assume a humped posture.[26] One species broods a few of its young and broadcasts the remaining eggs which cannot fit into the pouch.[24] In these brooding species, the eggs are relatively large, and supplied with yolk, and they generally (but not always[21]) develop directly into miniature starfish, without a larval stage. The developing young are called "lecithotrophic" because they get their nutrition from the yolk, as opposed to planktotrophic feeding larvae. In one species of intragonadal brooder, the young starfish get their nutrition by eating the eggs and embryos with them in their gonadal brood pouch.[27] Brooding is especially common in polar and deep-sea species, environments less favourable for larvae,[12][25] and in smaller species that produce few eggs.[28]
To increase their chances of fertilization, starfish may synchronize their spawning, gather in groups,[23] or form pairs.[29] This is called pseudo-copulation[30] when the males climb onto the females with their arms placed between those of the female, and release the sperm around them. Starfish may use environmental signals to coordinate timing 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 each other.[31] In some species, mature females produce chemicals to attract sperm in the sea water.[31]
Some species of starfish also reproduce asexually as adults either by fission[32] of their central discs or by the autotomy of their arms. The type of reproduction depends on the genus. Among starfish that regenerate whole bodies from their arms, some can do so even from fragments just 1 cm long.[33] Single arms that are regenerating the disc and other arms are called comet forms.[34] The division of the starfish either across their discs or at their arms is usually accompanied by changes that help them break easily.
The larvae of several starfish species can also reproduce asexually.[35] They may do this by autotomising some parts of their bodies or by budding.[36] When the larvae sense plentiful food, they favour asexual reproduction instead of directly developing.[37] Though this costs the larvae time and energy, it will allow a single larva to give rise to multiple adults when the conditions are right. Various other reasons trigger similar phenomena in the larvae of other echinoderms. These include the use of tissues that will be lost during metamorphosis or the presence of predators that target larger larvae.
Like all echinoderms, starfish are developmentally (embryologically) deuterostomes, a feature they share with chordates (including vertebrates), but not with most other invertebrates. Their embryos initially develop bilateral symmetry, again reflecting their likely common ancestry with chordates. Later development takes a very different path, however, as the developing starfish settle out of the zooplankton and develop the characteristic radial symmetry. As the organisms grow, one side of their bodies grows more than the other, and eventually absorbs the smaller side. After that, the body is formed into five parts around a central axis and the starfish has radial symmetry.[9]
The larvae of starfish are ciliated, free-swimming organisms. Fertilized eggs grow into bipinnaria and (in most cases) later into brachiolaria larvae, which either grow using a yolk or by catching and eating other plankton. In either case, they live as plankton, suspended in the water and swimming by using beating cilia. The larvae are bilaterally symmetric and have distinct left and right sides. Eventually, they settle onto the bottom of the sea, undergo a complete metamorphosis and grow into adults.[9]
The lifespan of starfish varies considerably between species, generally being longer in larger species. For example, Leptasterias hexactis (adult weight 2 grams) reaches sexual maturity in two years, and lives for about 10 years in total, while Pisaster ochraceus (adult weight 80 grams) reaches maturity in five years, and may live to the age of 34.[12]
Some (but not all[33]) species of starfish have the ability to regenerate lost arms and can regrow an entire new arm given time. Some of these can regrow the rest of their bodies from a single arm[38] while others need at least part of the central disc to be attached to the arm.[38] Regrowth can take several months to years.[33][38] Starfish are vulnerable to infections during the early stages after the loss of an arm.[33] Arms live off stored nutrients till they regrow mouths. Other than fragmentation carried out for the purpose of reproduction, the division of the body may happen inadvertently due to a predator or it may be actively shed by the starfish to escape (see autotomy). The loss of parts of the body in response to a predator is achieved by the rapid softening of a special type of connective tissue in response to nervous signals. [39] [40] This type of tissue is called catch connective tissue and is found in most echinoderms.
Most species are generalist predators, eating mollusks such as clams, oysters, some snails, or any other animal too slow to evade the 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 may consume coral polyps (the best-known example for this is the infamous crown-of-thorns starfish), sponges or even suspended particles and plankton (such as starfish of the order Brisingida).[41] The processes of feeding and capture may be aided by special parts; Pisaster brevispinus or short-spined pisaster from the West Coast of America may use a set of specialized tube feet to extend itself deep into the soft substrata to extract prey (usually clams).[42] Grasping the shellfish, the starfish slowly pries open the prey's shell by wearing out its adductor muscle, and then inserts (everts) its stomach into an opening to devour the organism.
About 1,800 living species of starfish are known, and they occur in all of the Earth's oceans. The greatest variety is found in the tropical Indo-Pacific. 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). Asterias is a common genus found in European waters and on the eastern coast of the United States; Pisaster, along with Dermasterias ("leather star"), are usually found on the western coast. Habitats range from tropical coral reefs, to kelp forests, to deep-sea floors, although none live within the water column; all species of starfish found are living as benthos. Echinoderms need a delicate internal electrolyte balance in their bodies; no starfish are found in freshwater environments.
Starfish move using a water vascular system. Water comes into the system via the madreporite.
Some of the better known starfish include:
The Northern Pacific starfish (Asterias amurensis), known as gohongaze, is considered an edible delicacy.[43]
Starfish and other echinoderms pump water directly into their bodies, via the water vascular system, as they find it. This makes them vulnerable to all forms of water pollution, as they have little ability to filter the water of toxins and contaminants. Oil spills and similar events often take a toll on echinoderm populations that carry consequences for the ecosystem.[44]
Astropecten lorioli – a species from the
Jurassic
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Star Fish at Montana de Oro.JPG
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The tube feet can be seen on this starfish.
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Caribbean seastar (Oreaster reticulatus), Bahia de la Chiva, Vieques, PR.
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