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Starfish
Fossil range: Ordovician–Recent
"Asteroidea" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Kingdom: Animalia
Phylum: Echinodermata
Subphylum: Asterozoa
Class: Asteroidea
Orders

Brisingida (100 species[1])
Forcipulatida (300 species[1])
Paxillosida (255 species[1])
Notomyotida (75 species[1])
Spinulosida (120 species[1])
Valvatida (695 species[1])
Velatida (200 species[1])

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".

There are over 1800 species of living species of starfish that occur in all the world's oceans, including the Atlantic, Pacific, Indian as well as in the Arctic and the Southern Ocean (i.e., Antarctic) 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 traits,such as regeneration and feeding on mussels. Starfish possess a wide diversity of body forms and feeding methods. The extent that Asteroidea can regenerate varies with individual species. Broadly speaking, starfish are opportunistic feeders, with several species having specialized feeding behavior, including suspension feeding and specialized predation on specific prey.

The Asteroidea occupy several important roles throughout ecology and biology. Sea stars, such as the Ochre 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) are voracious predators of coral throughout the Indo-Pacific region. Other starfish, such as members of the Asterinidae are frequently used in developmental biology.

Contents

Appearance

Red-knobbed starfish Protoreaster linckii, a sea star from the Indian Ocean
Schmedelian pin-cushion sea star (Culcita schmideliana) on Meedhupparu house reef in the Maldives
Closeup of the top surface of a starfish

Starfish express pentamerism or pentaradial symmetry as adults. However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Starfish, as well as other echinoderms, do exhibit bilateral symmetry, but only as larval forms.[3]

Most starfish typically have five rays or arms, which radiate from a central disk. 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 five or more rays due to developmental abnormalities.[4]

The bodies of starfish are composed of calcium carbonate components, known as ossicles. These form the endoskeleton, which takes on a variety of forms that are externally expressed as a variety of structures, such as spines and granules. The architecture and individual shape/form of these plates which often occur in specific patterns or series, as well as their location are the source of morphological data used to classify the different groups within the Asteroidea. . Terminology referring to body location in sea stars 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.

The body surface of sea stars often has 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 disk. This is a porous plate which is connected via a calcified channel to the animal's water vascular system in the disk. Its function is, at least in part, to provide additional water for the animal's needs, including replenishing water to the water vascular system.

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 surface. In forcipulate asteroids, such as Asterias or Pisaster, pedicellariae occur in pom-pom 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 to act as defense where others have been observed to aid in feeding. The Antarctic Labidiaster annulatus uses its large, pedicellariae to capture active krill prey. The North Pacific Stylasterias 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, which are thought to generate current in the burrows made by these infaunal sea star.[5]

Internal anatomy

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.[6] The water vascular system has many projections called tube feet on the ventral face of the sea star'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), closer to the top of the sea star and around the digestive system (the gastric hemal ring).[7] 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.

On the end of each arm or ray there is a microscopic eye (ocellus), which allows the sea star to see, although it only allows it to see light and dark, which is useful to see movement.[8] 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.[9]

Several types of toxins and secondary metabolites have been extracted from several species of sea star. Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.

Digestive system

The mouth of a starfish is located on the underside of the body, and opens through a short oesophagus 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.[10]

Many sea stars, such as Astropecten and Luidia swallow their prey whole, and start to digest it in the stomachs before passing it into the pyloric caeca[10]. However, in a great many species, the cardiac stomach can be everted out of the organism's body to engulf and digest food. In these species, the cardiac stomach fetches the prey then passes it to the pyloric stomach, which always remains internal.[11]

Some species are able to use their water vascular systems to force open the shells of bivalve mollusks such as clams and mussels by injecting their stomachs into the shells. With the stomach inserted inside the shell, the sea star is able to digest the mollusk in place. The cardiac stomach is then brought back inside the body, and the partially digested food is moved to the pyloric stomach.[12] Further digestion occurs in the intestine. Waste is excreted through the anus on the aboral side of the body.[13]

Because of this ability to digest food outside of its body, the sea star is able to hunt prey that are much larger than its mouth would otherwise allow, such as clams and oysters, arthropods, small fish, and mollusks. However, some species are not pure carnivores, and may supplement their diet 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.[10]

Some echinoderms can live for several weeks without food under artificial conditions. Scientists believe that they may receive some nutrients from organic material dissolved in seawater.

Nervous system

Echinoderms have rather complex nervous systems, but lack a true centralized brain. All echinoderms have a network of interlacing nerves called a nerve plexus which lies within, as well as below, the skin.[14] 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. The ring nerves and radial nerves coordinate the sea star's balance and directional systems.

Although the echinoderms do not have many well-defined sensory inputs, they are sensitive to touch, light, temperature, orientation, and the status of water around them.[15] The tube feet, spines, and pedicellariae found on sea stars are sensitive to touch, while eyespots on the ends of the rays are light-sensitive.[16] 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.[17]

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 body and are able to respond to light even when their eyespots are covered.[10]

Locomotion

The underside of a sea star. The inset shows a magnified view of the tube feet.

Sea stars 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 ampullae and provide suction to the tube feet. The tube feet latch on to surfaces and move in a wave, with one body section attaching to the surfaces as another releases. Most sea stars cannot move quickly. However, some burrowing species from the genera Astropecten and Luidia are capable of rapid, creeping motion: "gliding" across the ocean floor. This motion results from their pointed tubefeet adapted specially for excavating patches of sand.

Sea-star endoskeleton

Endoskeleton

Sea stars and other echinoderms have endoskeletons, suggesting that echinoderms are very closely related to chordates, animals with a hollow nerve chord that usually have vertebrae.[citation needed]

Respiration and excretion

Respiration occurs mainly through the tube feet, and through tiny structures called papullae that dot the body surface. These papullae 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.[10]

Excretion of nitrogenous waste is also performed through the tube feet and papullae, and there are no distinct excretory organs. 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 papullae where they are ejected into the surrounding water. Some waste may also be excreted by the pyloric glands and voided with the faeces.[10]

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 osmoregulation likely explains why starfish are not found in fresh water, or even in estuarine environments.[10]

Life cycle

Starfish are capable of both sexual and asexual reproduction. Most species are dioecious, with separate male and female individuals, but some are hermaphrodites. For example, the common species Asterina gibbosa is protandric, with individuals being born male, but later changing into females.[10]

Male and female sea stars are not distinguishable from the outside; one needs to see the gonads or be lucky enough to catch them spawning. Each arm contains two gonads, which release gametes through openings called gonoducts, located on the central body between the arms.

Reproduction

Fertilization takes place externally, both male and female releasing their gametes into the environment. The resulting fertilized embryos form part of the zooplankton in most species. However, some species brood their eggs, either by simply sitting over them, or using specialised brooding baskets on their aboral surface. In at least one species (Leptasterias tenera), the eggs are actually brooded inside the pyloric stomach. In these brooding species, the eggs are relatively large, and supplied with yolk, and they generally develop directly into miniature starfish, without a larval stage. Brooding is especially common in polar and deep-sea species, environments less favourable for larvae.[10]

Sea stars commonly reproduce by free-spawning: releasing their gametes into the water where they are fertilized by gametes from the opposite sex. To increase their chances of fertilization, sea stars probably gather in groups when they are ready to spawn, use environmental signals to coordinate timing (day length to indicate the correct time of the year, dawn or dusk to indicate the correct time of day), and may use chemical signals to indicate their readiness to each other.[18]

Some species of sea star also reproduce asexually by fragmentation, often with part of an arm becoming detached and eventually developing into an independent individual sea star. This has led to some notoriety. Sea stars can be pests to fishermen who make their living on the capture of clams and other mollusks at sea as sea stars prey on these. The fishermen would think they had killed the sea stars by chopping them up and disposing of them at sea, but each fragment would regenerate into a complete adult, ultimately leading to their increased numbers until the issue was better understood. A sea-star arm can only regenerate into a whole new organism if some of the central ring of the sea star is part of the chopped off arm.

Larval development

Like all echinoderms, starfish are developmentally (embryologically) deuterostomes; a feature they share with chordates (including vertebrates), but not with most other invertebrates. Their embryo initially develops bilateral symmetry, again reflecting their likely common ancestry with chordates. Later development takes a very different path, however, as the developing star fish settles out of the zooplankton and develops the characteristic radial symmetry. As the organism grows, one side of the body grows more than the other, and eventually absorbs the smaller side. After that, the body is formed into five parts around a central axis. Then the echinoderm has radial symmetry.

The larvae of echinoderms 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 — unlike adults, they have a distinct left and right side. Eventually, they undergo a complete metamorphosis, settle to the bottom, and grow into adults.

Lifespan

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 ten years in total, while Pisaster ochraceus (adult weight 80 grams) reaches maturity in five years, and may live to the age of 34.[19]

Diet

Sea star Pisaster ochraceus consuming a mussel in Central California

Most species are generalist predators, eating mollusks such 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 decomposed animal and plant material or organic films attached to substrate. Others may consume coral polyps (the best-known example for this is the infamous Acanthaster planci), sponges or even suspended particles and plankton (such as sea stars of the Order Brisingida).[20] 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)[21]. Grasping the shellfish, the sea star slowly pries open the shell by wearing out the adductor muscle and then inserts (also called evisceration) its stomach into an opening to devour the organism.

Distribution

There are about 1,800 known living species of sea star, and they occur in all of the Earth's oceans. The greatest variety of sea stars 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, kelp forests to deep-sea floor, although none of them live within the water column; all species of sea star found are living as benthos. Echinoderms need a delicate internal balance in their body; no sea stars are found in freshwater environments.

Diversity

Sea stars move using a water vascular system. Water comes into the system via the madreporite.

As mentioned above there are over 1800 species; with many species awaiting discovery. Some of the better known sea stars include:

The Northern Pacific sea star (Asterias amurensis) known as gohongaze is considered an edible delicacy[22].

Notes

  1. ^ a b c d e f g Sweet, Elizabeth (2005-11-22). "Asterozoa: Fossil groups: SciComms 05-06: Earth Sciences". http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/asteroz2/index_f/mod_fm.html. Retrieved 2008-05-07. 
  2. ^ Mooi, Rich. "Classification of the Extant Echinodermata." California Academy of Sciences - Research. <http://research.calacademy.org/research/izg/echinoderm/classify.htm>.
  3. ^ "Starfish." 16 May 2008. HowStuffWorks.com. <http://animals.howstuffworks.com/marine-life/starfish-info.htm> 16 January 2009.
  4. ^ You superstar! Fisherman hauls in starfish with eight legs instead of five, [[Daily Mail], 24 October 2009, accessed 3 January 2010.
  5. ^ Star Fish." South Central Service Co-op. 2001. <http://www.scsc.k12.ar.us/2001Outwest/PacificNaturalHistory/Projects/ReynoldsJ/Default.htm>.
  6. ^ "Wonders of the Sea: Echinoderms." Ceanside Meadows Institute for the Arts and Sciences. <http://www.oceaninn.com/guides/echino.htm>.
  7. ^ "Sea stars on Chek Jawa, Pulau Ubin, Singapore." Wildsingapore. <http://www.wildsingapore.org/chekjawa/text/p610.htm>.
  8. ^ "Animal Eyes." San Diego Natural History Museum--Your Nature Connection in Balboa Park. 31 Dec. 2002. <http://www.sdnhm.org/exhibits/eyes/overview.html>.
  9. ^ "Eye (invertebrate)" McGraw-Hill Encyclopedia of Science & Technology, vol. 6, p.790 2007
  10. ^ a b c d e f g h i Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 939-945. ISBN 0-03-056747-5. 
  11. ^ "Marine Biology Echinodermata - Sea Star." <http://home.earthlink.net/~huskertomkat/star.html>.
  12. ^ Nicholson, F. C. "How a Sea Star Gets Its Clam - Science Stories - HighlightsKids.com." HighlightsKids. <http://www.highlightskids.com/Science/Stories/SS0596_howseastargetsclam.asp>.
  13. ^ Dale, Jonathan. "Starfish Digestion and Circulation." 24 May 2009. Madreporite Nexus. <http://www.madreporite.com/science/digest.htm>.
  14. ^ "Star Fish." South Central Service Co-op. 2001. <http://www.scsc.k12.ar.us/2001Outwest/PacificNaturalHistory/Projects/ReynoldsJ/Default.htm>.
  15. ^ "Starfish brains, night length, space radiation." WonderQuest. 18 Apr. 2008. <http://www.wonderquest.com/sea-stars-nights-space-radiation.htm>.
  16. ^ Dale, Jonathan. "The Starfish Nervous System." Madreporite Nexus. 24 May 2009. <http://www.madreporite.com/science/fiverays.htm>.
  17. ^ Dale, Jonathan. "The Starfish Nervous System." Madreporite Nexus. 24 May 2009. <http://www.madreporite.com/science/orientation.htm>.
  18. ^ Chaet, Alfred B., American Zoologist 1966 6(2):263-271, "The Gamete-Shedding Substances of Starfishes: A Physiological-Biochemical Study" http://icb.oxfordjournals.org/cgi/content/abstract/6/2/263
  19. ^ Dale, Jonathan. "Starfish Regeneration." Madreporite Nexus. 24 May 2009. <http://www.madreporite.com/science/regeneration.htm>.
  20. ^ Dale, Jonathan. "Starfish Ecology." Madreporite Nexus. 24 May 2009. <http://www.madreporite.com/science/ecology.htm>.
  21. ^ Nybakken Marine Biology: An Ecological Approach, Fourth Edition, page 174. Addison-Wesley Educational Publishers Inc., 1997.
  22. ^ "Cooking Starfish In Japan". http://www.amakusa.tv/enewshito.html. Retrieved 2008-05-07. 

References

  • Blake DB, Guensburg TE; Implications of a new early Ordovician asteroid (Echinodermata) for the phylogeny of Asterozoans; Journal of Paleontology, 79 (2): 395-399; MAR 2005.
  • Gilbertson, Lance; Zoology Lab Manual; McGraw Hill Companies, New York; ISBN 0-07-237716-X (fourth edition, 1999).
  • Shackleton, Juliette D.; Skeletal homologies, phylogeny and classification of the earliest asterozoan echinoderms; Journal of Systematic Palaeontology; 3 (1): 29-114; March 2005.
  • Solomon, E.P., Berg, L.R., Martin, D.W. 2002. Biology, Sixth Edition.
  • Sutton MD, Briggs DEG, Siveter DJ, Siveter DJ, Gladwell DJ; A starfish with three-dimensionally preserved soft parts from the Silurian of England; Proceedings of the Royal Society B - Biological Sciences; 272 (1567): 1001-1006; MAY 22 2005.
  • Hickman C.P, Roberts L.S, Larson A., l'Anson H., Eisenhour D.J.; Integrated Principles of Zoology; McGraw Hill; New York; ISBN 0-07-111593-5 (Thirteenth edition; 2006).

External links

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

STARFISH, a popular term under which are included a large number of sea-animals, belonging all to the great group of Echino derms, but to three dis tinct divisions of that group: the Asterids, the Ophiurids and the Crinoids (see Echino Derma). The Asterids or starfish proper in clude the cross-fish, the b sun-star (see Echinoderma, fig. 17), the cushion-star, the butthorn, and many without a popular name. The common cross-fish or five-finger, Asterias rubens, of British seas, FIG. - An Asterid, Asterias rubens, may be taken as typical upper surface. (figs. i and 2), and the a, Madreporite.

a, The same magnified. description will apply b, Anus. also to the American This starfish may be 9-12 in. across. species A. forbesi and A. vulgaris. The animal consists of a central body or disk, produced into five arms or rays. The upper surface is covered with a leathery skin, strengthened by a rafter-work of little bones or plates, made of crystalline carbonate of lime, many of them bearing prickles of the same substance and small pincer-like bodies - the pedicellariae (see SEA-Urchin). In the middle of the body is a small anal opening, and near the angle between two rays is a furrowed plate pierced by many minute pores and called the madreporite. The under surface of the body has the mouth in the centre, and from it deep grooves radiate to the ends of the arms. At the bottom of each groove is a water-vessel, which gives off branches to the podia or suckingfeet on each side of it. A section across this groove is given in the article Echinoderma, fig. 1 2 B. The arrange and working of this FIG. 2. - Asterias rubens, under surface. ment hydraulic system is a, The arm-groove with its row of suckingfeet or podia.

essentially the same as b, End of a podium, magnified. in the sea-urchin, except for the presence of plates at the bottom of the groove beneath th.e radial water-vessel, and the absence of any plates covering the groove. At the end of each ray is, as in the urchin, a single tentacle surrounded by pigment and connected with a definite plate called "terminal." Thus the terminals of a starfish correspond to the oculars of a sea-urchin (see Echinoderma, fig. 3). The stomach is not a long coil, but a simple sac with branched blind tubes extending into each ray. A generative gland also passes down the side of each ray, and emits the milt or eggs when ripe through a pore near the body. Spawning takes place in spring or early summer. A starfish can crawl in any direction by means of its sucking-feet, whether the surface be hard or rough or polished, or the softest silt, whilst its supple body can squeeze through incredibly narrow crevices. The rate of progress is about six inches a minute.

The starfish are the scavengers of the sea, but unfortunately do. not confine their attentions to decaying matter; they eat oysters, clams, mussels, barnacles, sea-snails, worms, crustacea and even smaller starfish. There is constant war between oyster-fishers and starfish; no less than 42,000 bushels of starfish were removed from the oyster-beds of Connecticut in a single year, but not till they had worked damage to the amount of $631,500. The simplest way in which a starfish eats is by taking small bits of food into the stomach, and ejecting the refuse again through the mouth. But since the mouth is quite small and the food often large, the starfish finds it more convenient to turn its stomach inside out and to wrap it around the animal to be eaten, which is then digested quietly and the stomach withdrawn again. In the case of oysters and similar bivalves, the starfish first has to open them; and this it does by fixing the suckers of one or two rays to one valve and those of the opposite rays to the other valve, while it may get a purchase by also holding on to some neighbouring object. It then begins to straighten out its rays. The oyster can withstand a very strong pull, but it cannot hold out against a long pull, and the starfish does not hurry. At last the oyster gives way, and the starfish has its reward; but its companions often join in, and you may see a whole ball of them interlaced round half-digested molluscs and rolling about. Starfish begin to eat voraciously when quite young; one less than 5th in. across has been observed to eat over fifty young clams of half that length in six days. The more a starfish has to eat the quicker it grows, and it may become sexually mature in less than a year, then producing many thousands of young. Fortunately the increase is kept in check by many causes. The young, while still in the stage of free-swimming larvae, are swallowed in millions by various fish. When they settle down on seaweed their bright colours attract eels and many small fishes. Later in life they are attacked by parasites, while those which stray into shallow water are eaten by gulls and crows. Freshets and cold currents are also destructive.

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Probably the best way in which man can keep down the numbers of starfish is by dredging the seaweed in the latter half of July when it is covered with young; a single cartload thrown on shore would capture many millions. At a later stage tangles of hemp or cotton waste may be dragged over the oyster-beds, when the starfish will cling to them by their pedicellariae. They make excellent manure, but are of no further service to man. Fishermen who catch them in their nets or on their lines often tear them in half and throw them back into the sea. Some of these mutilated animals may, however, grow fresh rays, and thus one may find a starfish consisting of one large ray and four quite small ones, the whole shaped like a comet.

The Ophiurids (the name means "snake-tails") include the brittle-stars, sand-stars, and basket-fish or medusa-heads.

FIG. 3. - An Ophiurid, the Daisy Brittle-star (Ophiopholis aculeata); upper surface. (3 natural size.) The two former, which may often be found hiding under the rocks, or in the seaweed, or in pools at low tide, resemble the ordinary starfish in having five distinct arms. These, however, as shown in fig. 3, are long and serpent-like, and are attached to a relatively small body or disk. The digestive and generative systems do not extend to the rays but are confined to the body. The arms are cylindrical and have no groove on the under side such as exists in starfish; but the water-vessel traverses the solid bones that form the axis of the arm, and the podia pass out through special openings (see Echinoderma, fig. 18).

In Ophiurids it is the arms that are used for locomotion and not the podia, so that the latter have no terminal suckers. The axial ossicles, which correspond to the plates flooring the arm-groove in a starfish, resemble vertebrae connected by pairs of straight muscular bundles, and articulated by tenon-and-mortise joints, according to whose degree of development the arms vary in their power of coiling. These vertebrae are encased in the tough outer skin of the arm, in which are developed plates. Spines borne by these plates aid the animal in locomotion. The skin of the disk also bears small plates, which are often covered with prickles. The mouth is on the under surface of the disk, and round it are a number of short, flat processes, the mouth-papillae, which serve as strainers. Inside the mouth are seen the five tooth-plates, borne on a strong frame of complicated structure. In the sand-stars the rays are comparatively short, with their spines closely pressed to their sides, so that they look like lizards' tails; in the brittle-stars the rays are much longer and more flexible, with the spines standing out, so that they look like wriggling centipedes attached round a little sea-urchin. The brittle-stars are more active than the sand-stars, and can go more than two yards in a minute; some of them, if seized, break off their arms, which continue breaking into smaller pieces; but the body can soon grow new ones. Sand-stars and brittle-stars are found in all seas, usually occurring in quantities, but are most abundant in the rock-pools of the tropics. By constantly sweeping their arms over the sea bottom, they gather food consisting of minute animals. They eat the bait of fishermen, and their fish as well if they find any already dead, but they are themselves a favourite food with many fish, notably the cod.

The basket-fish or medusa-heads are Ophiurids whose arms branch several times, their ends often curling and interlacing. They live in deeper water and are often brought up clinging to fishermen's lines.

The feather-stars (fig. 4) have a central body and five arms, each forking at least once and fringed with small branches (pinnules) which give the feathery appearance. The mouth is in the middle of the body, and from it grooves pass along the arms and all their branches. The animal lives with the mouth upwards, and although it can crawl and even swim by movement of its arms, it generally fixes itself to a stone or seaweed or some zoophyte, by means of a bunch of small jointed and hooked processes (cirri) growing from the back or under side of the body. It gets its food in this way: the arm-grooves (Echinoderma, fig. 12, C) are lined with minute hairs (cilia) always waving in the direction of the mouth, towards which they drive a stream of water; this stream, containing minute organisms, constantly flows through the coiled gut, which extracts nourishment from it. The feather-stars were formerly placed with the starfish, but they really belong to another class of Echinoderms - the Crinoidea.

- ?? ?? = FIG. 4. - The Rosy Feather-star, Antedon bifida, attached by its cirri to a small stone, from which it is moving in the direction of the spectator by pushing with the branches of one arm and pulling with three branches of two arms. (Natural size.) In 1823 J. V. Thompson, of Cork, discovered that the featherstar when quite young was fixed by a stalk, just as are nearly all crinoids (see Echinoderma, figs. I and 2). The stalked crinoids are not so numerous as they once were, but feather-stars belonging to about half a dozen genera (Antedon, Actinometra, &c.) are found in all seas at all depths, often in enormous numbers.

(F. A. B.)


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Starfish
File:Red-knobbed.starfish.
Red-knobbed starfish from the Indian Ocean
Scientific classification
Kingdom: Animalia
Phylum: Echinodermata
Subphylum: Asterozoa
Class: Asteroidea
File:Ochre sea star on beach, Olympic National Park
Starfish on beach: about 25 cm (10 inches) in size

Starfish, or Sea stars, are Echinoderms, and belong to the Class Asteroidea.[1]p35

All of them live in the ocean, on the sea floor. Some live in deep water, and others live on beaches. They have five or more arms and can be quite large. The Sunflower sea star (Pycnopodia helianthoides) is the largest: fully grown, its arm-span is about a metre. This is a bit larger than the famous Crown-of-thorns starfish, which lives on coral polyps.

Although starfish are invertebrates, they do have a kind of skeleton. The bodies of starfish are composed of calcium carbonate plates, known as 'ossicles'. These form the endoskeleton, which takes on a variety of forms such as spines and granules.

They have a primitive nervous system, but not a brain. They also do not have blood, instead they use sea water to pump things around their bodies.

There are over 1500 different species of sea star. All of them live in salt water, usually on the floor of the ocean. Most sea stars are predators. They eat mussels, clams, and other bivalves. Occasionally, they might even catch small fish which are unable to get away from their attack.[2][1]

Contents

Symmetry

Most starfish have five arms, called rays that come out from a center circle (disk). If a starfish has more than five rays, it will often have rays in multiples of five; there could be 10, 15, 20, or even 30 rays on one starfish . This is called pentameral (five-fold) symmetry.

Skeleton

The degree of protection offered by the skeleton differs between species. If the skeleton becomes more rigid, it offers better protection against predators. However, this will tend to limit its feeding alternatives. Great flexibility is required for the process of external digestion used by many species to eat bivalves. The less well armoured species may adopt a life in places where they are not so open to predation. The heavily armoured ones can tough it out in the most competitive habitats:

"A heavily calcified sturdy skeleton capable of resisting both large and small predators has evolved in many tropical sea stars... a very firm body wall... still permits a degree of flexibility of the body and arms".[3]

Feeding

File:Asterias rubens (eating).jpg
Eating a mussel: note the tube feet

Senses

The movement of starfish is guided by their senses of touch and sight. There are five 'eyes', light-sensitive cushions, one at the end of each arm. These and the tube feet are connected to nerve fibres, so these animals are not quite so simple as might seem at first sight.

Tube feet

Although starfish started off as filter-feeders, they evolved to become major predators of shell-fish (the brachiopods and bivalves). They can also eat small crustacea and fish. Their tube feet developed suckers, perhaps originally to improve movement. Later, they were used to open shell-fish.

"Suckered tube feet may not have been present in any Palaeozoic sea star".[4]

Feeding methods

The shells of brachiopods and bivalves are held together by strong muscles. What the starfish does is clamp hold of them on either side with its tube feet, and apply a steady pull. The starfish, with its muscles and hydraulic system, can pull for much longer than any bivalve muscle can withstand. Apparently, ten minutes are usually enough to open the shell a bit. Then the starfish slips its stomach inside the shell. The stomach can get through a slot as narrow as 0.1mm.[5] The starfish then dissolves the mollusc where it lives, absorbing the nutrients. This digestion process takes much longer than opening the shell, perhaps a couple of days.[6]

Sometimes species swallow the shell whole, and dissolves the contents inside their stomach, then push out the shell afterwards.[7][1]p45

This ability of starfish appeared to develop in the Mesozoic, especially in the Jurassic and Cretaceous. This was part of the Mesozoic Marine Revolution, which transformed the sea-floor fauna. Weakly defended and static types disappeared, and more heavily armoured or more mobile shellfish flourished.[8][9][10]

Regeneration

Starfish, like many sea creatures, are able to regenerate (grow back) parts of their bodies. Starfish are better at regeneration than most other creatures. Not only can a new ray grow when a ray is torn off, but if the torn off ray has even a small piece of the central disk still attached, a whole new starfish can grow from the one ray.[1]p35

Because starfish like to eat clams and oysters, fishermen who gather shellfish have tried for years to get rid of them. To kill the starfish, fishermen would catch them, slice them right in half, and throw them back in the ocean. However, because starfish can grow back parts of their bodies, they were actually increasing the number of starfish.

References

  1. 1.0 1.1 1.2 1.3 Nichols, David 1962. Echinoderms. Hutchinson, London. ISBN 0-09-065994-5
  2. Sweet, Elizabeth (2005-11-22). "Asterozoa: Fossil groups: SciComms 05-06: Earth Sciences". http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/asteroz2/index_f/mod_fm.html. Retrieved 2008-05-07. 
  3. Blake D.B. 1984. Constructional morphology and life habits of the Jurassic sea star Sphaeroaster Quenstadt. Neues Jb. Geol. Palãont. Abb. 169, 74–101.
  4. Blake D.B. 1981. The new Jurassic sea star Eokainaster and comments on life habit and the origins of modern Asteroidea. J. Paleont. 55, 33–46.
  5. Vermeij G.J. 1987. Evolution and escalation: an ecological history of life. Princeton N.J. p153
  6. Jangoux M. and Lawrence J.M. (eds) 1982. Echinoderm nutrition. Balkema, Rotterdam.
  7. Nicholson F.C. "How a sea star gets its clam - Science Stories - HighlightsKids.com." HighlightsKids. <http://www.highlightskids.com/Science/Stories/SS0596_howseastargetsclam.asp>.
  8. Vermeij G.J. (1977). "The Mesozoic marine revolution; evidence from snails, predators and grazers". Paleobiology 3: 245–258. http://www.jstor.org/pss/2400374. Retrieved 2008-05-13. 
  9. Stanley, S.M. (2008). [Expression error: Unexpected < operator "Predation defeats competition on the seafloor"]. Paleobiology 34: 1–21. doi:10.1666/07026.1. 
  10. Stanley SM (1974). [Expression error: Unexpected < operator "What has happened to the articulate brachiopods?"]. GSA Abstracts with Programs: 966–967. 

Other websites

Look up Asteroidea in Wikispecies, a directory of species
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