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Sea stars (also often called "starfish") are echinoderms belonging to the class Asteroidea.Mooi, Rich. "Classification of the Extant Echinodermata." California Academy of Sciences - Research. /research.calacademy.org/research/izg/echinoderm/classify.htm>. The names "sea star" and "starfish" essentially refer to members of the Class Asteroidea. However, common usage frequently finds "sea star" and "starfish" also applied to ophiuroids which are correctly referred to as "brittle stars or "basket stars".

There are over 1800 species of living species of sea stars 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. Sea stars occur across a broad depth range from the intertidal to abyssal depths (>6000 m).

Sea stars are among the most familiar of marine animals and possess a number of widely known traits,such as regeneration and feeding on mussels that are largely based on the frequently encountered Atlantic Asterias. Sea stars possess a wide diversity of body forms and feeding methods. The extent that asteroids can regenerate varies with individual species. Broadly speaking, sea stars 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 sea stars, such as members of the Asterinidae are frequently used in developmental biology.

General anatomy

Red-knobbed sea star Protoreaster linckii, a sea star from the Indian Ocean


Sea stars express pentameral or pentardadial symmetry as adults. However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Sea stars, as well as other echinoderms, do exhibit bilateral symmetry, but only as larval forms."Starfish." 16 May 2008. HowStuffWorks.com. /animals.howstuffworks.com/marine-life/starfish-info.htm> 16 January 2009.

Most sea stars 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.

The bodies of sea stars are composed 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 posses 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.Star Fish." South Central Service Co-op. 2001. /www.scsc.k12.ar.us/2001Outwest/PacificNaturalHistory/Projects/ReynoldsJ/Default.htm>.

Internal anatomy

Dissection of Asterias rubens.


As echinoderms,sea stars possess a hydraulic water vascular system that aids in locomotion."Wonders of the Sea: Echinoderms." Ceanside Meadows Institute for the Arts and Sciences. /www.oceaninn.com/guides/echino.htm>. 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)."Sea stars on Chek Jawa, Pulau Ubin, Singapore." Wildsingapore. /www.wildsingapore.org/chekjawa/text/p610.htm>. 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."Animal Eyes." San Diego Natural History Museum--Your Nature Connection in Balboa Park. 31 Dec. 2002. /www.sdnhm.org/exhibits/eyes/overview.html>. 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.

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.

Digestion and excretion

Sea star digestion is carried out in two stomachs in the mouth: the cardiac stomach and the pyloric stomach. One that remains inside the body and another than can be protruded through the starfish's mouth (membranous or Cardiac stomach), and one normal pyloric stomach. The Cardiac stomach fetches the prey then passes it to the Pyloric stomach. The cardiac stomach is a sack-like stomach located at the center of the body and may be everted out of the organism's body to engulf and digest food."Marine Biology Echinodermata - Sea Star." /home.earthlink.net/~huskertomkat/star.html>. 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.Nicholson, F. C. "How a Sea Star Gets Its Clam - Science Stories - HighlightsKids.com." HighlightsKids. /www.highlightskids.com/Science/Stories/SS0596_howseastargetsclam.asp>. Further digestion occurs in the intestine. Waste is excreted through the anus on the aboral side of the body.Dale, Jonathan. "Starfish Digestion and Circulation." 24 May 2009. Madreporite Nexus. /www.madreporite.com/science/digest.htm>.

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.

Some echinoderms live several weeks without food under artificial conditions. It is believed that they may receive some nutrients from organic material dissolved in seawater.

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.

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 star include:

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

Physiology

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."Star Fish." South Central Service Co-op. 2001. /www.scsc.k12.ar.us/2001Outwest/PacificNaturalHistory/Projects/ReynoldsJ/Default.htm>. The esophagus is also surrounded by a number of nerve rings which send radial nerves that are 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."Starfish brains, night length, space radiation." WonderQuest. 18 Apr. 2008. /www.wonderquest.com/sea-stars-nights-space-radiation.htm>. 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.Dale, Jonathan. "The Starfish Nervous System." Madreporite Nexus. 24 May 2009. /www.madreporite.com/science/fiverays.htm>. 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.Dale, Jonathan. "The Starfish Nervous System." Madreporite Nexus. 24 May 2009. /www.madreporite.com/science/orientation.htm>.

Diet

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).Dale, Jonathan. "Starfish Ecology." Madreporite Nexus. 24 May 2009. /www.madreporite.com/science/ecology.htm>.The processes of feeding and capture may be aided by special parts; Pisaster brevispinus or short-spined pisaster from the West Coastmarker of America may use a set of specialized tube feet to extend itself deep into the soft substrata to extract prey (usually clams). 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.

Reproduction

Sea stars are capable of both sexual and asexual reproduction. Individual sea stars are male or female. Fertilization takes place externally, both male and female releasing their gametes into the environment. Resulting fertilized embryos form part of the zooplankton.

Sea stars are developmentally (embryologically) known as deuterostomes. Their embryo initially develops bilateral symmetry, indicating that sea stars probably share a common ancestor with the chordates, which includes the fish. Later development takes a very different path however as the developing star fish settles out of the zooplankton and develops the characteristic radial symmetry. Some species reproduce cooperatively, using environmental signals to coordinate the timing of gamete release; in other species, one to one pairing is the norm.

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.

Fertilized eggs grow into bipinnaria and 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.Some species of sea star brood their young: the males spawn gametes which fertilize eggs held by the females. The females may hold the eggs on their surface, in the pyloric stomach (as in Leptasterias tenera), or even attach them to the ground (as in Asterina gibbosa). Brooding is especially common in polar and deep-sea species, environments less favourable for larvae.

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. The gonads are located in each arm, and release gametes through gonoducts located on the central body between the arms.

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 presumably kill the sea stars by chopping them up and disposing of them at sea, 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.

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.


Regeneration

Some species of sea star have the ability to regenerate lost arms and can regrow an entire new arm in time. Most species must have the central part of the body intact to be able to regenerate, but a few can grow an entire sea star from a single ray. Included in this group are the red and blue Linckia star. The regeneration of these stars is possible due to the vital organs kept in their rays.Dale, Jonathan. "Starfish Regeneration." Madreporite Nexus. 24 May 2009. /www.madreporite.com/science/regeneration.htm>.

Larval development

The larvae of echinoderms are ciliated, free-swimming organisms. They tend to look like embryonic chordates because they organize themselves bilaterally. 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.

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 Statesmarker; 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.

Notes

  1. "Eye (invertebrate)" McGraw-Hill Encyclopedia of Science & Technology, vol. 6, p.790 2007
  2. Nybakken Marine Biology: An Ecological Approach, Fourth Edition, page 174. Addison-Wesley Educational Publishers Inc., 1997.
  3. 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


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



Gallery

Image:Starfish red komodo.jpg|Large red starfish found in Komodo National Parkmarker, Indonesiamarker.Image:Starfish komodo.jpg|Sea star from Komodo National Parkmarker, Indonesiamarker.Image:Crown of thorms starfish.jpg|Crown of thorns starfish from East Timormarker.Image:Sun flower sea star in tide pools.jpg|Sunflower starfish in tide pool in Californiamarker.Image:White Sea StarFish, Russia.jpg|White Seamarker starfish, Russiamarker.Image:Sculpin is eating Leptasterias hexactis.jpg|Tide pool sculpin eating Leptasterias hexactis.File:Blue starfish in Papua New Guinea.jpg|Blue starfish in Papua New Guineamarker.Image:Star Fish at Montana de Oro.JPG|Bat star, Patiria miniata.Image:Ochre sea star on beach, Olympic National Park USA.jpg|Ochre sea star, Pisaster ochraceus, on beach at Olympic National Parkmarker, USA.Image:Orange ochre star.jpg|An ochre star at Bamfieldmarker, CanadamarkerImage:Granulated sea star.jpg|Granulated sea star, Choriaster granulatus, on Meedhupparu house reef in the Maldivesmarker.Image:Guildings's sea star.jpg|Guilding's sea star, Linkia guildingi, on Meedhupparu house reef in the Maldivesmarker.Image:Sandstar 300.jpg|Pisaster giganteus, the giant sea star.Image:starfish.jpg|The tube feet can be seen on this starfish.Image:Cervena_morska_hviezdica.jpg|Caribbean seastar (Oreaster reticulatus), Bahia de la Chiva, Viequesmarker, PR.




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