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Brittle Stars – Why Are They Brittle?

Ocean life contains many hidden wonders in the form of dynamic plants and amazing animals. One cannot easily imagine what kind of animals dwell under the water of the ocean. Animal sizes vary from microscopic to large whales. One such sea creature that looks quite strange is the brittle star. The brittle star belongs to the kingdom Animalia and the phylum Echinodermata. The order brittle stars belong to is Ophiuroidea, so they are also called ophiuorids. They are closely related to sea stars. They have very flexible long arms with the help of which they crawl over the substrate of the seabed. They have very long arms that can reach 60 cm in length and are five in number. They are also known as snake stars.

They are not easily recognized by humans and remain unnoticed, but they share a different group. About 1,500 species of brittle stars are known today and are usually found at a depth of 500 meters down. Ophiuroids are believed to have evolved in the early Ordovician about 500 million years ago. They have adapted to a wide range of marine habitats from the tropics to the poles. They are also found attached to coral reefs and some species are also known to tolerate brackish water, their exoskeleton consists of ossicles. There are approximately 1900 extant species and 230 genera of brittle stars divided into three orders comprising the Oegophiurida, Phrynophiurida and Ophiurida. Stenourida is an order of the Paleozoic era and has become extinct.

Like all echinoderms, ophiuroids have a strong tendency toward pentagonal symmetry. The outline of the body of brittle stars is similar to that of starfish, with all five arms attached to the central disk with a slight difference that all arms are not completely connected to the central disk. This central disk contains all the organs of the body, and the various organs do not enter the arms as found in starfish. The underside of the central disk encloses the mouth surrounded by five jaws composed of skeletal plates. Madreporite is present between any of the jaw plates and is present on the lower side, but in starfish it is present on the upper side.

The water vascular system is very well marked in brittle stars as it is the characteristic feature of echinoderms. The vascular system of water provides force for movement. Tube feet are the terminations of the water vascular system. Usually a mandreporite is present in the aquatic vascular system, but species also lack a mandreporite. Corynes and ampullae are absent at the tube feet of brittle stars. The nervous system is very simple and consists of a main nerve ring that sends fine nerves down the length of the arms. The exact location of the nerve ring is in the central disc. Sensory organs are not well developed. They are also missing eyes. The nerve endings of the nerve ring perform the work of touch, taste, etc.

The mouth of the fragile star is surrounded by five jaws that carry the function of the anus as well as the mouth that serves both for the intake and for swallowing the food. Just behind the jaws is the esophagus and a blind stomach. Digestion proceeds in the stomach pouches and the stomach wall contains liver cells. Ophiuroids are considered primarily as scavengers or parasitivores, and small food particles are carried to the mouth with the help of tube feet. Sometimes they also feed on crustaceans and small worms. The bases of the arms contain cilia-lined sacs called stipules that are used for gas exchange. The cilia present in the follicles are the primary source for moving oxygen-rich water currents that circulate through the water vasculature. Follicles are also excretory organs as they contain the phagocytic cells involved in removing waste from the body. The exoskeleton of snakes is composed of calcium carbonate like all echinoderms which exists in the form of calcite. The calcite ossicles fuse to form plates which are later covered by the epidermis and take the form of syncytium. The sexes are generally separate, but a few species are also hermaphrodite. The gametes are stored in the follicles located on the arms and are released through the follicles. Fertilization is external and the gametes are shed into the surrounding water. A few species are also viviparous and show parental care by developing their immature larvae in the pouches. A few females of some species carry a male dwarf permanently attached to their mouth.

Brittle stars mature in two years and are fully grown in about three to four. Their average lifespan is only five years, but a few species may live longer than that. They have great powers of regeneration also called autotomy. Autotomy is the defense process in which snakes cut off one of their arms when attacked by a predator and run away. This lost arm is later created through the regeneration process. They move with the help of their arms that do not have webs. They are also attacked by parasites which include protozoa, nematodes, algae etc. Although they are non-toxic they are not used as food by humans. They are used by fish keepers as part of the aquarium.

Brittle stars are very fantastic as they are generally not noticed by predators.

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