Brittle star
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 "Ophiodea" from Ernst Haeckel's Kunstformen der Natur, 1904
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Brittle stars are echinoderms, closely related to sea stars. They crawl across the sea-floor using their flexible arms as "legs" for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to 60 centimeters (2 feet) in length on the largest specimens.
Ophiuroidea contains two large clades, Ophiurida (brittle stars) and Euryalida (basket stars). Many of the ophiuroids are rarely encountered in the relatively shallow depths normally visited by humans, but they are a diverse group.
There are some 1,500 species of brittle stars living today, and they are largely found in deep waters more than 500 metres (1,650 feet) down.
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[edit] Range
Ophiuroids can be found in all of the major marine provinces, from the poles to the tropics. In fact, crinoids, holothurians, and ophiuroids rule the floor of the deep oceans at depths below 500 m. Basket stars usually confined to the deeper parts of this range. Ophiuroids are known even from abyssal (>6000 m) depths. However brittle stars are also common, if cryptic, members of reef communities, where they hide under rocks and even within other living organisms. A few ophiuroid species can even tolerate brackish water, an ability otherwise almost unknown among echinoderms. A rare specimen of brittle starfish was found off the coast of Spain.
The ophiuroids diverged in the Early Ordovician, roughly 500 million years ago.
[edit] Disc and internal organs
Like all echinoderms, the Ophiuroidea possess a skeleton of calcium carbonate in the form of calcite. In ophiuroids, the calcite ossicles are fused to form armor plates which are known collectively as the test.
Of all echinoderms, the Ophiuroidea may have the strongest tendency toward 5-segment radial (pentaradial) symmetry. The body outline is similar to the Asteroidea, in that ophiuroids have five arms joined to a central disk called a calyx. However, in ophiuroids the central body disk is sharply marked off from the arms. The disk contains all of the viscera. That is, the internal organs of digestion and reproduction never enter the arms, unlike in the Asteroidea.
The mouth is rimmed with five jaws. Behind the jaws is a short esophagus and a large, blind stomach cavity which occupies much of the dorsal half of the disk. Ophiuroids have neither an intestine nor an anus. Digestion occurs within 10 pouches or infolds of the stomach. Gas exchange and excretion occur through cilia-lined sacs called bursae; each opens onto the interambulacral area (between the arm bases) of the oral (ventral) surface of the disc. Typically there are 10 bursae, and each fits between two stomach digestive pouches.
The sexes are separate in most species. Gonads in the disc open into the bursae. Gametes are then shed by way of the bursal sacs. Many species actually brood developing larvae in the bursae. The ophiuroid coelom is strongly reduced, particularly in comparison to other echinoderms. The nervous system consists of a main nerve ring which runs around the central disk. At the base of each arm, the ring attaches to a radial nerve which runs to the end of the limb. Ophiuroids have no eyes, as such. However, they have some ability to sense light through receptors in the epidermis.
Both the Ophiurida and Euryalida have five long, slender, flexible whip-like arms, up to 60 centimeters in length. They are supported by an internal skeleton of calcium carbonate plates that referred to as vertebral ossicles. These "vertebrae" articulate through ball-in-socket joints, and are controlled by muscles. The body and arms are also bear calcite plates and delicate spines. Euryalids are similar, if larger, but their arms are forked and branched. Ophiuroid podia generally function as sensory organs. They are not usually used for feeding, as in Asteroidea.
The vessels of the water vascular system end in tube feet. The water vascular system generally has one madreporite. However, some forms have none. Suckers and ampullae are absent from the tube feet.
Ophiuroids can readily regenerate lost arms or arm segments unless all arms are lost. Ophiuroids use this ability to escape predators, like lizards who automize, or deliberately shed, part of their tails to confuse pursuers.
[edit] Locomotion
Brittle stars use their arms for locomotion. They do not, like sea stars, depend on tube feet. Brittle stars move fairly rapidly by wriggling their arms which are highly flexible and enable the animals to make either snake-like or rowing movements. Their movement has some similarities with animals with bilateral symmetry.
[edit] Trophic levels
Many ophiuroids are scavengers or detritivores. Small organic particles are moved into the mouth by the tube feet. Ophiuroids may also prey on small crustaceans or worms. Basket stars in particular may be capable of suspension feeding, using the mucus coating on their arms to trap plankton and bacteria.
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