Sunday, December 12, 2010

Brittle Star!



Ophiuroids!!! or also known as Brittle Stars(:
by Sienna Purse

I. Introduction:
Brittle Star, also known as ophiuroids, are echinoderms that are closely related to starfish. Brittle stars crawl across the seafloor using their flexible arms for locomotion. They have 5 slender long arms, whip-like arms that may reach up to 24 inches in length in larger brittle stars. Surprisingly their arms are quite fragile. There are some 1,500 of brittle stars living today but they are usually in deep waters 1,650 feet deep. There are roughly 1,900 extant species in 230 genera, grouped in three orders: Oegophiurida, Phrynophiurida, and Ophiuroids. There is one species that is extinct, the Stenurida. An interesting fact is that brittle stars can readily regenerate lost arms or arm segments unless all arms are lost. They use this method to confuse then escape predators (somewhat how lizards can live without their tails.) They can also regenerate gut and gonad fragments. The ophiuroid coelom is strongly reduced just like other echinoderms.


II. Anatomy:
Like all echinoderms, Ophiuroids are radially symmetrical. One difference is that brittle stars do not have suckers on the undersides of their arms. Another difference is that brittle stars usually have spiny extensions on their arms. The body is similar to that of a starfish because ophiuroids have five arms joined to a central body. But in Ophiuroids, the central body disk is sharply marked off from the arms. The body disk contains all of the viscera. This means that digestion and reproduction do not at all enter the arms like they do in Asteroidea. The underside of the disk has a mouth with five toothed jaws formed from skeletal plates. The nervous system consists of a main nerve ring which rounds around the central disk. At the base of each arm, the ring attaches to a radial nerve which runs to the end of a limb. Ophiuroids have no eyes, or other specialized sense organs. But they do have several types of sensitive nerve endings in their epidermis, and are able to sense chemicals in the water, touch, and even the presence or absence of light.


III: Form & Function:
The mouth is rimmed with five jaws, and serves as an anus as well as a mouth. Behind the jaws is a short esophagus and a large stomach cavity which occupies most of the dorsal half of the disk. Digestion occurs within 10 pouches of the stomach, which are essentially ceca, but unlike sea stars, and never extend into the arms. Ophiuroids are generally scavengers or detritivores. Small organic particles are moved into the mouth by the tube feet. They also prey on small crustaceans or worms. Brittle stars eat suspended matter from prevailing seafloor currents. Gas excretion and excretion occur through cilia-lined sacs called bursae; each opens between the arms on the underside of the disc. Typically there are ten bursae and each fits between two stomach digestive pouches. Water flows through the bursae by means of cilia or muscular contraction. Oxygen is transported throughout the body via the hemal system, a series of sinuses and vessels distinct from the water vascular system.
The sexes are separate in most species however there are some that are hermaphroditic. The gonads are located in the disc, and open into pouches in between the arms, called genital bursae. Fertilization is external in most species, with the gametes being shed into the surrounding water by bursal sacs. There are some species that brood their young in the bursae and others that have free-swimming larvae.

IV: Impact On Humanity:
Other than something pretty to look at brittle stars do not really have a great impact on humanity. They are popular in fishkeeping and survive well in marine tanks. Sometimes if you look closely you might have a hitch-hiking micro brittle star tagging along on one of your live rocks.

V: Journal Review:
http://news.nationalgeographic.com/news/2008/05/080520-brittle-star.html
This article is about how they found a huge "city" of brittle stars on an undersea mountain chain in near Antarctica. Normally seamounts, underground mountains, are home to communities of sponges or corals but somehow the conditions were more favorable to the brittle stars. One of the theories for the brittle stars being there is because there is a fast current which supplies them with a great amount of food. There were two species of brittle stars found, one which is mainly native to New Zealand, and the other found almost worldwide.

Sources:
"Brittle Star." Wikipedia, the Free Encyclopedia Web. 12 Dec. 2010.. http://en.wikipedia.org/wiki/Brittle_star
"Brittle Stars." About Fish Online Web. 12 Dec. 2010.. http://www.aboutfishonline.com/articles/brittle-stars.html
"Brittle Star City Found On Anarctic Seamount" National Geographic News Web. 12 Dec. 2010.. http://news.nationalgeographic.com/news/2008/05/080520-brittle-star_2.html



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