Brittle stars

Brittle stars (or ophiuroids)

Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	Asterozoa
Class:	Ophiuroidea Gray, 1840

Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to 60 centimetres (24 in) in length on the largest specimens. They are also known as serpent stars.

Anatomy
Of all echinoderms, the Ophiuroidea may have the strongest tendency toward 5-segment radial (pentaradial) symmetry. The body outline is similar to that of starfish, in that ophiuroids have five arms joined to a central body disk. 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, as they do in the Asteroidea. The underside of the disk contains the mouth, which has five toothed jaws formed from skeletal plates. The madreporite is usually located within one of the jaw plates, and not on the upper side of the animal as it is in starfish.

The ophiuroid coelom is strongly reduced, particularly in comparison to other echinoderms.

Reproduction
The sexes are separate in most species, though a few are hermaphroditic or protandric. The gonads are located in the disk, and open into pouches in between the arms, called genital bursae. Fertilisation is external in most species, with the gametes being shed into the surrounding water through the bursal sacs. An exception is the Ophiocanopidae, in which the gonads do not open into bursae and are instead paired in a chain along the basal arm joints.

Many species brood developing larvae in the bursae, effectively giving birth to live young. A few, such as Amphipholus squamata are truly viviparous, with the embryo receiving nourishment from the mother through the wall of the bursa. However, there are some species that do not brood their young, and instead have a free-swimming larval stage. Referred to as an ophiopluteus, these larvae have four pairs of rigid arms lined with cilia. They develop directly into an adult, without the attachment stage found in most starfish larvae. The number of species exhibiting ophiopluteus larvae are fewer than those that directly develop.

In a few species the female carries a dwarf male, clinging to it with the mouth.

Locomotion
Brittle stars use their arms for locomotion. They do not, like sea stars, depend on tube feet, which are mere sensory tentacles without suction. 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. However, they tend to attach themselves to the seafloor or to sponges or cnidarians, such as coral. They move as if they were bilaterally symmetrical, with an arbitrary leg selected as the symmetry axis and the other four used in propulsion. The axial leg may be facing or trailing the direction of motion, and due to the radially-symmetrical nervous system can be changed whenever a change in direction is necessary.

Ecology
Brittle stars live in areas from the low-tide level downwards. Six families live at least 2 meters deep; the genera Ophiura, Amphiophiura, and Ophiacantharange below 4 meters. Shallow species live among sponges, stones, or coral, or under the sand or mud, with only their arms protruding. Two of the best-known shallow species are the green brittle star (Ophioderma brevispina), found from Massachusetts to Brazil, and the common European brittle star (Ophiothrix fragilis). Deep-water species tend to live in or on the sea floor or adhere to coral or urchins. The most widespread species is the long-armed brittle star (Amphipholis squamata), a grayish or bluish species that is strongly luminescent.


Credit: http://en.wikipedia.org/wiki/Brittle_stars