Reproductive biology

Most ophiuroids have separate sexes. When the gametes are mature they are discharged into the bursae and expelled into the water through the bursal slits. The eggs are usually small with a low yolk content and a diameter of 0.0039-0.07 in (0.1-0.18 mm). Following fertilization, the eggs develop into a ciliated blastula which then matures further into the characteristic ophiopluteus larvae with arms, a mouth, and an anus. The ophiopluteus larva is planktonic; the larval stage

A juvenile spiny brittle star (Ophiocoma paucigranulata) on a wide mesh sea fan (Gorgonia mariae). (Photo by ©Mary Beth Angelo/Photo Researchers, Inc. Reproduced by permission.)

lasts two to five weeks, during which the larvae feed on such microscopic particles as diatoms, dinoflagellates and other small flagellates. Subsequently the larval body goes through a remarkable metamorphosis into a young brittle star. The body flattens, the larval gut closes, and the anus disappears.

Other species develop via a vittelaria larva, which originates from eggs with a high yolk content. This larva does not feed; it develops into a young brittle star within approximately 12 days. There are also species that undergo direct development; the larval stages are suppressed and a juvenile brittle star emerges directly from the gastrula stage.

There are approximately 60 hermaphroditic species of ophiuroids that practice brood care. The eggs are retained and developed in the maternal body; in most cases, the bur-sae serve as brood chambers in which the young brittle stars also may be fed by the mother, as in Amphipholis squamata.

Lastly, there are about 45 species of brittle stars with a mixed life history that includes both sexual reproduction and clonal proliferation by fission. These brittle stars have six arms, and spontaneously divide into two halves. Each half then subsequently regenerates three new arms to form two new complete individuals. Although asexual species are uncommon in this class, they can be very successful, range widely, and occur in high densities in certain habitats. For most of these species, the asexual mode is associated with small body size, while sexual maturity is usually attained after an individual reaches a certain size.

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