Tropical brittle star

Ophiactis savignyi

ORDER Ophiurida

FAMILY Ophiactidae

TAXONOMY

Ophiactis savignyi Müller and Troschel, 1842, Egypt, presumably Red Sea.

OTHER COMMON NAMES English: Savigny's brittle star.

PHYSICAL CHARACTERISTICS

A very small brittle star with a disk only 0.039-0.11 in (1-3 mm) in diameter. It usually has six arms, but since it is fissi-parous some individuals that have not completed regeneration may have as few as two arms. Ophiactis has a conspicuous color pattern of green with darker markings and large bare radial shields.

DISTRIBUTION

Found in all tropical seas. Studies of its genetic pattern indicate that it has recently spread to the Atlantic Ocean from the Indo-Pacific, probably in ballast water or in fouling communities coming through the Panama Canal on ships.

HABITAT

Ophiactis savignyi is often associated with algal turfs and sponges but also hides in reef crevices.

BEHAVIOR

Ophiactis savignyi is cryptic and colonizes mainly algae or sponges; it probably receives shelter from predators among these organisms. It grows larger when it lives among sponges in comparison to algal turf, and may benefit from particles gathered by the sponge colonies from the water they take in. Ophiactis can reach high densities, sometimes more than 280 per deciliter of sponge.

FEEDING ECOLOGY AND DIET

The diet and feeding patterns of Ophiactis savignyi have not been studied as of 2003. Ophiactids, however, belong to a group that feeds mainly on small particles from the substrate or suspended in the water.

REPRODUCTIVE BIOLOGY

Ophiactis practices both sexual and asexual reproduction by fission. During fission, the mutable collagenous tissue of the disk softens, forms a groove, and tears irregularly across the disk. The two halves then regenerate into two new six-armed individuals. A single clone can in this way colonize a sponge or patch of alga. Several investigated populations seem to be maintained by asexual reproduction only. Sexual reproduction takes place when the brittle star has reached a size larger than 0.11 in (3 mm). The larva is a typical planktotrophic ophiopluteus.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS None known. ♦

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