Class OPHIUROIDEA

<i>Ophiothrix (Acanthophiothrix) purpurea</i> von Martens, 1867

Ophiothrix (Acanthophiothrix) purpurea von Martens, 1867

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Class OPHIUROIDEA

Brittle Stars

Compiler and date details

2012 - Tim O'Hara, Museum Victoria, Carlton, Victoria, Australia

2001 - Tim O'Hara, Museum Victoria, Carlton, Victoria, Australia

1995 - F.W.E. Rowe & J. Gates, Australian Museum, Sydney, New South Wales, Australia

Introduction

The echinoderms of the class Ophiuroidea, commonly called brittle or serpent stars and basket stars, comprise some 18 families, 244 genera and 2000 species worldwide. Fifteen families, 94 genera and 320 species are known to occur in Australian waters.

Ophiuroids are similar in appearance to the asteroids or sea stars but have their arms clearly delimited from the central disc. They are the most mobile of all echinoderms. The central disc ranges from less than 10 to 100 mm diameter while the length of arms ranges widely. Colour varies and some species have mottled or banded patterns. The majority of ophiuroids are epibenthic, on all substrates; some are epizoic, ectocommensal or infaunal in soft substrates. They may be carnivorous or microphagous feeders, the latter strategy including deposit, suspension or filter feeding.

Ophiuroids are represented at all depths and in all seas worldwide. They range from the Lower Ordovician to Recent. The Ophiuroidea is diagnosed as: free-living, radially symmetrical, with a central disc from which extend five long and slender or dichotomously branched arms. The toothed mouth and madreporite of the water vascular system are on the under (ventral) side of the disc-like body; no anus.

The water vascular system comprises a ventral madreporite leading, via the stone canal, to the circum-oral ring canal from which arise five radial water canals, four to five polian vesicles and ten subdivided canals which lead to 20 buccal tube feet. Ambulacral tube feet arise either side of the radial canals, paired, not alternating. The radial canals lie internal to the ventral arm plates.

Coelomic cavities, the gonads and sac-like digestive system are usually restricted to the central disc. An ambulacral groove occurs on the under side of the arms but is closed by ventral plates.

The endoskeletal elements of the disc are usually scales, which may bear variously shaped granules or spines. The arms comprise a series of vertebra-like plates, each surrounded by a lateral (each side) and ventral plate and generally by a dorsal plate as well. The lateral plates usually bear a fan of prominent spines. The ventral surface of the disc is invaginated on each side of the arm bases to form ten bursal sacs used as respiratory chambers, as outlets for sexual products, or as brood chambers.

There are five to many gonads, opening into the bursal sacs. Asexual reproduction includes fissipary and parthenogenesis. Sexual reproductive strategies range through broadcast of gametes to brooding of juveniles to viviparity. Ophiuroidea are usually dioecious with the sexes indistinguishable externally; occasionally they are hermaphroditic; some species are dimorphic, with dwarf males. The feeding larval stage is an ophiopluteus.

Detailed descriptive and illustrative information on the class Ophiuroidea can be obtained from a range of texts, including those listed in the 'References' below.

The grouping of families into orders adopted here follows that of Spencer & Wright (1966).