Class CRINOIDEA

<i>Oxycomanthus bennetti</i> (Müller, 1841)

Oxycomanthus bennetti (Müller, 1841)

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Class CRINOIDEA

Feather Stars, Sea Lilies

Compiler and date details

30 April 2012 - Tim O'Hara, Museum Victoria, Carlton, Victoria, Australia; update

30 June 2001 - Tim O'Hara, Museum Victoria, Carlton, Victoria, Australia; update

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

Introduction

The Crinoidea, commonly called feather stars and sea lilies, are the smallest of the major extant echinoderm classes, with some 24 families, 144 genera and 600 species worldwide. Eighteen families are represented in Australia by a total of 63 genera and 136 species.

The common names applied to the two forms of crinoid aptly describe their general appearance. Sizes range up to 0.5 m across the arms in the feeding posture. Some species, especially among the feather stars, are brilliantly coloured, others are duller browns, reds and purples. They are epibenthic or epizoic on soft or hard substrates and are passive rheophilic suspension feeders; some species may be deposit feeders.

Crinoids occur in all seas: feather stars (comatulids) generally from the lower shore to about 200 m, rarely deeper, while stalked forms may extend from about 70 m to about 6000 m. They range from Mid-Cambrian to Recent (sea lilies), or Lower Jurassic to Recent (feather stars).

Crinoids are the only living echinoderm with the mouth directed upwards and are diagnosed as: radially symmetrical; either free-living (comatulids) or attached to the substrate by a jointed column or stalk which may or may not bear cirri and which has a basal expansion or holdfast (isocrinids, cyrtocrinids and bourgueticrinids). The main body (theca) comprises a dorsal, plated calyx or cup from which five simple or complexly branched arms or brachia arise. A ventral tegmen or membrane overarches the calyx to protect the viscera. On the calyx of free-living forms a dorsal plate, the centrodorsal, may bear cirri for grasping the substrate. In life, the tegmen with the mouth and anus are directed upwards; the mouth is central or eccentric (comatulids), with ambulacral grooves leading from it to the upper (i.e. ventral) surface of the arms; the anus is interradial, the tegmen bilaterally symmetrical. Short, jointed pinnules arise on each side of alternate arm joints; there are three main types: the proximal, protective oral pinnules, the gonad-bearing genital pinnules, and food-collecting distal pinnules. Break points (syzygies) occur at specific points along the arms.

The water vascular system comprises a circumoesophageal ring from which arises a pair of oral tube feet from each interradius, together with numerous ciliated pores connecting the water vascular system with the coelom. Five radial canals arise from the ring and divide along the upper surface of the arms, genital and distal pinnules, to the arm tips. Tube feet, in groups of three, arise from the radial canals. There is no external madreporite, but the tegmen is pierced by numerous ciliated funnels (hydropores) establishing a connection between the water vascular system and the external environment.

The coelom is reduced, divided, and contains the coiled digestive tract.

Crinoidea are dioecious, the sexes are usually indistinguishable (except by colour and appearance of the distended gonads in very ripe crinoids of both sexes or the occurrence of brooding females). Gonads are external, restricted to the upper surface of genital pinnules on the arms. Reproduction is usually by broadcast of gametes; a few species brood juveniles or are considered viviparous, although the mechanism of transfer of sperm to the ovary is not known. In comatulids, non-feeding doliolarian larvae metamorphose through stalked cystidean and penta-crinoid stages in all forms prior to stalk loss (except centrodorsal plate).

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

Major revisions of the family Comasteridae have been carried out by Rowe et al. (1986) and Hoggett & Rowe (1986). A.H. Clark's identification of comasterid taxa was confused. Thus, the taxonomy of the remaining families is considered by the senior author (FWER) to be in a state of some flux, and requiring a detailed reassessment.

The classification adopted for this group follows Rasmussen & Sieverts-Doreck (1978).