Class ASTEROIDEA

<i>Astropecten monacanthus</i> Sladen, 1883

Astropecten monacanthus Sladen, 1883

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Class ASTEROIDEA

Starfish

Compiler and date details

July 2012 - Tim O’Hara, Museum Victoria

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

2001 - Tim O'Hara, Museum Victoria, Carlton, Victoria, Australia (update)

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

Introduction

Members of the second largest of the echinoderm classes, the Asteroidea, are commonly known as sea stars or starfish. Worldwide, they are represented by about 32 families, 303 genera and 1800 species. Some 25 families, 111 genera and 279 species occur in Australian waters.

Asteroids are similar to ophiuroids in having a central disc and a basic five-star form but the arms are not clearly demarcated from the central disc and lack vertebral ossicles; some have very short arms. Often asteroids are brightly coloured in red, orange, blue, purple or green or combinations of these colours. They are epibenthic on all substrates but some species are infaunal in soft substrates. Their variety of feeding strategies includes ingestion of substrate, browsing on algae and active predation.

Asteroids are represented in all depths and in all seas worldwide. They range from Lower Ordovician to Recent.

The Asteroidea are diagnosed as: free-living, radially symmetrical, with a central disc from which radiate five (rarely fewer) or more stout to slender arms. Some species may be pentagonal to circular in outline, without projecting arms. The body is usually aborally arched from a flat oral surface; rarely more flattened to wafer-like. The mouth and ambulacral grooves are on the under (oral) side of the body, the anus (rarely absent) and madreporite on the upper (aboral) side.

The water vascular system comprises the aboral interradial madreporite leading via the stone canal to the circum-oral ring canal. Five pairs of Tiedemann's bodies and, in some asteroids, Polian vesicles arise interradially from the ring canal. Five radial canals extend from the circum-oral ring along the five ambulacra to the arm tips with the ampulla/tube foot system arising from alternate lateral branches on either side of each of the radial canals. The ampullae are internal to the ambulacral plates.

The coelomic cavities are capacious; the gonads and branches of sac-like digestive system extend into the arms.

Endoskeletal ossicles are variously arranged aborally, as an open reticulum or as imbricating or abutting plates which support variously shaped granules or spines and, in some species, pedicellariae. Orally, the ambulacral groove comprises two radial and adjacent series of ambulacral plates forming an arched furrow and between which the tube feet protrude. Superficially and external to each ambulacral row lies a row of adambulacral plates bearing spines capable of closing over and protecting the furrow. The margin of the body is demarcated by two rows of (often prominent) plates, the upper superomarginal row and lower inferomarginal row. Internally, in an interradial position and associated with each of five pairs of oral plates, is a T-or Y-shaped ossicle (odontophore).

Usually ten gonads are present, two to each arm, each gonad with its own gonopore opening interradially and aborally; in some species the gonopores open orally in the interradial area. Some species have more numerous gonads arranged serially along the arms, each with its own gonopore opening laterally. Species are usually dioecious, rarely hermaphroditic, and sexes are usually indistinguishable externally except where gonopore size is determinable (e.g. Caymanostellidae). Reproduction includes asexual autotomy, fission or parthogenesis and a range of sexual strategies including broadcast of gametes, brooding of juveniles, and viviparity, involving the concomitant repression of larval stages. Feeding larval forms include bipinnaria and (sometimes) brachiolaria stages.

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

The arrangement of families within orders is indicated in the following family introductions and is taken mainly from Blake (1987). Family diagnoses are adapted from Spencer & Wright (1966).