Class HOLOTHUROIDEA

Bohadschia argus (Jaeger, 1833)

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Class HOLOTHUROIDEA

Sea Cucumbers, Trepang

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 class Holothuroidea includes the echinoderms commonly known as sea cucumbers, trepang or bêche de mer, and comprises about 25 families, 200 genera and 1400 species worldwide. Nineteen families, 87 genera and 274 species are known in Australia.

Holothuroids are more or less cylindrical, generally black, brown or olive green, though some may be brightly coloured; adults range in length to more than a metre. They are suspension or deposit feeders, and are epibenthic on all substrates, or infaunal in soft substrates. Their movement is slow; some species are capable of swimming movements and a few are pelagic in oceanic waters. Extant forms are found at all depths and in all seas. Fossil taxa are known from Late Silurian to Recent.

Holothuroids are diagnosed as: free-living, cucumber-shaped or vermiform echinoderms with mouth at anterior end and anus at posterior end of body; and with radial symmetry masked by secondary bilateral symmetry. Ambulacra, delimited by five longitudinal bands of tube feet, may be restricted, more or less, to three bands on the under (ventral) side; the upper (dorsal) two bands are often modified as sensory papillae. Alternatively, the tube feet may be scattered over the body, or if the tube feet are absent, the five ambulacra are marked by internal longitudinal muscle bands.

A ring of modified tube feet, the tentacles, surround the mouth and are used for feeding. The pharynx is supported by a characteristic set of (usually) 10 calcareous plates, the calcareous ring.

The water vascular system comprises a circum-oral ring canal from which arises, interradially, one (or more) dorsal stone canals leading to one (or more) madreporic bodies lying in the coelom (in a few species the madreporite opens externally in a mid-dorsal position, close to the gonopore, at the base of the tentacles), and one or more polian vesicles. Five radial canals arise from the circum-oral ring, extend anteriorly and then run posteriorly along the ambulacra, giving rise to the tube foot/ampulla system. The tentacles (with their ampullae) arise directly from the circum-oral ring canal. Ampullae of the tube feet lie in the coelom and the tube feet themselves extend through the body wall. In the Apodida only the circum-oral ring and tentacles occur; radial canals and tube feet are absent.

The coelom is large, containing a long digestive tract in an S-shaped loop; a single, dorsally positioned gonad with gonoduct leading to a gonopore opening mid-dorsally close to the base of the tentacle; a branched respiratory organ (present only in Aspidochirotida, Dendrochirotida and Molpadida) and cuvierian (defence) organs (present in some Aspidochirotida) which both arise from the cloaca.

The endoskeleton comprises microscopic ossicles embedded in the dermis. These exhibit a wide range of shapes.

Holothuroidea are dioecious, with the sexes usually indistinguishable externally, except for brooding females; a few species are hermaphroditic. Reproduction is usually by broadcast of gametes but a number of species brood young either externally (e.g. amongst tentacles), in external brood pouches or internally within the gonad or coelom. Asexual reproduction by fission is known in some species. Feeding larval stages are auricularia.

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