AbstractThe class Holothuroidea includes some 1200 echinoderms divided into six orders: Apodida, Aspidochirotida, Dactlyochirotida, Dendrochirotida, Elasipodida and Molpadiida.
With the exception of the apodids, holothurians typically have 5 ambulacral rows of tube feet. Modification and/or reduction of these tube feet has produced a differentiation between the orsal and ventral surfaces, resulting in secondary bilateral symmetry in some species, e.g. Pawsonia saxicola (Brady and Robertson) (McKenzie, 1991). The ventral surface is composed of 3 ambulacral rows and is termed the bivium, the dorsal surface has 2 ambulacral rows and is termed the bivium. The mouth is positioned at the anterior end of the body and is encircled by branching tentacles which are modified buccal podia and represent an extension of the water vascular system; these tentacles are characteristic for different classes of holothuroidea and reflect the mode of feeding.
Holothurian tentacular structure is used to separate the group at the subclass or order level (Pawson, 1966). Dendritic, tree-like tentacles are used by suspension-feeding holothurians to filter particulate matter from the water; they are found in dendrochirotids as described by McKenzie (1987). Pinnate tentacles are very fine and feather-like and occur in synaptids (Class: Apodida). Digitate tentacles possess finger-like processes in various forms, these are used to shovel sediment into the mouth and are found in species of Aspidochirotida, Apodida, Molpadiida and Elasipodida. Peltate tentacles bear end-discs which vary in their extent of development; they occur in holothurians which feed by scraping the surface layers of the sediment and are found in aspidochirotids and elasipodids (Massin, 1982a).
The digestive tract of holothurians is long and looped, with the exception of some synaptids and elasipodids (Feral and Massin, 1982). Their relatively long gut compared to other echinoderms (Choe, 1962 cited in Fish, 1967b) could be a reflection of their low quality diet (Lawrence, 1982) and an adaptation for continuous feeding (Ferguson, 1969). The gut shows great regional differentiation (Feral and Massin, 1982) and is functionally heterogeneous throughout its length (Filimonova and Tokin, 1980).
Holothurians are conspicuous components of many marine communities, being widespread in all seas and at all depths (Pawson, 1966). They occur at both tropical and temperate latitudes, they are numerous in shallow-water environments and are also dominant in temperate abyssal regions; they are one of the few groups that penetrate the deepest recesses of the ocean (Billett, 1991). Aspidochirotids, being dependent upon particles on the substratum, are most prominent in the tropics (Pawson, 1970; Bakus, 1973) and at bathyal depths (Hansen, 1975). The occurrence of suspension feeding dendrochirotids is related to the availability of suspended material (Lawrence, 1982); they are most prominent in temperate and sub-tropical regions but not in clear tropical waters (Pawson, 1970; Bakus, 1973) or in the deep sea (Hansen, 1975). Elasipodids and most molpadiids are exclusively found in deep-sea environments (Billett, 1991). Dactylochirotids occur in shallow waters, although one of the three families, the Ypsilothuriidae, is found in the deep sea (Billett, 1991). Apodids are also essentially shallow water holothurians but have representatives in the deep sea (Billett, 1991).
|Date of Award||Jul 1996|
|Supervisor||Dai Roberts (Supervisor)|