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ABOUT SEA CUCUMBERS

Holothuroidea

The Holothuroidea, or sea cucumbers, are an abundant and diverse group of worm-like and usually soft-bodied echinoderms. They are found in nearly every marine environment, but are most diverse on tropical shallow-water coral reefs. They range from the intertidal, where they may be exposed briefly at low tide, to the floor of the deepest oceanic trenches. The oldest undoubted fossils of sea cucumbers are of isolated spicules from the Silurian (ca. 400 million years ago; Gilliland, 1993). Considerable diversification has occurred since then with about 1400 living species in a variety of forms. Some of these are about 20 cm in length, though adults of some diminutive species may not exceed a centimeter, while one large species can reach lengths of 5 m (Synapta maculata). Several species can swim and there are even forms that live their entire lives as plankton, floating with the ocean currents.

Sea cucumbers are members of the class Holothuroidea (phylum Echinodermata), and are related to sea stars, brittle stars, basket stars and sea urchins. This class is divided further into six orders: the Dendrochirotida, Dactylochirotida, Apodida, Molpadida, Elasipodida and Aspidochirotida. Of the 1200 holothurians that have been described, 49 species (belonging to 36 genera) are present in New Zealand waters (Pawson 1970).

Economically, sea cucumbers are important in two main ways. First, some species produce toxins that are of interest to pharmaceutical firms seeking to learn their medical value. Some compounds isolated to date exhibit antimicrobial activity or act as anti-inflammatory agents and anticoagulants. Second, as a gourmet food item in the orient, they form the basis of a multimillion-dollar industry that processes the body wall for sale as beche-de-mer or trepang. However, the high value of some species, the ease with which such shallow-water forms can be collected and their top-heavy age structures all contribute to over-exploitation and collapse of the fisheries in some regions.

The most important feature distinguishing the sea cucumbers is a calcareous ring that encircles the pharynx or throat. This ring serves as an attachment point for muscles operating the oral tentacles and for the anterior ends of other muscles that contract the body longitudinally. Sea cucumbers are also distinct as echinoderms in having a circlet of oral tentacles. These may be simple, digitate (with finger-like projections), pinnate (feather-like), or peltate (flattened and shield-like). A third key feature, found in 90% of living species, is the reduction of the skeleton to microscopic ossicles. In some species, the ossicles may be enlarged and plate-like.

Some sea cucumbers possess organs not found in other invertebrates. In some Aspidochirotida, the respiratory trees display Cuvierian tubules. In most species, these are apparently defensive structures. They can be expelled through the anus, whereupon they dramatically expand in length and become sticky, entangling or deterring would-be predators, such as crabs and gastropods. Many forms, with the exception of members of Elasipodida and Apodida, possess respiratory trees used in gas exchange. These are paired, heavily branched tubes attached to the intestine near the anus. This type of breathing ("cloacal breathing") is also present in an unrelated group, the echiuran worms.

References

Overview: Aspects of Sea Cucumber Industry Research and Development In The South Pacific., Andrew Morgan & Jeff Archer, Secretariat of The Pacific Community - SPC

Holothuroidea, Alexander M. Kerr, Marine Laboratory, University of Guam Mangilao, Guam