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Sea cucumbers are members of a group of marine animals known as echinoderms (“spiny skins”). The 7,000 species of echinoderms that live in today’s oceans also include the starfish, sea urchins, brittle stars, and sea lilies. The 1,500 species of sea cucumbers, or holothurians, can be found in all oceans and at all depths, in a great variety of habitats – some burrow deep into mud or sand, while others may spend their entire lives swimming in midwater. It is in the dark reaches of the deep sea where the sea cucumbers rule. Here, a group known as the elasipods, of which Scotoplanes globosa is an example, can be found in enormous numbers (Dave Pawson, in litt. December 2009).
The bizarre deep-sea sea cucumbers were first described in wonderful detail by Swedish zoologist Hjalmar Théel in 1882, when he wrote a monograph of the astonishing collections amassed by the British research ship HMS Challenger in her round-the-world cruise of 1872-1876. Théel described about 65 new species which he placed in a new Order, the Elasipoda. The so-called elasipods are restricted to deep and cold parts of the world ocean, where they are the dominant large animals in most areas, often comprising more than 95% of the total weight of animals on the deep-sea floor. They are of great importance in the general economy of the deep sea, for as they feed on sediments, and move along on the seafloor, they introduce oxygen into the sediments, thus making them habitable by myriad small animals (Dave Pawson, in litt. December 2009).
Elasipods can vary greatly in shape. Most are more or less cylindrical, but some are quite flat, resembling flatfish in general appearance. They are often very fragile, for their bodies incorporate a large amount of water. As a result, many elasipods may break into gelatinous fragments when they are collected in nets and dredges. Several of these neutrally buoyant elasipods are capable of actively swimming for short to long periods of time. Unique features can include the presence of several very large “walking legs”, which are greatly modified fluid-filled tube feet (tube feet are hydraulically operated locomotory organs possessed by all echinoderms). The upper body surface may carry few to many whip-like so-called papillae, which are also modified tube feet. It is believed that these papillae have a sensory function, helping the animal to “smell” its way to tasty sediments (Dave Pawson, in litt. December 2009).
Additional information and images can be found here: http://blogspotarchive.blogspot.com/2009/07/echinoblog.html.