Some species of coral-reef sea cucumbers within the order Aspidochirotida can defend themselves by expelling their sticky cuvierian tubules (enlargements of the respiratory tree that float freely in the coelom) to entangle potential predators. When startled, these cucumbers may expel some of them through a tear in the wall of the cloaca in an autotomic process known as evisceration. Replacement tubules grow back in one-and-a-half to five weeks, depending on the species. The release of these tubules can also be accompanied by the discharge of a toxic chemical known as holothurin, which has similar properties to soap. This chemical can kill any animal in the vicinity and is one more way in which these sedentary animals can defend themselves.
They can be found in great numbers on the deep sea floor, where they often make up the majority of the animal biomass. At depths deeper than 5.5 mi (8.8 km), sea cucumbers comprise 90% of the total mass of the macrofauna. Sea Cucumbers form large herds that move across the bathygraphic features of the ocean, hunting food. The body of some deep water holothurians is made of a tough gelatinous tissue with unique properties that makes the animals able to control their own buoyancy, making it possible for them to either live on the ocean floor or to float over it to move to new locations with a minimum of energy, for instance Enypniastes eximia, Peniagone leander and Paelopatides confundens.
In more shallow waters, sea cucumbers can form dense populations. The strawberry sea cucumber (Squamocnus brevidentis) of New Zealand lives on rocky walls around the southern coast of the South Island where populations sometimes reach densities of 1,000 animals per square metre. For this reason, one such area in Fiordland is simply called the strawberry fields.
Sea cucumbers extract oxygen from water in a pair of 'respiratory trees' that branch off the cloaca just inside the anus, so that they 'breathe' by drawing water in through the anus and then expelling it. A variety of fish, most commonly pearl fish, have evolved a commensalistic symbiotic relationship with sea cucumbers in which the pearl fish will live in sea cucumber's cloaca using it for protection from predation, a source of food (the nutrients passing in and out of the anus from the water), and to develop into their adult stage of life.
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