Holothurians, informally known as “sea cucumbers”, are the seemingly immobile “sausages” littering the sandy bottoms of our divesites. There are some 1,250 known species of sea cucumbers, which are closely related to sea stars, brittle stars, and sea urchins. They can be easy to ignore, but holothurians are a magnificent class of animals and when given a closer look will reveal spectacular sights.
A multitude of species of sea cucumbers, with elongated bodies and leathery skins, are found on the sea floor worldwide. Sea cucumbers can live between 5 and 10 years and generally are scavengers, feeding on debris in the benthic zone of the ocean. The diet of most cucumbers consists of plankton and decaying organic matter found in the sea water or in the bottom sand. Marbled sea cucumbers common to Koh Tao position themselves in currents and catch food that flows by with their open tentacles. Others sift through the bottom sediments using their tentacles. Sea cucumbers communicate with each other through sending hormone signals through the water which others pick up. A remarkable feature of these animals is the catch collagen that forms their body wall. This can be loosened and tightened at will, and if the animal wants to squeeze through a small gap, it can essentially liquefy its body and pour into the space. To keep itself safe in these crevices and cracks, the sea cucumber hooks up all its collagen fibres to make its body firm again. Sea cucumbers have hundreds of tiny suction-cup tube feet that they use to crawl across the sea floor.
Sea cucumbers obtain oxygen from water in a pair of respiratory organs 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 symbiotic relationship with sea cucumbers in which the pearl fish will live in sea cucumber’s cloaca using it for protection from predation. Many bristled worms and some crabs have also specialized to use the cloacal space of sea cucumbers for protection by living inside the animal.
Some species of coral-reef sea cucumbers can defend themselves by expelling their sticky cuvierian tubules (enlargements of the respiratory organs that float freely in the colon) 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.
Many holothurian species and genera are targeted for human consumption. The harvested product is also referred to as sea cucumber, or as trepang, bêche-de-mer, balate, or sea slug, though you shouldn’t expect to be able to purchase any of these dishes on Koh Tao.