Sea cucumbers are unusual marine invertebrates that comprise the class Holothuroidea. They often have leathery skin and a weak calcified endoskeleton just underneath. In some species, the endoskeletal plates are enlarged, forming armor for the creature. Sea cucumbers are bottom-feeding recyclers, breaking down dead organic matter and other sea floor detritus for further use by the ecosystem. At the deep sea floor, sea cucumber species can make up an overwhelming majority of the ecosystem’s biomass. These sedentary creatures are known for releasing tubules and toxic substances when threatened, but are nonetheless predated upon by a wide number of other creatures.
Sea cucumbers have a vaguely oblong body shape that looks much like the vegetable after which they are named. They have no arms or appendages other than a small ring of tentacles surrounding the mouth. These tentacles help the cucumber collect seafloor detritus that it eats. The body of the sea cucumber is highly flexible, and it can even control the elastic collagen of the body wall, allowing it to nearly liquefy its own body in order to move in between a tight space or passage.
Sea cucumbers are benthic feeders that scavenge for nutrients on the sea floor. They roll through various sediments and find whatever organic materials they can to eat. Plankton forms a large part of the sea cucumber diet, as well as scavenged material falling from higher oceanic depths. It is not uncommon for certain species to position themselves with the undersea currents to catch passing nutrients with an open mouth as the water brings them. Other species will bury themselves under the sea floor and expose only their tentacles as they drift and search for food.
Sea cucumbers often reproduce by releasing sperm and ova into the surrounding waters. A single organism will often produce thousands of individual gametes. With luck, some of these will find their way to an individual of the opposite sex, who will then fertilize the egg, which then turns into a free-swimming larva. At this stage, the larva is highly vulnerable to predators but those that survive will grow into fully adult sea cucumbers. A very small number of sea cucumbers will brood their young inside of their own body cavity in a kind of invertebrate pregnancy period, but this is a rare case confined only to a few exceptional species.