Sea cucumbers (class Holothuroidea) are part of the phylum Echinodermata, a group of animals that also includes starfish, sea urchins and sand dollars. There are over 1,000 known species of sea cucumbers. They resemble cucumbers or sausages in shape and are often covered with warty bumps or spines. These animals use many small tube feet to crawl across the deep-sea floor, and they are typically soft bodied and extremely flexible. Sea cucumbers can defend themselves from predators by ejecting their internal organs from their bodies, and they have the ability to quickly regenerate lost organs. Different species of sea cucumbers come in many beautiful colors and patterns.
Depending on the species, sea cucumbers have between eight and 30 tentacles, which are actually modified tube feet, surrounding the mouth. They feed on plankton and small particles of decaying organic matter, using their tentacles to obtain food and bring it to the mouth. They can sit in an ocean current, holding out their tentacles to filter food from the water. They also use their tentacles to dig through the sea floor and filter out particles of organic material from the sand.
Massive herds of sea cucumbers can be found grazing on the bottom of the deep sea, and they represent the vast majority of the living biomass in some deep sea habitats. Sea cucumbers occupy an ecological niche in the ocean that is similar to the role earthworms play in terrestrial ecosystems.