Found in almost all marine environments, Sea Cucumbers are members of the following families: Cucumariidae, Phyllophoridae, Holothuriidae, Stichpodidae, or Synaptidae. Like their cousins the Starfish and the Urchins, they have a pentagonal symmetry, with five rows of tube feet for locomotion. They have internal plates made up of calcium and spicules, which are often the best way to identify a specimen. Most of the motile cucumbers will have a dorsal (top) and ventral (bottom) side, with a worm-like or cucumber body shape. The selection for aquariums is limited by the fact that many Sea Cucumbers grow up to 6 feet long. Aquarium specimens will usually range between 4 to 12 inches.
The motile species generally have lobed tentacles to sift through the substrate for food. Sea Apples are sessile species which are usually more rounded, with the mouth pointed up, and generally will be 3 to 6 inches in diameter. They will extend large, highly branched tentacles to trap floating plankton. While motile cucumbers will usually be on the bottom of the tank, sea apples will usually climb near the top and point their tentacles at the light or in a strong current. It is not unusual to open the lid and find them partially out of water.
Sea Cucumbers are best kept in reef or invertebrate aquariums. Water quality must be maintained at the highest levels, and care must be taken with any water intakes/overflows to ensure that the Sea Cucumber cannot become attached. Sea Cucumbers are popular additions to reef aquariums that incorporate live sand.
Many motile species have specialized tubules that can be expelled from their undersides as a form of defense, either by trapping the predator or in some cases being toxic to the predator. The last line of defense is to expel all of its internal organs, escape, and then regenerate the organs. Unfortunately, if either of these events occurs in the aquarium, it is toxic enough to kill everything in the tank. However, if Sea Cucumbers are maintained with fish that do not harass invertebrates, this rarely occurs. Before acquiring a Sea Cucumber, determine how toxic that particular species may be.