Sea Cucumbers

Scientific name: Holothuroidea

Sea Cucumber - Holothuroidea
Sea Cucumber - Holothuroidea. Photo © Vittore / Bigstock.

Sea cucumbers (Holothuroidea) are a group of echinoderms that include about 1250 living species. Sea cucumbers have an internal skeleton or endoskeleton that consists of tiny ossicles, small particles of calcium carbonate embedded in the body wall. Sea cucumbers have an elongated body and thick skin. Their mouth is located at the front of their body and is surrounded by tentacles, the number and structure of which varies among the different subgroups of sea cucumbers.

Sea cucumbers generally grow to lengths of between 4 and 12 inches, although a few species are smaller or larger. Most sea cucumbers are scavengers that feeding on plankton and decaying organic matter that falls to the sea floor. Sea cucumbers have a variety of body shapes ranging from cylindrical to spherical. Unlike brittlestars and starfish, sea cucumbers do not have arms. Sea cucumbers also differ from other echinoderms because because their mouth and anus are located at either end of the animal instead of on the top and bottom surfaces as they are in brittlestars and starfish.

The mouth of a sea cucumber is surrounded by a ring of tentacles that can be pulled inside the mouth. Circular muscles around the body of the sea cucumber can be contracted to enable the sea cucumber to elongate its body and move.

Sea cucumbers have a worldwide range. Most live on the sea floor, but some species burrow into the substrate or hide in crevices or under rocks.

A small number of species are free-swimming. Sea cucumbers sometimes eject their entire digestive tract either through their mouth or anus (depending on species) and this behavior is thought to be a form of defense or to distract predators. The lost organs are usually regenerated.

Sea cucumbers are scavengers that feed on tiny particles of detritus such as algae, tiny aquatic animals, and other waste materials that gather on the sea floor.

They play an important role in the ecosystems to which they belong by helping to break down and recycle nutrients.

Sea cucumbers have a flexible layer of collagen inside their body wall that they can manipulate and loosen in such a way as to enable the sea cucumber to become extremely pliant and thus squeeze their body through small crevices. Alternatively, the sea cucumber can tighten the collagen layer to form a rigid shape.

Some species of sea cucumbers can go into a state of dormancy known as aestivation if water temperatures become too warm. During such dormant periods, the sea cucumber's metabolism slows and the animal stops feeding. When water temperatures return to a more suitable range, the sea cucumber comes out of aestivation and returns to its normal behavior.

Sea cucumbers do not have a true brain. Instead they have neural tissue that encircles their mouth and nerves that stretch into tentacles and other parts of their body. Sea cucumbers extract oxygen from the water using a pair of respiratory trees they extend into the water.


Sea cucumbers are classified within the following taxonomic hierarchy:

Animals > Invertebrates > Echinoderms > Sea Cucumbers

Some of the better known groups of sea cucumbers include:

  • Apodida - There are about 270 species of Apodida alive today. Members of this group lack respiratory trees and have no tube feet. They inhabit both shallow and deep water.
  • Elasipodida - There are about 141 species of Elasipodida alive today. Members of this group have respiratory trees and shield-like tentacles. All Elasipodidas live in deep water.
  • Aspidochirotida - There are about 340 species of Aspidochirotida alive today.
  • Molpadiida - There are about 95 species of Molpadiida alive today. Members of this group ore known as the rat-tailed sea cucumbers. They have respiratory trees and simple tentacles. Most Molpadiida species live in shallow water.
  • Dendrochirotida - There are about 550 species of Dendrochirotida alive today. Members of this group have respiratory trees and extensively branched tentacles. They live attached to bottom substrate or burrow into soft sea floor sediments.
  • Dactylochirotida - There are about 35 species of Dactylochirotida alive today. Members of this group are known as the plated sea cucumbers. They have respiratory trees and simple tentacles.


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Klappenbach, Laura. "Sea Cucumbers." ThoughtCo, Jun. 6, 2015, Klappenbach, Laura. (2015, June 6). Sea Cucumbers. Retrieved from Klappenbach, Laura. "Sea Cucumbers." ThoughtCo. (accessed November 22, 2017).