Black Sea Cucumber, Holothuria forskali

Image Caption: Black Sea Cucumber, Holothuria forskali. Credit: Rpillon/Wikipedia (CC BY-SA 3.0)

The black sea cucumber (Holothuria forskali), also known as the cotton-spinner, is a species that can be found in the Mediterranean Sea and eastern areas of the Atlantic Ocean. Its range includes the waters around the Azores and the Canary Islands. It prefers to reside in shallow waters at depths of up to 164 feet and can be found on rocky, vertical surfaces. In 1969, Rowe classified it within the Panningothuria subgenus, where it is known as the typetaxon, or type species, for that subgenus.

The black sea cucumber, and all sea cucumber, received its common name from its shape, which resembles a cucumber.  It is typically black or rich brown in color, with yellow occurring most often on the underbelly. Its skin is soft yet tough and is covered with flesh-like, white tipped papillae. These are thought to serve as sensory organs that can detect touch and possible chemicals in the surrounding water. The front side of this species holds twenty retractable tentacles around the mouth that are yellow in color. It uses hydrostatic pressure to move around by using its tube feet to create suction. The back end holds two cotton glands, or Cuvierian tubules, near the anus. These structures are used to ward off predators by releasing a white, sticky thread like substance.

The black sea cucumber cannot be identified as male or female until the gonads mature, which typically takes a long time. After the gonads mature, gametes are released into a water column, typically in later spring. It is thought this timing occurs due to an increase in water temperature. The gametes, or larvae, intermingle with the zooplankton in the water column and later grow into cucumbers and settle on the sea floor. Young members of this species are not often seen, most likely choosing to reside in crevices during the day to avoid predators.

The black sea cucumber is most active at night. It is known as a detritivore, a species that consumes the decaying or fresh excrement of other species. It uses the papillae within its mouth to create a vacuum by completely surrounding the sediment with its mouth and sucking in the nutrients and other materials using rings of papillae to gather the food. After processing the food, the remains are released as a sausage like dropping. It is difficult to distinguish the back end and front end of this species when it is not feeding. This species is known to harbor Asterocheres boecki, a parasitic copepod, within its body.