Holothuria forskali

Holothuria forskali, called the black sea cucumber or cotton-spinner, belongs to the family Holothuriidae. It lives in shallow waters of the eastern Atlantic Ocean and the Mediterranean Sea. In 1969, Rowe assigned it to the subgenus Panningothuria, and it is the type species for that subgenus.

Sea cucumbers are marine invertebrates related to sea urchins and starfish. These groups share radial symmetry and a water vascular system that uses hydrostatic pressure, allowing movement via many tube feet. Sea cucumbers typically have leathery, gherkin-like bodies and a cluster of short tentacles at one end.

Description

The black sea cucumber has a cylindrical body reaching up to thirty centimeters long. Its color is usually deep brown or black, sometimes with yellowish mottling underneath. The skin is soft but coarse and tough, covered with fleshy papillae often tipped with white. These papillae are thought to sense touch and possibly chemicals in the water.

The underside has three rows of tube feet for walking and climbing; the upper side has two rows of rudimentary suckers. At the front, twenty yellowish, short, retractable tentacles surround the mouth. Inside the rear body cavity lies a bundle of Cuvierian tubules (cotton glands) that can be ejected as sticky white threads to entangle or confuse predators.

Distribution

This species occurs along the Atlantic coasts of northwest Europe, the Canary Islands, the Azores, and in the Mediterranean Sea. It lives on boulders and rocks, especially vertical surfaces, from the intertidal zone down to about fifty meters deep.

Biology

The black sea cucumber is a detritivore that feeds mainly at night. To feed, it bends its body toward the substrate, presses its oral papillae against the surface, and opens its mouth wide. It repeatedly extends and retracts its short tentacles, vacuuming up sediment, digesting the nutritious parts, and excreting the rest as sausage-like strings. When not feeding, the mouth closes and tentacles retract, making the head end hard to identify.

Adults are usually either male or female. Their gonads take a long time to mature, and they release gametes synchronously into the water in early spring, likely triggered by rising water temperatures. Larvae become part of the zooplankton. After several molts, they grow tentacles and settle on the seafloor.