Holothuria forskali
A black sea cucumber that ejects sticky white threads to deter predators.
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.
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.
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.
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.
Quick Facts
- Taxon
- Holothuria forskali
Facts from the source article.
Lore & Background
This sea cucumber has a cylindrical body, usually deep brown or black, sometimes with yellowish mottling on the underside. Its skin is soft yet coarse and tough, covered with fleshy papillae often tipped with white, believed to be sensory organs. The underside has three rows of tube feet for walking and climbing, while the upper side has two rows of rudimentary suckers. At the anterior end, a bunch of twenty yellowish short, retractable tentacles encircles the mouth. Inside the posterior body cavity, a bundle of Cuvierian tubules (cotton glands) can be ejected as a tangle of sticky white threads to confuse or enmesh predators.
The black sea cucumber occurs around the Atlantic coasts of northwest Europe, the Canary Islands, the Azores, and in the Mediterranean Sea, on boulders and rocks, particularly vertical surfaces, from the intertidal zone down to about fifty metres. It is a detritivore that feeds mostly at night, bending its body down, pressing oral papillae against the substrate, and vacuuming up sediment. When not feeding, the mouth is closed and tentacles retracted, making it difficult to tell which end is the head.
Adults are normally either male or female. Gonads take a long time to mature, and gametes are released synchronously into the water column in early spring, probably due to rising water temperatures. Larvae become part of the zooplankton, then after several moults grow tentacles and settle. Juveniles are seldom seen, so it is surmised they live by day in crevices and under rocks for protection, while adults take no particular care to remain hidden.
Reader's Guide
The black sea cucumber is significant for its defensive and chemical adaptations. It possesses Cuvierian tubules that can be ejected as sticky white threads to entangle predators, a trait that gives it the common name 'cotton-spinner'. Additionally, twenty-six saponins (triterpene glycosides) are present in the Cuvierian tubules and twelve in the body wall. During stressful times, such as when a predator is near, these saponins are released into the surrounding water. Though insufficient to harm a potential predator, they are thought to serve as a warning that the cucumber is unpalatable. The species also hosts the parasitic copepod Asterocheres boecki as an endoparasite. Its placement as the type taxon of the subgenus Panningothuria by Rowe in 1969 underscores its taxonomic importance within the Holothuriidae family. The species' nocturnal feeding behavior and the synchronous spawning in early spring, likely triggered by rising water temperatures, highlight its ecological role as a detritivore in shallow eastern Atlantic and Mediterranean waters.
Did You Know?
- The black sea cucumber can grow to thirty centimetres long.
- It ejects a tangle of sticky white threads from Cuvierian tubules to confuse or enmesh predators.
- Twenty-six saponins are present in its Cuvierian tubules and twelve in its body wall.
- The parasitic copepod Asterocheres boecki is an endoparasite of this species.
More in Sea Cucumbers and Brittle Stars, Part 2 1-24
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