Holothuria poli

The white spot cucumber, Holothuria (Roweothuria) poli, belongs to the family Holothuridae and the subgenus Roweothuria. Italian doctor and naturalist Stefano delle Chiaje first described this sea cucumber in 1824. Its known range covers the Mediterranean Sea, the Red Sea, and the Bay of Biscay. The species likely reached the Red Sea through anti-Lessepsian migration, with its larvae passing via the Suez Canal.

Description

This sea cucumber has a cylindrical body, reaching about 25 cm (10 in) in length and 5 cm (2 in) in width. A crown of retractable tentacles surrounds the mouth at the front end, while the cloacal opening sits at the rear. Its thick, leathery skin contains embedded calcareous sclerites and displays blackish spots on a mottled brown and grey background.

The underside is somewhat paler than the upper side, which bears numerous low tubercles tipped with white. Longitudinal rows of tube feet secrete mucus that attracts sand and detritus, often hiding the animal’s appearance. This species is frequently mistaken for Holothuria tubulosa, which shares a similar distribution.

Distribution and habitat

Holothuria poli inhabits the East Atlantic Ocean between the Canary Islands and the Bay of Biscay, as well as the Mediterranean and Red Seas. It lives on the seabed, usually at depths down to about 20 m (70 ft), though off the coast of Tunisia it has been observed on muddy substrates at much greater depths, from 80 to 250 m (260 to 820 ft). It occurs on sand or rock and favors seagrass meadows and areas with green seaweed like Caulerpa.

Ecology

As a limnivore, this sea cucumber uses its oral tentacles to gather sediment and shovel detritus into its mouth. The gut sorts the material: organic particles are digested, while sand and other indigestible matter mix with mucus and form into balls, which are expelled through the anus in a string.

Calculations show that sea cucumbers in a square meter of sand can process nearly 20 kg (44 lb) of sediment per year. The worm-like sea snail Entoconcha mirabilis sometimes parasitizes this species. Few predators threaten it, thanks to its thick leathery skin and the bony sclerites in its integument.

The sexes are separate, and reproduction occurs between November and January. Individuals rear up into L-shaped postures to release clouds of gametes into the water, with some synchronization mechanism apparently timing this release.