Alcyonium coralloides

Alcyonium coralloides, called false coral, is a soft coral that lives in colonies and belongs to the family Alcyoniidae. Its most striking feature is how differently it grows depending on location: along the northeastern Atlantic coast it forms sheets or small lobes on rock surfaces, but in the Mediterranean Sea it acts as a parasite, spreading over and killing sea fans.

The Russian naturalist Peter Simon Pallas first described this species in 1766, giving it the name Parerythropodium coralloides. Later, scientists looked at its growth forms, the shape of its tiny skeletal spicules, and the channels running through its coenenchyme—the tissue that connects the polyps—and decided it belonged in the genus Alcyonium, so it was renamed Alcyonium coralloides.

This coral is abundant in the Mediterranean but less common along the Atlantic coast of Western Europe and in the English Channel, with Scotland marking its northern limit. In the Atlantic, colonies stay small and attach directly to vertical rock faces, under overhangs, and inside caves. In the Mediterranean, they usually grow on sea fans like Eunicella, Paramuricea, and Leptogorgia, as well as on the tunicate Microcosmus and on coralline algae. Commonly colonised hosts include Eunicella singularis, Eunicella cavolinii, Eunicella verrucosa, Paramuricea clavata, and Leptogorgia sarmentosa.

Growth habits vary. In the Atlantic, the coral sometimes forms red encrusting sheets over rock, dotted with white or yellow polyps and occasionally bare patches. More often it appears as short, finger-like lobes up to 4 cm long, pale pink with white polyps. It rarely encrusts sea fans there. In the Mediterranean, the coral typically spreads as an encrusting layer over the surface of a gorgonian. Because the sea fan has a rigid skeleton, Alcyonium coralloides doesn’t need its own supportive skeleton, but it does have spicules in its surface layers that make it feel rough. Colony colour is often purple, though white, pink, or yellow forms occur; the white and pink ones, mostly found in deep water, are considered mutants. Polyps are usually white, cream, or yellow, up to 5 mm long, and generally larger than those of the sea fan it overgrows. The coral is considered parasitic because it kills the sea fan’s tissues and clings to the underlying skeleton, though how it does this is not understood.