Phyllodesmium briareum (Bergh, 1896)

Location: Paradise 1, Mabul, Borneo, Malaysia
Date: 2011/10/09
Length: 20mm
Depth: 6.0m
Water temperature: 28.0℃

Description

A medium-sized aeolid; living animals 13-20 mm in length. Ground colour ranges from light brown to olive-brown, with the dorsum scattered with darker spots. The areas around the rhinophores, the nuchal region and the anterior foot margin are paler and unspotted. The oral tentacles are markedly longer than the rhinophores; both are simple and cylindrical. The cerata are arranged in four groups along each side of the dorsum, with the anterior two groups close to each other. Each ceras is slender, straight, and club-shaped with a swollen apex; the surface is smooth. The anterior head margin is broad, with two triangular lateral expansions. As in other members of the genus, cnidosacs are absent — a key character of Phyllodesmium.

Distribution

Type locality: vicinity of Ambon Island, Indonesia (Mer des Indes, Amboine). Widely distributed across the tropical to subtropical Indo-West Pacific. The species is associated with shallow-reef colonies of the soft (octocoral) genus Briareum, on or close to which it is typically found.

Etymology

The specific epithet briareus derives from Briareus (Briareos) of Greek mythology, one of the Hekatoncheires — the hundred-handed giants. The name alludes to the many cerata that adorn the dorsum, likening the slug to the many-armed giant. The genus Briareum Blainville, 1830 (its prey octocoral) is itself named after the same mythological figure, so the species name coincidentally matches the host genus.

Remarks

Originally described by Bergh as Ennoia briareus, in a new genus, based on a single specimen collected during the Malay Archipelago expedition of Maurice Bedot and Camille Pictet 1890. The genus name Ennoia refers to the Gnostic concept of "Divine Thought" — Bergh footnoted Flaubert's "La tentation de Saint-Antoine" (1895 edition) as the source. Ennoia was subsequently treated as a synonym of Phyllodesmium Ehrenberg, 1831, establishing the current combination; the parentheses in the author citation reflect this generic transfer. Like other members of Phyllodesmium, the species lacks cnidosacs and instead practises kleptoplasty: it sequesters zooxanthellae from its octocoral prey within its cerata, becoming one of the well-known "solar-powered" aeolid nudibranchs.