Phorbas dives (Topsent, 1891)


Family : Hymedesmiidae

Form: Crust to cushions to about 1cm thick; spreading in patches to more than 1m across.

Colour: Creamy to brownish or greenish yellow, orange.

Smell: A slight 'spongy'/"fresh marine" smell.

Consistency: Compressible, fairly resilient; breaks when bent through ca. 90o.

Surface: More or less smooth with conspicuous broad subsurface channels converging on the oscules.

Apertures: Quite numerous, of moderate size.

Contraction: Slight.

Internal characters

Skeleton: A plumose skeleton of ascending multispicular fibres of acanthostyles, with smaller accessory acanthostyles. Occasionally spicules at the end of the fibres penetrate the surface. Tornotes common in the surface, more or less perpendicular to it. Abundant sigmata in the endosomal tissues and chelae arcuatae, most abundant near the surface.

Spicules: The megascleres of the main skeleton are acanthostyles (a), 180-(211)-275Ám, sparsely spined especially away from the head. The auxilliary acanthostyles (b) are 60-(92)-110Ám long, and relatively strongly spined. The ectosomal tornotes (c) are 132-(147)-155Ám long. Microscleres are sigmata and arcuate isochelae. The sigmata (e) are usually in two size groups, ca. 15-18Ám and 35-40Ám - the latter are more numerous. The isochelae (d) are mostly of two size classes, ca. 13-15Ám and ca. 20Ám, with a few ca. 35Ám and a very few ca. 8-10Ám (the latter may be difficult to detect).

Habitat: Vertical hard substrate faces, from shallow sublittoral (well-shaded and wave exposed) to 75m; on overhangs and under cobbles and boulders. Among Cystoseira (Topsent, 1891); may be associated with Axinella dissimilis.

Distribution: Known recently from Anglesey, Selsey Bill, Roscoff, Channel Isles, Lough Hyne. Apparently restricted to the south west of the British Isles?

Distribution Map: NBN map : National Biodiversity Network mapping facility, data for UK.

Identity: The appearance, with conspicuous subsurface channels, is not very distinctive. The spicule complement, however, is fairly distinctive: there are apparently several species of Stylostichon with similar spiculation, but differing in the size and relative abundance of the isochelae, and the size, shape and spination of the acanthostyles. Stylostichon bihamigera is very similar to this species, but has abundant chelae of two size classes in a surface layer. The channels are more noticeable in close-up photographs in situ.

Editors: D. Moss, B.E. Picton.

 Picton, B.E., Morrow, C.C. & van Soest, R.W.B., 2011. [In] Sponges of Britain and Ireland