Phorbas fictitius (Bowerbank, 1866)


Family : Hymedesmiidae

Form: Thin sheets to cushions, up to 14mm thick when contracted. Forms patches up to 30cm across.

Colour: Variable. Grey, pale orange pink, to deep red. This is a species where the amount of pigment present generally reflects prevailing water clarity and illumination levels. On the west coast of Ireland it is normally bright red, while in turbid water in Stra

Smell: Present, but not strong.

Consistency: Firm, "compressible".

Surface: Smooth to slippery, with circular depressions containing inhalent pore sieves similar to those in Hemimycale columella, but the rims are the same colour as the rest of the sponge. Unlike Hemimycale the inhalent depressions contract in disturbed and collected specimens. The depressions are smaller at ca. 1mm (max 2mm) than in Hemimycale when out of the water, and are more uniform in diameter in any given specimen.

Apertures: Oscules are distinct, with raised translucent rims; raised oscular channels may be visible whilst in situ but disappear on preservation. Regularly distributed.

Contraction: Distinct: 25 - 50%.

Internal characters

Skeleton: Plumose. Main skeleton of ascending multispicular fibres of tornotes echinated and almost concealed by acanthostyles. Tornotes also form the ectosomal skeleton. Chelae are present, abundant in the ectosomal region. Spongin does not encase the spicules in the ascending fibres as much as in the Crellinae (q.v.)

Spicules: Megascleres are tornotes with dissimilar, oxeote, ends (a), 310-(330)-350Ám. Acanthostyles are of two sizes, the larger (b) are basally spined, 330-(430)-490Ám and the smaller (c) entirely spined, 130-(155)-195Ám. Microscleres are arcuate isochelae (d), 18-25Ám.

Habitat: In exposed or tide swept areas on rock. Most often recorded in the infralittoral zone, on gully walls. "Can be locally abundant, in the littoral and to 19m; on hydroids, oyster shells and under stones."

Distribution: "British Isles; France; Spain, Mediterranean." Known recently from Strangford Lough; Tiree; Western Ireland; Saltee Is., Co. Wexford; Isle of Man; Lleyn peninsula. A fairly common species.

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

Identity: This species has been confused in situ with Hemimycale but its raised oscular rims, conspicuous oscules and the lack of white rims to the uniformly-sized depressions make it quite easy to distinguish from Hemimycale. A number of Phorbasinid and Hymedesmid species could be confused superficially with this species, so the spiculation and skeleton must be checked for positive identification. Both spiculation and colour are similar to Hymedesmia jecusculum (q.v.), but this species is permanently a thin sheet. Phorbas is distinguished from Crellid spp. by having echinating spicules on the ascending fibres, and ectosomal spicules of tornotes rather than acanthostyles.

Voucher: BELUM : Mc468. Tiree, Hebrides.

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

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