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(Bowerbank, 1866)

Species Overview

Phorbas fictitius (Bowerbank, 1866) is a thickly encrusting red sponge covered with small (1-2 mm) rounded pore fields, similar to those of Hemimycale columella but of uniform colour (whereas Hemimycale has white rims). It is a common species in shallow rocky areas, occurring under stones and on the wall of tidal gullies. It is a southern species occurring on the west coasts of the British Isles, France, Spain and into the Mediterranean.

Taxonomic Description

Colour: Variable. Deep red, grey, pale orange pink. This is a species where the amount of pigment present generally reflects prevailing water clarity and illumination levels: for example on the west coast of Ireland it is normally bright red, while in turbid water in Strangford Lough (east coast) it is pale pink. In alcohol the sponge and alcohol go the same colour, "a greyish yellow".
Shape, size, surface and consistency: (Phorbas fictitius MCS2) (Phorbas fictitius MCS3) Thin sheets to cushions, up to 14 mm thick when contracted. Forms patches up to 30 cm across. Surface smooth to slippery, with circular depressions ("areolae") combining 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. 1 mm (max. 2 mm) than in Hemimycale when out of the water, and more uniform in diameter in any given specimen. Oscules are distinct, with raised translucent rims; raised oscular channels may be visible whilst in situ but disappear on preservation. Oscules regularly distributed. Contraction distinct: 25-50 %. Consistency firm, compressible. Smell present, but not strong.
Spicules: (Phorbas fictitius spics) Megascleres are tornotes with dissimilar, oxeote, ends: 230-350 x 3-5 µm. Acanthostyles are of two sizes, the larger are basally spined: 330-490 x 8 µm, and the smaller entirely spined: 120-195 x 5 µm.
Microscleres are arcuate isochelae: 18-25 µm.
Skeleton: (Phorbas fictitius skel) Tornotes form the ectosomal skeleton. Choanosomal skeleton a plumose arrangement of ascending multispicular fibres consisting of tornotes, echinated and almost concealed by acanthostyles. Chelae are abundant in the ectosomal region. Spongin does not encase the spicules in the ascending fibres as much as in Crella spp.
Ecology: 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 19 m; on hydroids, oyster shells and under stones. A fairly common species.
Distribution: British Isles, France, Spain, Mediterranean.
Etymology: fictitius (Latin) = artificial or false, referring to the artificiality of its original assignment to genus Microciona, already acknowledged by Bowerbank.
Type specimen information: The type is in the Natural History Museum, London: BMNH: 1910.1.1.72 (dry), Norman Collection, Guernsey + slide unregistered (Bk. 423). MCS voucher BELUM Mc468, Tiree, Hebrides.

Remarks

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 anchinoid and hymedesmiid 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, but this species is permanently a thin sheet. Phorbas is distinguished from crellid species by lacking the tangential surface crust of acanthostyles/-oxeas.
Source: Ackers et al., 1882 (B.E. Picton, S.M. Stone, R. Earll, D. Moss)

Phorbas fictitius