Petrosia crassa

(Carter, 1876)

Species Overview

Petrosia crassa (Carter, 1876) is an irregularly massive, lobate deep water sponge. It has a stony, brittle incompressible consistency and a smooth surface. Characteristically, the lobes bear a single, compound oscule. The ectosome consists of a detachable crust. The choanosome is crumb-of-bread like, with very little spongin. Spicules consist of several categories of oxeas. Reported from the Faroes and Norway.

Taxonomic Description

Colour: Dirty white with a yellowish tinge, both alive and in alcohol.
Shape, size, surface and consistency: (Petrosia crassa B and W) (P. crassa Lundbeck) Irregularly roundish masses. Surface smooth but rough to the touch. Oscules on the summit of lobes, few, large (8-10 mm in diameter), circular, slightly sunken. Individual canal openings (1-3 mm in diameter) are visible, making the oscules distinctly compound. Size of largest specimens up to 9 cm long and 4-5 cm thick. Consistency very firm, but somewhat crumbly.
Spicules: (Petrosia crassa spicules) Three size categories of oxeas: (1) 300-355 x 12-19 µm, (2) 100-200 x 2.5-7.5 µm, and (3) 50-100 x 2-4.5 µm. All oxeas are evenly curved with rather short but sharp point. No strongyles.
Skeleton: (Petrosia crassa drawings) (Petrosia crassa surface) Ectosomal skeleton: irregular reticulation of loosely organized single spicules, partially obscured by the underlying peripheral spicule tract reticulation (see figures). Choanosomal skeleton: regular alveolar reticulation of multispicular tracts, meshes 400-500 µm in diameter.
Ecology: Deep water, down to 600 m.
Distribution: Faroes, Norway.
Etymology: The name refers to the thickly lobate shape.
Type specimen information: No data.

Remarks

Similar to Petrosia ficiformis and P . clavata from the Mediterranean Atlantic region but differing in habit and spicule sizes and form, which was already pointed out by Lundbeck (1902). Topsent (1928: 324) suggested this is a junior synonym of P . ficiformis . P . ficiformis and P . clavata are predominantly shallow-water forms. Their skeletons are less coarse, with fewer, more irregular and thinner spicule tracts. However, the differences are subtle and the strongest argument for the existence of separate species lies in the lack of overlap between the northern crassa (not known south of the Shetlands and Norway) and the southern ficiformis (Mediterranean, Canary Islands and Azores).
Source: De Weerdt, 1985.