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(Fabricius, 1780)

Species Overview

Grantia compressa (Fabricius, 1780) is a very characteristic white, flattened, calcareous sponge occurring usually in groups of half a dozen or more. Its surface looks smooth but feels rather rough; its consistency is rather firm and it can be bent without breaking. It is common on all rocky shores of Western Europe and is most frequently found on brown and red seaweeds in the midlittoral and lower intertidal.

Taxonomic Description

Colour: White or occasionally brown.
Shape, size, surface and consistency: Flat, like a small hot water bottle (Grantia compressa MCS2), but may be tubular, or composed of contorted flat lobes; with a very short stalk. Surface smooth, has a moderately high friction when stroked. There is usually a single, smooth-rimmed terminal oscule. Occasionally there may be several oscules along the margins. Contraction not noticeable. Consistency moderately firm, can be bent through 180° without breaking.
Spicules: (Grantia compressa spics) Calcareous. Ectosomal triactines, rays: 100-120 x 8 µm. Overlapping triactines, paired rays: 80-120 x 8 µm and basal rays 200-300 µm. Choanosomal triactines: 100-150 x 5-8 µm; tetractines: 100-150 x 5-8 µm, with apical rays 40-80 x 8-12 µm. Diactines, resembling hockey sticks: 100-300 x 8 µm.
Skeleton: The ectosomal skeleton is a tangential layer of triactines with 'tufts' of diactines projecting beyond the surface. The skeleton of the chamber layer consists of regularly overlapping triactines. There is a choanosomal skeleton of paired rays of triactines and a tangential layer of triactines and tetractines lining the central atrium.
Ecology: The main habitat is the sea shore where specimens are found attached to the underside of overhangs, often in abundance; they also occur commonly in the shallow sublittoral, e.g. on kelp stipes. In the East Atlantic from littoral to ?288 m.
Distribution: A common species in the British Isles; Arctic to Channel Islands.
Etymology: The name refers to the flattened habit.
Type specimen information: No type material in BMNH; MCS voucher BELUM: Mc104. Portrush, Co Antrim.

Remarks

Identity may be established on sight, provided the specimen is simple, flattened and has a smooth surface. The only real confusion possible is with small specimens of Sycandra utriculus, but normally these exceed G. compressa in size without much overlap. Other growth forms exist and these need to be checked microscopically. There is some overlap between tubular instead of flat G. compressa and Sycon ciliatum which may have a smooth outer surface instead of the normal finely papillate one.
Source: Ackers et al., 1992 (editors S.M. Stone, J. D Guiterman, D. Moss, B.E. Picton)

Grantia compressa