(Bowerbank, 1862)
Species Overview
Clathria (Microciona) atrasanguinea (Bowerbank, 1862) is one of several reddish thinly encrusting sponges, showing venal patterns and slightly raised oscules. The identity can only be reliably determined from the study of the spicules. The present species is so thin that the structure of the substrate is visible through the sponge body. The venal pattern consists of darker lines. The surface is velvety and the consistency is soft. It is common on boulders and rocks all along the coasts of Europe.
Taxonomic Description
Colour: Red, blood red, or brown. The photo is typical of the form found in deeper water.
Shape, size, surface and consistency: Thin sheet covering extensive areas, typically 3 mm thick by up to 30 cm diameter. It has been reported as a cushion, but this may be the result of the sponge growing over a saddle oyster. Typically looks like a patch of red paint. Surface with little structure—conforms closely to the underlying substrate. Excurrent channels are clearly visible as dark branching lines converging on the oscules. Slightly velvety appearance; minutely porous. Tuberculate, hispid, often creviced according to Lévi (1960). Oscules have slightly raised transparent rims, apparent mainly because of converging excurrent channels. These oscules are numerous and regularly spaced over the surface. No noticeable contraction. Consistency difficult to determine because of thinness of sheet but fairly soft; difficult to scrape off—it crumbles and tears.
Spicules: (Clathria atrasanguinea V) Megascleres : Slender ectosomal subtylostyles: ca. 225-355 µm. Principal spicules of the main skeleton are styles (occasionally subtylostyles, rarely sparingly spined at the base): 220-470 µm x 10 µm (average ca. 300 µm long). Lévi (1960) gives a size range of 110-375 µm. Auxiliary megascleres are entirely spined acanthostyles: ca. 80-135 µm.
Microscleres : Palmate isochelae: ca. 8-12 µm; smooth tipped toxa, which may be numerous, in two size categories: ca. 100-125 µm and 18-40 µm.
Skeleton: In the juvenile form the skeleton is hymedesmoid, with both the principal spicules of the main skeleton and also the auxiliary spicules (acanthostyles) standing up from a basal sheet of spongin, the bases of the spicules being implanted in the spongin and the points directed towards the surface. Slender accessory spicules (subtylostyles) are scattered throughout. In the adult form as the sponge thickens the skeleton becomes plumose. The main skeleton then consists of plumose columns of spicules incorporating spongin rising up from the basal layer of the juvenile form. These are in turn echinated by smaller spicules (acanthostyles). Slender ectosomal subtylostyles are present in the form of brushes at the surface.
Reproduction: September; larvae have a bare posterior pole (Lévi, 1956; Wapstra and Van Soest, 1987).
Ecology: Vertical surfaces of rocks or wrecks. Also encrusting pebbles in areas of strong current (e.g. Menai Straits). Occurs in brackish water.
Distribution: Atlantic coasts of Europe from Shetlands southwards; Mediterranean.
Etymology: Atrasanguinea (Latin) = dark blood-red, referring to the life colour.
Type specimen information: The type is in the Natural History Museum, London. MCS voucher: BELUM Mc821.
Remarks
Littoral or shallow sublittoral Clathria (Microciona) species and other microcionids often form thin reddish sheets. The dendritic branching of the excurrent channels of this species (photo) may be characteristic but the spicules should always be checked for a positive identification. Superficially it may be confused with Clathria seriata , but note the regular arrangement of oscules in the latter species which is not so apparent in C . atrasanguinea .
Sources: Lévi, 1960; Ackers et al., 1985 (B.E. Picton, S.M. Stone), 1992.