Clathria (Microciona) osismica (Cabioch, 1968) is one of the (presumably) red encrusting Microcionidae, which can only be reliably identfied by microscopic examination of the spicules. It is close in spiculation to C. (M.) strepsitoxa and C. (M.) ascendens, differing in minor details of spicule sizes and categories. So far this species is only known from a single record from Roscoff, NW France.
Colour: Yellowish in alcohol.
Shape, size, surface and consistency: Encrusting, very thin: about 1 mm, on barnacles.
Spicules: Megascleres: Ectosomal subtylostyles, with microspined base: 150-335 x 2-4 µm; large acanthostyles, mostly entirely smooth except for conical spines on the head: 160-370 x 6-8 µm; echinating acanthostyles, entirely spined, with spines generally fine, the larger ones curved (those at the head are curved towards the point, those along the shaft are curved towards the head): 70-130 x 4-7 µm. Microscleres: Palmate isochelas: 11-16 µm; toxas in two distinct categories, 1) long, straight, with a very small inflexion (not twisted): 150-380 x 1-1.3 µm, and 2) short, deeply curved, finely rugose at the apices: 15-125 x 0.5-1.5 µm.
Skeleton: Ectosomal subtylostyles scattered confusedly in the surface and among the choanosomal spicule tracts. They form bouquets at the end of the choanosomal spicule tracts. The choanosomal skeleton consists of short plumose columns of styles bound by some spongin, but not enveloped in it. The columns divide and extend obliquely towards the surface; they are echinated by acanthostyles.
Ecology: On rocky substrate in deeper water.
Distribution: Known so far only from Roscoff, France.
Etymology: Osismi are a tribe of Breton people in the time of the Roman occupation of France.
Type specimen information: The type is in the collections of the Station Biologique at Roscoff.
This species is similar to C. (M.) strepsitoxa (Hope, 1889) differing from it in the clearly smaller sizes of most of the spicules. From C. (M.) ascendens (Cabioch, 1968) it differs in the possession of small toxas next to the long straight ones, the absence of a twist in the long toxas, and in the microspined condition of the subtylostyle heads.
Source: Cabioch, 1968.