Hymedesmia (Hymedesmia) basispinosa Lundbeck, 1910 is a thinly encrusting sponge of unknown live colour described originally from deep water off Greenland, but subsequently recorded from the Plymouth area. Its occurrence in shallow water in the British Isles needs to be confirmed since no description of that material was provided.
Colour: Dark brownish in alcohol.
Shape, size, surface and consistency: Encrusting. Size up to 15 mm in lateral expansion, thickness about 0.5 mm. The surface is smooth, without projecting spicules. It bears some conical papillae of about 1 mm high, which are presumably the oscules. Consistency presumably soft.
Spicules: (Hymedesmia basispinosa spics) Megascleres: Ectosomal tornotes are oxeote, nearly straight, sometimes polytylote, with sharp points at both ends: 320-500 x 4-7 µm; long acanthostyles, almost entirely smooth, with only the heads spined: 420-590 x 20-25 µm; short acanthostyles, entirely spined: 90-230 x 14-21 µm. Microscleres: Arcuate chelae, slender, only slightly curved (resembling a palmate chela in that respect): 25-30 µm.
Skeleton: Ectosomal: bundles or short tracts of tornotes stretch from the lower parts up to the ectosomal membrane. Papillae walls consists of a dense mass of tangential tornotes. Choanosomal: hymedesmoid with larger and shorter acanthostyles erect on the substrate.
Ecology: Under stones at ELWS (Plymouth); encrusting brachiopod shells at 750 m (off Greenland).
Distribution: Plymouth, off Greenland.
Etymology: The name refers to the acanthostyles.
Type specimen information: The type is in the Copenhagen Museum.
Occurrence of this deep water Arctic species under stones at the low water mark near Plymouth, SW England needs to be confirmed.
Source: Lundbeck, 1910.