(Hentschel, 1929)
Species Overview
Crella derma (Hentschel, 1929) is a thin smooth crust of unknown live colour, which may be only reliably recognized by examination of the spicules. It is an Arctic-Boreal species.
Taxonomic Description
Colour: Yellow-grey to colourless in alcohol.
Shape, size, surface and consistency: Thinly encrusting with smooth surface. Size up to 1 cm in lateral expansion and 0.5 mm in thickness. Surface membrane easily detachable. No obvious oscules. Consistency fragile.
Spicules: Megascleres : Choanosomal tornotes, fusiform, smooth, with endings sharp, grading into blunt, sometimes polytylote: 200-232 x 6 µm; ectosomal acanthostyles, curved, fusiform, not swollen at the base, strongly spined all over: 96-168 x 5-7 µm; choanosomal basal acanthostyles, with strongly spined swollen basal end, lighter spined along the shaft: 126-210 x 6-13 µm.
Microscleres : Arcuate chelae (Hentschel says "ancorae spatuliferae", but his drawing shows a clear arcuate chela), tridentate, gradually curved shaft: 17-19 µm.
Skeleton: Ectosomal skeleton a parchment-like multiple-layered crust of tangential acanthostyles. Choanosomal skeleton consists of plumose bundles of tornotes, 70 µm in diameter, rising up from a basal plate of spongin in which acanthostyles are embedded with their heads, shafts upwards. Microscleres concentrated in the choanosome, rather rare.
Ecology: On worm tubes and pebbles, 60-120 m
Distribution: Faroes; Arctic.
Etymology: Derma (Greek) = skin, referring to the tangential skeleton of Crella .
Type specimen information: Hentschel's material is in the Zoological Museum of Hamburg.
Remarks
This species is ill-known (has in fact been described only once), and seems to be close to other likewise ill-known species such as C . albula and C . basispinosa . The combination of thinly encrusting habit, oxea-like tornotes and evenly curved chelae distinguish the present species from these others.
Koltun (1959) synonymized C . derma with C . imparidens (Rezvoj, 1925) from the Barents Sea. This is not followed here.
Source: Hentschel, 1929.