Clathria (Microciona) gradalis Topsent (1925a) is one of the red encrusting Clathria species which are not recognizable unless a microscopic preparation is made. It is originally described from the Mediterranean but is reported to occur in the the Channel area and the North Sea. It is characterized by the possession of long styles up to 700 µm.
Shape, size, surface and consistency: Thinly encrusting, microhispid.
Spicules: Megascleres: Ectosomal subtylostyles, microspined heads: 100-500 x 2-4 µm; choanosomal styles with rare spines on the heads: 700 x 8-20 µm; echinating acanthostyles, curved, strongly spined, especially in the lower half: 65-200 x 5-6 µm. Microscleres: Palmate isochelas with short alae: 11-14 µm; toxas, smooth, deeply curved: 90-200 x 1.5-2.5 µm.
Skeleton: Leptoclathriid or hymedesmoid, i.e. styles and acanthostyles erect on the substrate, or microcionid i.e. coring and echinating short spongin fibres.
Ecology: On the walls of sublittoral caves, encrusting Bryozoans.
Distribution: Roscoff, North Sea coast of Scotland; Western Mediterranean.
Etymology: The name refers to the grade in skeletal development from single spicules to spicules arranged in columns observed in the three type specimens.
Type specimen information: The type specimens from Naples and Monaco are in in the Paris Museum.
The species is similar to Clathria armata in spiculation, but has larger and almost smooth choanosomal styles and smaller palmate chelas.
Source: Lévi, 1960.