Clathria (Microciona) toxitenuis Topsent (1925a) is one of the many red encrusting Microcionidae species which can only be reliably identified by microscopic examination of the spicules. It stands out among these Microcionidae by the rarity (or absence) of isochelae. The species has been reported from Western Europe only once, at 50 m depth off the coast of Brittanny.
Colour: Bright orange.
Shape, size, surface and consistency: Thinly encrusting. No size data available.
Spiculation: Megascleres: Ectosomal subtylostyles, straight, elongated heads, sometimes with a few spines: 200-410 x 2-5 µm; main choanosomal acanthostyles, spined only at the rather feebly developed heads: 300-700 x 11-14 µm; echinating acanthostyles, spined all along the shaft but especially dense in the lower pointed end: 100-130 x 6-10 µm. Microscleres: Palmate isochelae absent or very rare (and then 13-15 µm); toxas, one very variable category, thin, small central flexion: 100-300 x 1µm.
Skeleton: Mostly hymedesmoid, with single spicules (especially the smaller acanthostyles) implanted erect on the basal spongin plate. Some groupings of megascleres bound by spongin are also observed.
Ecology: On tunicates (Naples) or pebbles (Roscoff), in deeper water (50 m).
Distribution: Known from only a single record from Roscoff and a few records in the Mediterranean (Naples, Mediterranean).
Etymology: tenuis (Latin) = thin, referring to the delicate toxas.
Type specimen information: The type is in the Muséum National d'Histoire Naturelle, Paris.
Clathria (M.) bitoxa and C. (M.) normani share the apparent lack of chelae; however, Lévi (1960) says it has very rare chelae of 13-15 µm. Further skeletal details remind of Clathria (M.) strepsitoxa (Hope, 1889).
A deep-water species not treated here [C. tenuissima (Stephens, 1916 as Eurypon) from 700 m off the west coast of Ireland] apparently is close to this species (fide Topsent, 1925a). Another Mediterranean species C. toximajor Topsent (1925) is very close and differs only in some details of toxa size.
Source: Topsent, 1925a; Lévi, 1960.