Clathria (Microciona) basifixa (Topsent, 1913) is a thinly encrusting Microcionidae distinguished from many other such sponges by its darker colour: dark brownish red. Its spicule complement is also distinctive in lacking chelae and having entirely smooth choanosomal styles. It is usually a boreal deep-water species but has been recorded once from 11 m depth.
Colour: Dark brownish red.
Shape, size, surface and consistency: Thinly encrusting, less than 1 cm2 in size. Surface microhispid. No apparent oscules. Consistency soft, easily torn.
Spicules: Megascleres: Ectosomal subtylostyles with smooth heads: 320-760 x 3-7 µm; choanosomal styles robust, curved, smooth, perhaps slightly roughened heads: 285-1185 x 15-31 µm (Van Soest and Stone, 1986 report dubious rare acanthostyles). Microscleres: Toxas, shallow-curved, robust, in two distinct size categories: 70-115 µm and 19-33 µm. Apparently no chelae.
Skeleton: Leptoclathriid or hymedesmoid, i.e. with styles of widely different sizes erect on the substrate, the larger ones protruding through the skin. Ectosomal subtylostyles confusedly arranged.
Ecology: Encrusting dead corals, mostly at greater depths, but reported from 11 m depth.
Etymology: The name refers to the hymedesmoid skeleton.
Type specimen information: The type is in the Monaco Museum; a slide is found in the Muséum National d'Histoire Naturelle.
This species stands out among the encrusting Microcionidae because of its dark colour (dark brownish red (Van Soest and Stone, 1986), "noiratre" (Topsent, 1913), and because of the lack of chelae. The smooth choanosomal styles are also unusual.
Sources: Topsent, 1913; Van Soest and Stone, 1986.