Chondrosia reniformis Nardo, 1833 is a massive lobate, smooth sponge coloured in variable shades of grey and brown-violet on top, lighter at the sides. When cut in half a distinct darker rim (cortex) is visible on the peripheral parts and the interior is whitish. Its consistency is described as cartilaginous. There are no spicules or fibres in this sponge. It is a southern species dubiously recorded from the Kattegat as its northermost record.
Colour: Mottled brown-violet to grey, darker on the upper parts, lighter on the sides.
Shape, size, surface and consistency: Massive-lobate, forming rounded knolls or elongately rounded shapes. Size up to 5 cm high, 10 or more cm in lateral expansion. Surface smooth, glistening, slippery. Oscules simple, with slightly elevated rims (Chondrosia reniformis Ates). Consistency cartilaginous.
Skeleton: Absent. In cross section there is a distinct darker coloured peripheral layer, which is in fact an organic cortex consisting of layers of collagenous tissue, becoming almost leathery in consistency. The choanosome is lighter coloured, whitish and contains also collagen with scattered elliptical choanocyte chambers. Apopyles and prosopyles relatively long. No spongin fibres or other discrete supporting structures.
Ecology: Shallow water, exposed habitats.
Distribution: Portugal; Mediterranean; ?Kattegat.
Etymology: The name refers to the kidney-shaped form.
Type specimen information: The type is lost.
The record from Kattegat by Arndt (1935) is dubious, since no further specimens have ever been reported.
This is one of four species in the area which do not have any discrete skeleton (no spicules, no spongin fibres). The other species are Halisarca dujardini, which is thinly encrusting, soft and smooth with a clearly punctate surfae, Oscarella lobularis, which forms characteristic small lobules and is likewise soft, and Hexadella racovitzai which has a conulose surface. All three are easily told apart from Chondrosia reniformis.
The species most similar (and most closely related) is Chondrilla nucula. This is not yet recorded from the area but is sympatric with C. reniformis in the Mediterranean. C. nucula has sphaerasters in the cortex.
Source: Arndt, 1935.