Female carapace subovate in lateral view, ventral margin nearly straight; inflated posteriorly. Male carapace more elongate and not inflated posteriorly. Valves variably pitted to almost smooth, sometimes nodose with up to seven sites of tubercular development on each adult valve. Dorsomedian sulcus weak. Posteroventral spine usually strongly developed on right valve. Colour of living specimens yellowish brown to dark brown.
Found in a wide range of salinities from almost freshwater to over 60 ä in inland ponds, lakes, lagoons, estuaries, fjords, deltas and other marginal marine environments, down to a depth of around 30 metres. It appears to prefer a mud or sandy mud substrate but is sometimes found on algae.
A comprehensive historical review of the complex nomenclatural problems surrounding this species has been given by Sandberg (1964). Here both noded (tuberculate) and smooth forms of C. torosa are regarded as a single species; the nature of variation and their occurrence in the same population precludes the possibility of there being two separate subspecies.
The considerable literature concerning the connection between noding/ tuberculation and low salinity has been reviewed by Kilenyi (1972) with particular reference to this species. He concluded that this is a case of balanced genetic polymorphism rather than a straightforward physiological reaction to lowered salinity. In C. torosa nodes can occur at seven discrete sites on the valve; node 2 may be the only one developed, but is usually accompanied by 1 and 3, forming the 'basic triangle', while the others are less frequently developed. We have only seen the full complement of seven in the type material.
Widespread throughout Europe, W and Central Asia, the Mediterranean region of the Middle East and N Africa and as far north as Iceland; also found in lakes in Central Africa. The only living records of noded forms in Britain are from the Norfolk Broads and W Ireland.