Unlike its congenerous one, H. arctica, this species has got no byssus. Recent studies (see bibliography), concerning the discovery of several specimens of this species inside Spongia officinalis sponge (Linnaeus, 1758), have pointed out the main difference between H. rugosa and H. arctica due to the presence of a small tooth in the cardinal margin on both H. arctica valves, missing in H. rugosa. Considering that H. rugosa lives into cavities, it does not need a real hinge; the ligament is enough to hold the valves together. it is a species having a remarkable morphological variability. This is the reason for variety names such as: pholadis, trans¬versa, striata, cylindrica and so on, of no taxonomic value. The average measures are abt. 12 mm.

Very common all over the Mediterranean. It lives up to remarkable depths. Shells were found on rocky substrata and on fishing boats operating in the Tuscan Archipelago (Italy). Mainly it lives into cavities of rocky substrata or of madrepora as Cladocora cespitosa.