Bivalve living in bivalve shells: The hidden occurrence of the date mussel Lithophaga lithophaga

Abstract

The date mussel Lithophaga lithophaga is a protected, long-lived endolithic bivalve typically associated with calcareous rocky substrata. In this study, we report its occurrence within the shells of two bivalve species, opportunistically collected in Mali Ston Bay, Adriatic Sea. Twelve Ostrea edulis and five Spondylus gaederopus specimens were analysed by carefully examining their shells and extracting the boring bivalves. The shell length, abundance and position of L. lithophaga within each host valve were recorded, together with visible alterations of the host shell. In O. edulis, L. lithophaga ranged from 2.2 to 36.4 mm in shell length, with up to ten individuals recorded in a single host. Larger specimens were restricted to the thicker region of the left valve near the hinge, whereas smaller individuals also occurred in other parts of the shell and in the right valve. Only small individuals were observed in S. gaederopus. These findings indicate that the thick shells of bivalves can serve as substrata for L. lithophaga. The concealed presence of this strictly protected species within shells of commercially interesting bivalves also raises practical conservation and regulatory questions, because its presence may remain undetected until the host shell is opened or processed.