Functional morphology of Rocellaria dubia (Bivalvia: Gastrochaenidae) with new interpretations of crypt formation and adventitious tube construction, and a discussion of evolution within the family

Abstract

Rocellaria dubia bores into subtidal rocks of karsted limestone in the Adriatic Sea and elsewhere. It also bores into the shells of various bivalve species. The mechanism of boring has hitherto been debated, but examination of occupied shells suggest that this is achieved by mechanical (the shell) abrasion and chemical etching using secretions produced from glands in the anterior mantle. Fast growing bivalves such as Ostrea edulis and Pinna nobilis carry heavy R. dubia burdens and encapsulate the borer in secreted calluses. Slow growing bivalves such as the burrowing Venus verrucosa and Glycymeris violacescens carry low R. dubia burdens, are less able to encapsulate the borers and probability incur enhanced mortalities as a result. Individuals of R. dubia removed from their limestone boreholes re-secreted adventitious tubes around their siphons, probably from glands in the posterior mantle. The lifestyle of R. dubia is now better understood and its ability to bore bivalve shells in particular suggests how the more advanced tropical gastrochaenids Cucurbitula and Eufistulana have evolved from initial (as juveniles) bivalve shell borers into occupants of adventitious crypts and tubes, respectively. It is further argued that the Gastrochaenidae show convergent similarities with the similar crypt and tube building representatives of the Clavagelloidea