The biology and functional morphology of Septifer bilocularis and Mytilisepta virgata (Bivalvia: Mytiloidea) from corals and the exposed rocky shores, respectively, of Hong Kong

Abstract

There are fossil Mytiloidea assigned to the Septiferinae such as Coxesia mezzalirai and species of Admytilus and Assytilus. Extant species of the Septiferinae are assigned to Septifer and Mytilisepta and date from the end of the Triassic, >200 mya, and the end of the Cretaceous, >65 mya, respectively. All septiferines possess shells characterised by an external radially bifurcate ribbing and internal umbonal septa associated with which is the anterior adductor muscle. The Indo-West Pacific Septifer bilocularis and Mytilisepta virgata are described herein. Uniquely, both possess accessory posterior adductor muscles and posterior pedal retractor muscles that are either absent (M. virgata) or virtually so (S. bilocularis). It is hypothesised that the former muscles have evolved from modified pallial retractors. Gene sequencing evidence suggests that M. virgata is related to the Brachidontiinae and ongoing research further suggests that S. bilocularis is more related to the Mytilinae. This implies parallel evolution and argues that species of Septifer can be retained within the Septiferinae, as currently defined, whereas M. virgata, as the type species, should possibly be placed in its own subfamily – the Mytiliseptiferinae – and, again possibly, allied with the Brachidontiinae. But with both subfamilies retained within the Mytilidae. Uniquely too for the species-diverse marine Mytiloidea, M. virgata possesses ectopic pallial eyes located at the apex of papillae that line the inhalant aperture to the mantle cavity.