A morphological and genetic comparison of Septifer bilocularis, Mytilisepta virgata and Brachidontes variabilis (Bivalvia: Mytiloidea) from Hong Kong and erection of the Mytiliseptiferinae sub-fam. nov.

Abstract This is a comparative study of two extant, umbonally septate, marine, Mytiloidea — the Indo-West Pacific coral associated Septifer bilocularis and the rocky intertidal Mytilisepta virgata with the non-septate, also intertidal, Brachidontes variabilis. Mytilisepta virgata has most recently been associated genetically with the Brachidontinae and this study examines the anatomical and genetic case for this. The three species share many mytiloid characteristics including an heteromyarian shell form that equips them for life within crevices and a radially and bifurcatingly ribbed, or lirate, shell. Only the two septiferines possess an interior umbonal septum in each valve with an anterior adductor muscle located between them. And only the septiferines possess an accessory posterior adductor muscle. It is concluded that S. bilocularis should be retained within the Septiferinae, as currently accepted, whereas M. virgata, though genetically (but not morphologically) linked with representatives of the Brachidontinae, should be included in its own new subfamily – the Mytiliseptiferinae – herein proposed. It thus appears that the two septate mytilids have evolved separately from a common ancestor but, today, share ancestral features of a radially and bifurcatingly ribbed, septate shell and the possession of accessory posterior adductor muscles. How the septum of M. virgata has been formed is described.