Unravelling the Brachidontes variabilis species complex (Bivalvia: Mytilidae) of the Indo-Pacific region

Members of the radially ribbed mytilid genus Brachidontes are often dominant filter feeders on hard intertidal substrata in the Indo-Pacific region. However, there is still a great deal of uncertainty and confusion over their species identities, taxonomy and phylogenetic relationships. At least six species names ( crebris- triatus, karachiensis, mutabilis, pharaonis, ustulatus and variabilis) have at one time or another been synonymized with each other and applied to these common intertidal mussels. They occur widely across the Indian and Pacific Oceans from East Africa to Hawaii. Our results, based on sequences of five genes and anatomical features, suggest there are at least eight closely related sibling species in three monophyletic clades: (1) Brachidontes variabilis-form A in the western Indian Ocean; (2) Brachidontes pharaonis in the Red Sea; (3) Brachidontes variabilis-form B, found from India to East Asia; (4) Brachidontes mutabilis, restricted to southern Japan and China; (5) Brachidontes crebristriatus, in the Indian Ocean through to Southeast Asia and Hawaii; (6) Brachidontes ustulatus, an endemic species in western and southern Australia; (7) Brachidontes maritimus, so far known only from Hawaii; and (8) Brachidontes erosus, a southern Australian endemic. The latter two species form a small clade (Clade 1) that is sister to two larger Clades 2 and 3, each comprising three species. Clade 2 contains Brachidontes crebristriatus, Brachidontes mutabilis and Brachidontes ustulatus, whilst Clade 3 is made up of Brachidontes pharaonis, Brachidontes variabilis-form A and variabilis-form B. The shells of these eight species are not only variable but can greatly overlap in morphology. Many are also nearly impossible to distinguish without geographical information. However, the absence or presence of plicate organs in the mantle cavity, and the shape of guard papillae lining the inhalant aperture, may serve as important diagnostic morphological characters when used alongside geographical provenance.