Use of shell-shape to discriminate betweenBrachidontes rodrigueziiandBrachidontes purpuratusspecies (Mytilidae) in the transition zone of their distributions (south-western Atlantic)
Mussels are important components of rocky shore assemblages throughout the world. Several mytilid species are found as multilayered beds in the intertidal along the coasts of the south-western Atlantic. However, in contrast to the north communities in the Pacific and Atlantic, those of the south-western Atlantic are dominated by species of small size, locally named mejillines. These mid-intertidal beds attached to consolidated substrates are dominated by virtual monocultures of two small-sized species ofBrachidontes: B. rodriguezii(d'Orbigny, 1842) andB. (Perumytilus) purpuratusLamarck, 1819, respectively distributed in the warm and cold temperate sectors of the south-western Atlantic; both coexisting in the transition zone between 40° and 44°S latitude. Nevertheless, there has been some confusion about the separation ofB. rodrigueziiandB. purpuratus, as well as about the boundaries of their distribution ranges in the south-western Atlantic. Here, on the basis of a morphogeometric analysis of shell morphology, we describe a feasible way for the identification of both species.