Identificação genética e morfológica de Brachidontes exustus (Linnaeus, 1758) (Bivalvia, Mytilidae) em diferentes ambientes do Oceano Atlântico Sul Ocidental

Abstract

Along the coast of the West Atlantic Ocean, Brachidontes exustus (Linnaeus, 1758) (Mollusca, Mytilidae) was registered in several localities, but including other species as synonyms. Taxonomic studies on B. exustus based on shell morphology attributed variation of these characteristics to phenotypic plasticity. This fact has imposed difficulties on the identification of species and generated taxonomic confusion. At the moment, B. exustus corresponds to a species complex found in Florida, Gulf of Mexico and in Caribbean waters, but its taxonomic status still remain without clarification in some regions of the West Atlantic. The present study aims to contribute for the clarification of the taxonomic status of populations of B. exustus from the South West Atlantic, through a molecular and morphological approach. Mussels from four localities (Cartagena, in Colombia; Salvador in Bahia, Arraial do Cabo in Rio de Janeiro and Fernando de Noronha archipelago in Pernambuco) were studied. Morphometric study consisted in a Canonical Discriminant Analysis of five shell linear measurements (width, height, anterior and posterior dorsal length) relativized by the total length. Canonical roots were used to observe the distribution of mussels in the discriminant space and to evaluate their morphological discrimination. Classification functions were obtained and the percentage of correct hits were evaluated by species. Molecular analyses were based on partial sequences of the mitochondrial genes 16S and COI, which were used for the construction of haplotype networks with PopArt software and phylogenetic trees based on maximum likelihood and Tamura-Nei distance. The P-distance and Tamura-Nei distance were obtained for all pair-wise populations and with sequences of reference species obtained in the GenBank. For these analyses, it was applied 1,000 bootstrap resampling and the MEGA software was used for these purposes. Sequences of the 16S rRNA gene were generated from the studied species, which will be submitted to the Genbank. Mussels form Cartagena and Salvador were identified as B. exustus and showed low genetic distance between them (0.002 ± 0.001). These mussels were grouped in the COI network and phylogenetic tree together with the Miami form of the species, to which they showed a distance of 0.005 ± 0.002.The population of Arraial do Cabo stream was clustered with B. darwinianus from Uruguay, to which it shows a low distance (0.006 ± 0.003), confirmating of belonging to this species. Fernando de Noronha population was grouped in an independent clade formed by B. puniceus from Cape Verde (in the base of the clade) and by the Bahamas form of B. exustus, showing a mean distance between them of 0.050 ± 0.010; and a distance with the other species of Brachidontes, here studied, of 0.220, indicating that the correct name of this clade should be B. puniceus. The discriminant function allowed the partial segregation of the identified species with correct hits of at least 85% of the cases. It is possible to conclude that B. exustus extends its distribution to Salvador, Brazil, expanding the recently verified distribution of the species. It was confirmed the occurrence of B. darwinianus in rocky shores of Praia do Pontal in Arraial do Cabo, Brazil, in the lower intertidal zone at the channel output of brackish water. Fernando de Noronha mussels correspond to B. puniceus, which is the same species found in Cape Verde, Bahamas and Bermuda Islands. The low genetic distance among them and the degree of isolation of Fernando de Noronha population allow to propose that this population should be named as the subspecies Brachidontes puniceus noronhensis.