Molecular and morphometric analysis of nominal Brachidontes exustus (Mollusca, Mytilidae) in Brazilian waters

Abstract

Brachidontes exustus (Mollusca, Mytilidae) is mainly distributed in Central America, where it has been recognized as a _lataforma species. This study aimed to determine whether B. exustus extends beyond the Amazon Barrier and southward along the Brazilian West Atlantic coast. Mitochondrial genes coding for cytochrome-c oxidase, subunit I (COI) and 16S subunit of ribosomal _lataforma__ cid (16S rRNA) were analyzed with _lata parameters on Brazilian populations (Salvador, Arraial do Cabo and Fernando de Noronha) of scorched mussels previously recorded as B. exustus. Multivariate morphometric _latafor showed partial discrimination of species. Molecular _latafor confirmed B. exustus at Salvador, a population highly similar to Cartagena (Colombia), both belonging to the Atlantic Clade of the B. exustus complex. This fact adds evidence to the idea of the Amazon outflow as a semipermeable barrier. In the southeast of Brazil, B. exustus was not found; instead, B. darwinianus is the species represented at Arraial do Cabo (state of Rio de Janeiro), associated with brackish _lataf. Scorched mussels from Fernando de Noronha are most closely related to B. puniceus from Cape Verde with 4.4% differentiation. Demonstrating an independent evolutionary history since at least the beginning of the Pleistocene, its proposed new name is B. noronhensis.

Figure 1 -. Sampling sites (black circles) showing scorched mussel morphotypes (right column). Major South Atlantic current systems are based on Muller-Karger et al. (2017): red arrows = warm waters; blue arrows = cold waters (1-7).

Figure 2 -. Linear shell dimensions obtained from scorched mussels: (TL) length, maximum distance between the umbone and the posterior margin of the shell; (H) height, the maximum distance between dorsal and ventral margins; (W) width, the maximum distance between valves; (DAL) anterior dorsal angle length, the maximum distance between the umbone and the dorsal angle; and (DPL) posterior dorsal angle length, the maximum distance between the dorsal angle and the posterior margin of the shell.

Figure 3 -. The maximum likelihood phylogenetic tree based on Tamura-Nei distances of COI nucleotide sequences, showing the phylogenetic positions of scorched mussels obtained in this study (in blue), branch length (in italics), and bootstrap branch confidence. Lee and Foighil (2004, 2005) clades of B. exustus complex, most represented by consensus sequences (1). For GenBank accession numbers, see Table S1

Figure 4-. The TCS network showing COI haplotypes of scorched mussels from Salvador, Cartagena, and from taxa of B. exustus complex belonging to the Gulf of Mexico and Atlantic clades (Lee and Foighil, 2004, 2005). Haplotypes are represented by circles, the size of which is not proportional to haplotype frequency. GenBank accession numbers can be found in Table S1.

Figure 5 -. The TCS network showing COI haplotypes of scorched mussels from Fernando de Noronha and from taxa of B. exustus complex belonging to The Bahamas and Antillean clades (Lee and Foighil, 2004, 2005). Haplotypes are represented by circles, the size of which is not proportional to haplotype frequency. GenBank accession numbers can be found in Table S1.

Figure 6 -. Canonical discriminant space showing the dispersion of scorched mussels based on relative shell parameters: A) scorched mussels allocated to their respective localities, B) scorched mussels allocated to their respective species.