Phylogeographic patterns in Macrodon ancylodon sampled from 12 locations across all its range were investigated using mitochondrial DNA cytochrome b sequences, and analysed together with patterns of morphometric differentiation. Populations of the North Brazil and the Brazil currents, with warmer waters, form a clade (tropical clade) separated by 23 fixed mutations from the populations that inhabit regions of colder waters influenced by the Brazil and Malvinas currents (subtropical clade). No gene flow exists between the tropical and subtropical clades, and most likely also between the two groups of the tropical clade. Distribution of these clades and groups is correlated with flow of currents and their temperatures, and is facilitated by larval retention and low adult migration. Despite differentiation at the molecular level, fishes analysed from all these current-influenced regions are morphometrically homogeneous. Throughout its range M. ancylodon inhabits the same, or very similar niche; thus, stabilizing selection probably promotes the retention of highly conserved morphology despite deep genetic divergence at the mitochondrial DNA cytochrome b.

The king weakfish (pescada-gó in Portuguese -Macrodon ancylodon (Sciaenidae), a demersal (bottom-feeding) species found in South America Atlantic coastal waters from the Gulf of Paria in Venezuela to Baia Blanca in Argentina, is an economically important species because of its abundance and wide acceptance by consumers. Because of its wide distribution this fish may be subject to geographic isolation and this may have resulted in distinct populations along its coastal range. Considering that this species represents an important economic resource, confirmation of whether M. ancylodon is a single species or there are different genetic stocks spread over its wide distribution would be an important contribution to conservation policies and population management of the king weakfish. To investigate differences between king weakfish populations we used the cytochrome b and 16S rRNA genes to characterize M. ancylodon specimens caught throughout its South American range from Venezuela to Argentina. Our results clearly distinguished two genetically different groups which show nucleotide divergence and genetic structuring patterns that strongly suggest they may be different species, disagreeing with the widely accepted traditional taxonomy that accepts only one species of Macrodon in the western Atlantic.

Red snappers (Lutjanus purpureus in Brazil and Lutjanus campechanus in USA and Gulf of Mexico) are both under clear effect of overfishing. Because of their high morphological similarity it has already been suggested that they could possibly be considered as a single species. To investigate the degree of similarity and the genetic structure of red snapper populations we constructed a common dataset of partial D-loop mtDNA sequences of L. purpureus from Brazil (Amapá, Pará and Maranhão) and L. campechanus from the Atlantic coast of the USA (Florida, Louisiana and Mississippi). Phylogenetic and population genetic analyses surprisingly depicted high similarity between L. campechanus and L. purpureus, compatible with the hypothesis of a single species of red snapper for the Western Atlantic Ocean. These preliminary but very curious findings open an important discussion regarding the legislation involved on the capture of this overexploited fish resources as well as regarding their taxonomy.

Fragments of mitochondrial (COI and rRNA 16S) and nuclear (Tmo-4C4) genes were sequenced to evaluate the phylogenetic relationships among 15 genera of the western South Atlantic Sciaenidae, two freshwater genera and the northwest Pacific Larimichthys crocea. Our results suggest a great diversification for the western Atlantic assemblage; the monophyly of Macrodon, Menticirrhus and Plagioscion genera; the distinctiveness of Bardiella, Stellifer and Ophioscion as belonging to the same clade; the possible existence of two distinct groups in Stellifer; the closer relationship between Lonchurus and Paralonchurus; the non-monophyly of Cynoscion; and the remarkable diversification of Larimus breviceps populations in the Brazilian coast. This is the most comprehensive study evaluating the phylogenetic relationships of the western Atlantic sciaenid and provides a guide for future studies within this family.

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