Deep genetic divergences and few morphological changes support the cryptic speciation in Larimus breviceps (Sciaenidae, Acanthuriformes) from the western South Atlantic.

We evaluate the evidence of cryptic speciation in Larimus breviceps, a species widely distributed in the western South Atlantic, from the Greater Antilles to Santa Catarina in Brazil. Mitochondrial (COI, Cyt b, and Control Region) and nuclear (IGF1 and Tmo-4C4) sequences were obtained from populations in the western South Atlantic. The analysis revealed two genetically distinct, sympatric lineages with no gene flow, with L. breviceps lineage II (LII) being closer to Larimus pacificus than to the L. breviceps lineage I (LI). The most recent common ancestor (TMRCA) for the L. breviceps LI and L. pacificus/L. breviceps LII clade dates from 12.3 Ma, whereas TMRCA for the L. pacificus and L. breviceps LII dates from 3.4 Ma, indicating that speciation processes may be related to the rise of the Isthmus of Panama. Despite these profound genetic differences, morphometric analyses found only subtle differences between lineages, with specimens of the LI being slightly larger than those of the LII, suggesting the existence of cryptic species. Within each lineage there is a pattern of panmixia within the study area. Therefore, we suggest that it is necessary a taxonomic revision of Larimus from the western Atlantic to validate the species status of such lineages.