RESUMEN
The presence of morphologically identical individuals that are genetically divergent, known as cryptic species, is a phenomenon that is likely to be of importance at the theoretical level, for speciation studies, and at the applied level, for management purposes. Cryptic species are probably common in the marine environment, and they have been described in a variety of habitats. Here we present the first case of cryptic speciation in the mesopelagic environment for the Benthosema lanternfishes. Based on mitochondrial and nuclear sequences, we describe the presence of widely divergent Benthosema pterotum cryptic species that diverged at least 5 million years ago. Our results have important theoretical, as well as practical implications, related to developing mesopelagic fisheries.