Resolving the ambiguities in the identification of two smooth-hound sharks (Mustelus mustelus and Mustelus punctulatus) using genetics and morphology

Elasmobranch species are among themost threatened marine fish, and sound biological data for many of them are lacking. In this context, the smooth-hounds (Mustelus spp., Triakidae, Carcharhiniformes) represent an intriguing genus, being characterized by unclear and sometimes contrasting diagnostic traits. Here, we present new insights into the identification of two closely related species, M. mustelus and M. punctulatus.The use of three different molecular markers on a dataset of 588 specimens highlighted a genetic distinction between the two species. However, whereas microsatellites provided unambiguous results in all the samples, the other two markers were not able to assign a fraction of the individuals (6.6% for cytochrome c oxidase subunit 1, 14.4% for Internal Transcribed Spacer 2), suggesting the occurrence of heteroplasmy and introgression due to past hybridisation events. The comparison between morphological traits, previously suggested as diagnostic for species identification, and genetic identification, enabled validation of the most reliable and practical morphological traits for species identification. The shape of the dermal denticles represents the most reliable trait, but its use during fieldwork may be impractical. The black spots on body sides were present almost exclusively in M. punctulatus, allowing its identification; however, their absence is not diagnostic for M. mustelus. The distance of the nostrils and the shape of the mouth are useful to distinguish the two species during fieldwork. Our results will allow an accurate identification of the two species leading to the collection of reliable data on their distribution, life history traits, exploitation and status of conservation.