Avaliação molecular sobre o status taxonômico e populacional de um recurso pesqueiro criticamente ameaçado de extinção: Isogomphodon oxyrhynchus (Valenciennes, 1839).

Abstract

The Carcharhinidae family, made up of widely known shark species, faces debate over its internal classification, especially in relation to the genus Carcharhinus. The genus Carcharhinus Blainville, 1816, is paraphyletic in relation to morphologically distinct taxa. Among them, the duck shark, Isogomphodon oxyrhynchus (Valenciennes, 1839), is a species endemic to the Western Atlantic, distributed between Trinidad and Tobago and the Gulf of Maranhão in northern Brazil, one of the smallest distribution areas of any species of elasmobranchs and has faced a drastic population reduction in recent decades due to anthropogenic impacts, a decline that resulted in its classification as critically endangered by the IUCN in the last assessment made for the species in 2019. However, there is considerable debate about the validity of the species ( I. oxyrhynchus) and the status of Isogomphodon Gill, 1862 as a distinct taxon of the genus Carcharhinus. Arising from the need to understand this relationship, the present molecular study was conducted to evaluate the genetic validity of I. oxyrhynchus, using mitochondrial and nuclear markers. The analyzes confirmed the paraphyly of the genus Carcharhinus, grouping I. oxyrhynchus with Carcharhinus porosus, corroborating the reclassification of I. oxyrhynchus as Carcharhinus oxyrhynchus, as well as Prionace glauca (Linnaeus, 1758) should be Carcharhinus glaucus, respectively. The divergence of C. oxyrhynchus from C. porosus occurred during the Miocene, associated with geomorphological events on the north coast of South America, especially related to the configuration of the Amazon River. Its distinctive features are autapomorphic ecological adaptations to the Amazon plume habitat, not indicating a significant systematic distinction from other members of Carcharhinus. In addition, recent studies have explored genetic variability in sharks through mitochondrial DNA, highlighting the control region (RC) as a valuable tool for phylogenetic inferences. Despite the low genetic variation in some groups of sharks, CR had the potential to aid analyzes and provide support for phylogenetic data. For this, 40 species of sharks from four families of the order Carcharhiniformes were analyzed, seeking to identify patterns in the structure of the RC, defining functional and conserved blocks. DNA extraction and sequencing were performed, mapping conserved and variable segments, to describe the CR for the four families. The results subdivided CR into three domains, highlighting functional blocks such as TAS-1, TAS-2, CSB-F, CSB-1, CSB-2 and CSB-3, with CSB-F predominant in the central portion. Compared to mammals and teleosts, sharks showed variation in nucleotide compositions in the 5', central and 3' domains. The rate of variation in Carcharhiniformes mtDNA was remarkable, revealing common patterns in vertebrate sequences. These findings expand the potential of CR for phylogenetic and population research in sharks.