Evolutionary History and Taxonomic Reclassification of the Critically Endangered Daggernose Shark, a Species Endemic to the Western Atlantic

The family Carcharhinidae includes the most typical and recognizable sharks, although its internal classification is the subject of extensive debate. In particular, the type genus, Carcharhinus Blainville, 1816, which is also the most speciose, appears to be paraphyletic in relation to a number of morphologically distinct taxa. Isogomphodon oxyrhynchus (Valenciennes, 1839) (the daggernose shark) is a carcharinid, which is endemic to a limited area of the Western Atlantic between Trinidad and Tobago and the Gulf of Maranhao in northern Brazil, one of the smallest ranges of any New World elasmobranch species. In recent decades, I. oxyrhynchus populations have been decimated by anthropogenic impacts, which has led to the classification of the species as critically endangered by the IUCN. However, there is considerable debate on both the validity of the species (I. oxyrhynchus) and the status of Isogomphodon Gill, 1862 as a distinct entity from the genus Carcharhinus. The present study is based on a molecular assessment of the genetic validity of the I. oxyrhynchus that combines mitochondrial and nuclear markers, which were used to identify the biogeographic events responsible for the emergence and dispersal of the species in northern Brazil. The genetic distance analyses and phylogenetic trees confirmed the paraphyly of the genus Carcharhinus, recovering a clade comprising Carcharhinus+I. oxyrhynchus+Prionace glauca (Linnaeus, 1758). Our results indicate not only that the daggernose shark is actually a member of the genus Carcharhinus, but that it is genetically more closely related to Carcharhinus porosus (Ranzani, 1839) than it is to the other Carcharhinus species analyzed. Given this, I. oxyrhynchus and P. glauca are therefore reclassified and recognized as Carcharhinus oxyrhynchus and Carcharhinus glaucus. The daggernose shark, Carcharhinus oxyrhynchus, diverged from C. porosus during the Miocene, when significant geomorphological processes occurred on the northern coast of South America, in particular in relation to the configuration of the Amazon River. It is closely associated with the area of the Amazon plume, and its distinctive morphological features represent autapomorphic ecological adaptations to this unique habitat and do not reflect systematic distinction from Carcharhinus.