Combining geometric morphometrics and genetic analysis to identify species of Opisthonema Gill, 1861 in the eastern Mexican Pacific

Studies focusing on the specific identification of the taxonomic cast within the genus Opisthonema are scarce and contradictory, in spite of this species commercial importance and of the potential ecological impact that fisheries may have on the pelagic ecosystem. In this study, the specific composition of thread herrings Opisthonema (O. bulleri, O. medirastre, and O. libertate) in the southeastern region of the Gulf of California was identified using meristic characters, morphometric data, and mtDNA sequences. In this paper we supported the hypothesis that the commercial catch comprises three evolutionary entities with meristic and morphometric differences. The results were based on the number of ceratobranchial gill rakers, the presence of spicules on the gill rakers, the form of insertion of the gill rakers base onto the ceratobranchial segment, the geometric morphometrics of body shape, and the genetic distances from a fragment of Cytochrome Oxidase Subunit I gene (COI) of mtDNA. Specimens of Pacific thread herring were obtained from landings of the small pelagic fleet at Mazatlan, Sinaloa during 2011–2012. Taxonomic categories were assigned to 1060 individuals according to the measurement of meristic characters. A total of 228 thread herrings O. bulleri, (n = 76), O. medirastre (n = 76), and O. libertate (n = 76) were used in the morphometric analysis, and 25 specimens (O. bulleri n = 8, O. medirastre n = 7, and O. libertate n = 10) were used for genetic comparisons. The morphometric results showed differences among the three groups that had been previously identified using meristic characters, giving support to the existence of three discrete morphotypes. The percentage of sequence divergence of the COI gene supports the existence of three clades. Genetic distances were considerably lower within each clade than between clades. The results of this study provide sufficient evidence for the existence of three Pacific thread herring species in the southern Gulf of California.