Identification of Bothus ocellatus and Bothus robinsi larvae using morphological and molecular approaches leads to a distribution record for the Gulf of Mexico

The genus Bothus includes small flatfishes classified as left-eyed flounders. Within the Gulf of Mexico (GoM), B. ocellatus and B. robinsi are the most widely distributed species, whose differentiation using morphological characters has been a challenge limiting their accurate identification. Larvae were sampled during two summer seasons (2015 and 2017) in the GoM. Morphological characters allowing identification of both larval species were limited, so they were classified as Bothus spp. Two morphotypes were observed based on the melanophore’s presence on the tip of the caudal fin. Species identification was made using a portion of the cytochrome oxidase I (COI) gene. COI sequences for 126 larvae were identified using Basic Local Alignment Search Tool (BLAST) searches of the National Center for Biotechnology Information (NCBI) GenBank and the Barcode of Life Data (BOLD) System repositories. COI sequences for larvae and reference sequences from databases were used to build phylogenetic trees and perform genetic distance analyses. COI analyses identified 21 larvae as B. ocellatus and 105 as B. robinsi. Both morphotypes were detected in both species and no diagnostic characteristics were observed. The geographic distribution of the species was similar, but B. robinsi abundance was higher than B. ocellatus in contrast to findings of previous studies. These differences between the results in this study and those of previous ones could have been due to errors in identifications based solely on morphological characters. Therefore, the results obtained demonstrate the power of molecular approaches to improve accuracy of the information generated for Bothus genus larvae.