The urohyal bone, located in the central part of the mandibular skeleton, plays an important role in the mouth openingclosing mechanism of fish, and is considered a synapomorphy in teleostean fish. Morphology of the urohyal bone in six species of Gerreidae (Diapterus brevirostris, D. auratus, Eugerres lineatus, E. plumieri, Eucinostomus entomelas and Gerres cinereus) from the Pacific and Atlantic coasts of México was compared, using size and shape measurements. The main goal of the study was to explore the effectiveness of urohyal measurements in discriminating Gerreidae species. Morphological variation of urohyal bones, in terms of size and shape parameters, allowed species differentiation. Discriminant function analysis (DFA) significantly separated the six species with high classification rates (overall mean 92%). The G-test and Cohen's kappa confirmed the high rates of classification success obtained by DFA. Circularity, Feret minimum, roundness, rectangularity and area were the main urohyal measurements explaining inter-specific variability. These results suggest the usefulness of urohyal bone morphology in differentiating Gerreidae species analyzed, highlighting the taxonomic value of the urohyal bone, until now never quantitatively evaluated as a diagnostic character in the classification of teleostean fish.

The checklist presented in this study includes the latest taxonomic and systematic modifications and updates (early 2018) for the Chondrichthyes that inhabit the Exclusive Economic Zone (EEZ) of Mexico. The list is based on a literature review of field-specific books, scientific publications and database information from collections and museums worldwide available online such as, the Ocean Biogeographic Information System (OBIS), Global Biodiversity Information Facility (GBIF), Encyclopedia of Life (EOL), iSpecies, FishBase and the National Biodiversity Information System (SNIB–CONABIO). Information was cross-referenced with digital taxonomic systems such as the Catalog of Fishes of the California Academy of Sciences, the World Register of Marine Species (WoRMS), and the Integrated Taxonomic Information System (ITIS). There is a total of two subclasses two divisions, 13 orders, 44 families, 84 genera, and 217 species that represent approximately 18% of all living and described species of chondrichthyans worldwide. For the Mexican Pacific and the Gulf of California, 92 species of chondrichthyans are listed compared to 94 species for the Gulf of Mexico and the Caribbean Sea. Additionally, 31 species listed occur on both coasts of Mexico. The species richness of the Mexican chondrichthyans will surely continue to increase, due to the exploration of deep-water fishing areas in the EEZ.

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For many years in America, many authors have considered that the Gerres genus contained a single species, the yellowfin mojarra Gerres cinereus (Walbaum, 1792); however, others have recognized two species in this genus in the Neotropics. In this study, specimens of G. cinereus (sensu lato) caught throughout its geographic range on both coasts of America were compared for the first time to analyse divergence among the sampled populations. A genetic analysis based on DNA sequences from three mitochondrial genes and one nuclear gene revealed a clear separation of two clades, one in the Pacific and one in the Atlantic. Geometric morphometric differences, based on body shape, sagittae otoliths and urohyal bone features, were significant between the two groups, including separation by an accurate classification a posteriori by a canonical variate analysis (CVA) > 82% in all data sets (overall mean >91%). Integration of the results of these approaches indicates that there is sufficient evidence to support species status for Gerres simillimus Regan, 1907, and it should be considered a valid species in the neotropical Pacific.