Morphology, ethology, new records, and ecological niche modeling for Hydromorphus and Tretanorhinus (Squamata, Colubridae) in Panama

The genera Hydromorphus and Tretanorhinus (Colubridae) comprise uncommon, poorly studied aquatic snakes, largely because of their nocturnal and aquatic habits. In Panama, four species have been recorded: Hydromorphus concolor , H. dunni , Tretanorhinus mocquardi , and T. nigroluteus . Although more than a century has passed since the first Panamanian record, knowledge of their distribution, morphology, ecology, and behavior remains limited. Through an exhaustive literature review and fieldwork, we updated their distribution and provided seven new records for H. dunni , 16 for H. concolor , six for T. mocquardi , and five for T. nigroluteus . For H. dunni , the only previously known record was the holotype from the original description. We documented substantial range extensions, especially for H. dunni , with new records extending its known distribution by 228 km northeast to Donoso (Colón Province) and 273 km southeast to Altos de Campana National Park, representing the most distant records for the species to date. We report new localities in Darién Province for T. mocquardi and in Bocas del Toro Province for T. nigroluteus , collectively refining the known distribution of these aquatic snakes in Panama. We identified previously unreported variation in the arrangement and number of cephalic scales, particularly in H. dunni , strengthening the morphological basis for taxonomic diagnosis. We also provide an updated morphological dichotomous key. In addition, we contribute to knowledge of the trophic ecology of T. nigroluteus by documenting a direct case of necrophagy during captive feeding trials. Overall, this study provides new information on these taxa in Panama, improves their ecological characterization, and may support future conservation efforts.