Diana Estela González-Trejo, Manuel Elías-Gutiérrez
Lake Bacalar is an oligotrophic body of water considered an extreme environment due to its unusual chemistry, with many species yet to be discovered. Mysids were recently collected in this system using light traps. Most previous records of these organisms in the continental Yucatán Peninsula are from closed cenotes (sinkholes or dolines) and caves. We applied integrative taxonomy, considering morphological characters of mature males and females, their geographical distribution, and mitochondrial cytochrome C oxidase subunit I (COI) gene sequences to describe Mayamysis bacalarensis n. gen., n. sp. The morphological analyses included scanning electron microscopy (SEM) and bright-field microscopy observations. We found that M. bacalarensis n. gen., n. sp. has a unique combination of morphological characters within the tribe Gastrosaccini, such as the absence of rostrum, the terminal structure of the third pair of pleopods in males, which are highly developed and have a reduced endopod. Both sexes have a lamellar projection on the fifth abdominal segment and numerous setae (35–40) and spines (22) on the margins of the exopod and endopod of the uropods. In addition, the telson has a posterior cleft margin, bearing a total of 32 spines, the apical and lateral ones being the largest. Furthermore, the genetic sequences of these mysids indicate that the organisms from Bacalar are unique, showing an average of 15% genetic divergence from the closest relative, represented by Chlamydopleon dissimile, and over 20% to more than 50% divergence with other mysids sequenced from Mexico. The geographic range of M. bacalarensis n. gen., n. sp. includes the northern part of Chetumal Bay, a brackish oligotrophic lagoon, where organisms identical to those from Lake Bacalar were collected. This finding indicates a possible permanent connectivity through subterranean water flows. Based on the evidence, we conclude that this is a new genus and species, possibly microendemic to southeastern Quintana Roo, Mexico.