Rediscovery and redescription of <i>Cynodonichthys myersi</i> from the mangroves of northern Yucatan Peninsula, Mexico (Cyprinodontiformes, Rivulidae)

The Cynodonichthys tenuis species group has one of the largest distribution ranges among the Central American rivulids, ranging from the Atlantic slope of southeastern Mexico to Colombia. The enigmatic species Cynodonichthys myersi was described by Hubbs in 1936 from Yucatan. Since then, no further collection of C. myersi specimen has been reported. C. myersi was later considered only a junior synonym of C. tenuis. Here, we report the rediscovery of C. myersi in northern Yucatán mangroves and validate its status as a distinct species through morphological and molecular analyses. The average cytochrome oxidase 1 (cox1) genetic distance between C. myersi and other species from C. tenuis group samples was 8.7%. Our phylogenetic analysis revealed that C. myersi is more closely related to Cynodonichthys species distributed in Panama (Cynodonichthys brunneus, Cynodonichthys weberi, Cynodonichthys montium by ~1700 km distant) than to C. tenuis contradicting expectations based on geographic proximity, suggesting independent ancestral dispersal routes between C. myersi and C. tenuis in Mexico. C. myersi can be differentiated by distinct colouration patterns: males exhibit dark spots over the anal fins on iridescent greenish blue (cyan), whereas females display a spotted pattern in caudal fin. The fish were found at the soil-water interface and out of water, revealing a potential amphibious behaviour.