The Golfo Dulce yellow sea snake (Elapidae: Hydrophis platurus xanthos) from morphological and molecular perspectives

The yellow sea snake Hydrophis platurus xanthos is found only in Costa Rica’s South Pacific embayment of Golfo Dulce, confined to a <215-m-deep inner basin. This endemic population is geographically separated from the pelagic sea snake Hydrophis platurus platurus by >20 km and has distinctive morphological characters, suggesting potential phylogenetic divergence. Our study confirms morphological diagnosability of the Golfo Dulce population using coloration (predominantly yellow vs. dorsally black) and consistently small body size (<60 cm in total length). Several significant differences in cephalic and caudal scale counts are also documented. Seven preserved yellow specimens collected outside Golfo Dulce in the 1970s are morphologically consistent H. p. xanthos, suggesting that they originated from inside the gulf. Despite this, when we used reduced representation sequencing to examine single-nucleotide polymorphisms, targeted squamate conserved loci, and mined mitochondrial DNA, our molecular analyses provided no evidence that H. p. xanthos and H. p. platurus are separately evolving lineages. Indeed, we found near-complete lack of structure both within and between these populations. The absence of genetic differentiation, which suggests regular gene flow despite contrary morphological and biogeographical factors, creates an intriguing paradox. Recent separation and/or high selection pressure might be in effect.