Convergent morphological and genetic patterns delimit Evolutionary Significant Units in endangered Aipysurus sea snakes

Abstract

Background: Identifying significant adaptive variation within species is a key focus of efforts to preserve evolutionary potential under environmental change. We analysed genome-wide SNPs and ecomorphological traits to define units for the conservation of Aipysurus sea snakes. This group contains nine fully-marine taxa that are primarily endemic to the northern Australian region, including two critically endangered and one endangered listed species. Results: Results show convergent patterns of genetic and morphological divergence between coastal and offshore populations in four of five co-distributed Aipysurus lineages. In A. apraefrontalis, A. duboisii, A. foliosquama, and the A. fuscus-A. tenuis complex, individuals from the WA Coast and remote Timor Sea reefs form distinct genetic clusters with gene-tree based divergence estimates of five hundred to nine hundred and fifty thousand years. Coastal populations in all four of these lineages have longer bodies, higher vertebrate counts, and lighter colour patterns compared to offshore-reef populations. In A. laevis (the fifth co-distributed lineage), body length and colour pattern variation mirrored the coastal-offshore divisions seen in the other Aipysurus, but this species showed different patterns of geographic genetic structure that are incongruent with current species boundaries. Conclusions: This striking genetic and morphological convergence implicates shared patterns of biogeographic isolation and suggests local adaptation related to locomotory performance and crypsis in open soft-bottomed coastal habitats versus structurally complex reefs. We recommend that coastal and offshore-reef populations of A. duboisii, A. apraefrontalis and A. foliosquama be recognised as evolutionarily significant units (ESUs), while the current taxonomy of A. fuscus, A. laevis, A. pooleorum and A. tenuis be retained.