Saturday 8 December 2012

A cryptic Sea Snake from Australia.

The Beaked Sea Snake Enhydrina schistosa is known from the Arabian Gulf to Southeast Asia and south to Australia and Papua New Guinea. It is distinctive, with an elongated central scale on its lower jaw, overlapping the upper jaw to form a sort of beak, used to tackle spiney Fish. The Snake has a bad reputation; its habits lead to it frequently being caught in fishing nets, where it is hard to detect. To make matters worse it is notoriously bad tempered, and has an extremely venomous bite. About 50% of all Sea Snake bites are the work of Enhydrina schistosa, and around 90% of fatalities.

The Beaked Sea Snake, Enhydrina schistosa. Avinash Shanbhag/Indian Snakes.

In a paper published in the journal Molecular Phylogenetics and Evolution on 5 October 2012, a team of scientists led by Kanishka Ukuwela of the School of Earth and Environmental Sciences at the University of Adelaide publish a study of the population genetics of Enhydrina schistosa, which suggests that the Snake is in fact two sepperate, convergently evolved species, that arose within the morphologically disticnt genus Hydrophis, the two Snakes having apparently setttled on the same form due to similar ecologies (convergent evolution).

This is an important discovery, and not just from a taxonomic point of view; there is no reason to suppose that two Snakes that have evolved similar morphologies will have evolved similar venoms, and therefore an antivenom developed to counter the venom of one Snake is unlikely to have any effect on the venom of the other. Fortunately the Snakes do not seem to have overlapping ranges, with one species found in Australian waters and the other around the coast of South Asia.

The taxonomic position of the snakes is now complicated; Ukuwela et al. report the early description of a second species in the genus Enhydrina, Enhydrina zweifeli, described from a single specimen from Papua New Guinea in 1985, but not generally recognised as a valid species by herpetologists. The Australian population of Enhydrina schistosa is reported to have fewer bands on the body than the Asian population, and lower but broadly overlapping ventral scale count; a description matching that of Enhydrina zweifeli, and Ukuwela et al. suggest this name should be used to describe the Australian population. 

However this would make the genus Enhydrina polyphyletic (having more than one origin) and the genus Hydrophis paraphyletic (not including all the species descended from the last common ancestor of the group) whereas modern taxonomy usually requires that taxomic groups be monophyletic (all descended from a common ancestor, and including all the species descended from that ancestor), so it is likely that this taxonomy will change.

The multiple origins of Enhydrina schistosa within the genus Hydrophis. Ukuwela et al. (2012).