Sea Snake 'Shrunken Heads' May Have Evolved Recently

While bigger is better for some sea snakes, others seem to prefer to stay on the small side. A new study shows that some of these snakes have evolved "shrunken heads" and why they may have smaller physical features than their larger cousins.

Sea snakes are a subfamily of venomous elapid snakes that inhabit marine environments for most of their lives. They range across the globe and some of them can inflict fatal bites in humans. Two particular species, the Asian sea snake and the Australian sea snake, possess venom that is more potent than that of a cobra.

The research, published in the journal Molecular Ecology, examined how normal-shaped sea snakes could evolve shrunken heads relatively rapidly. Since sea snakes usually swallow large, spiny fish, a large head would seem indispensible to the species. Yet the new study demonstrates that under some circumstances, a large head could be more of a hindrance than a help. Some sea snakes feed by probing their front ends into narrow, sand eel burrows. Because of this, smaller heads are an advantage.

In order to further study this adaptation, Kate Sanders, Mike Lee and their team investigated the genetic differences across all sea snakes. In particular, they noticed that the blue-banded sea snake, Hydrophis cyanocinctus and the slender-necked sea snake, Hydrophis melanocephalus, were almost genetically indistinguishable. It's possible that these two species, despite being drastically different in size and shape, could have once been the same species in the not-too-distant past.

"This suggested they separated very recently from a common ancestral species and had rapidly evolved their different appearances," said Lee in a press release. "One way this could have happened is if the ancestral species was large-headed, and a population rapidly evolved small heads to probe eel burrows--and subsequently stopped interbreeding with the large-headed forms."

The scenario is certainly a possibility. Small-headed populations are usually much smaller in absolute size, which means they avoid interbreeding with large-headed ancestors. Because of this, speciation can rapidly occur.

"Our results highlight the viviparous sea snakes as a promising system for studies of speciation and adaptive radiation in marine environments," said Sanders in a press release. The study is not only relevant for sea snakes; it could hold broader implications for the study of evolution and could allow scientists to view the process in a marine environment.