Tag Archives: evolutionary convergence

A Remarkable Convergence of Species: The Deadliest Sea Snake

ResearchBlogging.orgSea snakes are true snakes that look a little like eels because of their horizontally flattened rudder-like tails, and they spend a lot of time…for most species, their entire lives…in the ocean. Only one species seems to be able to move on land at all. They seem to all be venomous, some extremely so. They are all tropical or near-tropical, and there are numerous species distributed among about 15 genera.

One species is Enhyrina schistosa, known as the Beaked Sea Snake, or the Hook-Nosed Sea Snake. It lives in the waters near Indonesia and Australia. This is known to be the most venomous of all of the sea snakes, and a certain number of people are bitten by them. In fact, most people who die of sea snake bites were bitten by Enhyrina schistosa. How many people get bitten by them? Hard to say. In Australia, between 1942 and 1950, 56 people died from sea snake bites. What is the meaning of that number? Hard to say; it is just one of those esoteric bits of information one finds in Wikipedia. These snakes probably don’t bite very many people, but when they do, you have a problem.

The Beaked Sea Snake feeds mainly on spiny catfish and blow fish, and as such benefits from have a large gape. Selection for the large gape has altered the morphology of this snake in a way that probably contributes to it’s beaked nose and a couple of other features that are used to distinguish it from other sea snakes and thus identify it to species. The problem is, this selection pressure seems to have caused two distinctly different groups of snakes (actually, three … see below) to converge on a single morphology. So, what we have been calling Enhyrina schistosa, the beaked sea snake, is clearly two distinct species that look enough alike to have been confused as one. This is destine to be a classic example of evolutionary convergence.

This is all being reported in a paper due out soon in Molecular Phylogenetics & Evolution by Kanishka D.B. Ukuwela, Anslem de Silva, Mumpuni, Bryan G. Fry, Michael S.Y. Lee and Kate L. Sanders. Caroline Bird of the University of Queensland Communications Office provided some background and the great snake picture.

From the abstract of the paper:

We present a striking case of phenotypic convergence within the speciose and taxonomically unstable Hydrophis group of viviparous sea snakes. Enhydrina schistosa, the ‘beaked sea snake’, is abundant in coastal and inshore habitats throughout the Asian and Australian regions … Analyses of five independent mitochondrial and nuclear loci for populations spanning Australia, Indonesia and Sri Lanka indicate that this ‘species’ actually consists of two distinct lineages in Asia and Australia that are not closest relatives. As a result, Australian ‘‘E. schistosa’’ are elevated to species status and provisionally referred to Enhydrina zweifeli. … Our findings have important implications for snake bite management in light of the medical importance of beaked sea snakes and the fact that the only sea snake anti-venom available is raised against Malaysian E. schistosa.

Have a look at this diagram:

Fig. 4. Bayesian multi-locus coalescent species tree. Asian and Australian Enhydrina schistosa lineages form separate and distantly-related clades (each with affinities to geographically proximate taxa). Nodes with Bayesian posterior probability >0.9 are indicated. Outgroup Hemiaspis damelii is not shown. (Scale bar = substitutions per site).

You can see the two populations, from Australia (top) and Southeast Asia (bottom), separated by numerous other species that look very different. And, if you look at just the Southeast Asian group, they cluster into two subgroups as well. Apparently this genetic divergence and grouping was not noticed by prior researcher dividing the snakes up into species and genera on the basis of morphology. Having said that, it is also true that the sea snakes are a bit dicy in their overall phylogeny, and are understudied. This, apparently, is being rectified.

There are other potential explanations for this pattern that should be considered, involving the genetics. It is possible to come up with a genetic tree that inaccurately represents the actual phylogeny of the species at hand. This study, however, used multiple methods and multiple DNA sites, involving both mitochondrial and nucleic DNA, so the species tree you see here is probably reasonably close to accurate, and the conclusion that Enhydrina schistosa consists of two groups that are not monophyletic is strong.

“This mixup could have been medically catastrophic, since the CSL sea snake antivenom is made using the venom from the Asian snake based on the assumption that it was the same species,” noted Bryan Fry, one of the study’s authors. “Luckily, the antivenom is not only very effective against the Australian new species but actually against all sea snakes since they all share a very stream-lined fish-specific venom.”

Wear a wet suit!

Ukuwela, K., de Silva, A., Mumpuni, ., Fry, B., Lee, M., & Sanders, K. (2012). Molecular evidence that the deadliest sea snake Enhydrina schistosa (Elapidae: Hydrophiinae) consists of two convergent species Molecular Phylogenetics and Evolution DOI: 10.1016/j.ympev.2012.09.031

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