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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How awesome! Isn't evolution grand??

Yes Gwen,yes it is. :)

By Handsome Jack (not verified) on 18 Nov 2012 #permalink

The above commenters wouldn't happen to be Gwen Cooper & Jack Harkness, would they?

You just made my day Artor.

use it

By Spellcheck (not verified) on 20 Nov 2012 #permalink

Checkmate, non-atheists!

"Only one species seems to be able to move on land at all..."
"They seem to all be venomous..."
"and a certain number of people are bitten..."
"How many people get bitten by them? Hard to say..." "What is the meaning of that number? Hard to say..."

Can we be a bit more definitive in our facts in future.

Jmac0585: I would love to be. You have identified a literary device called cynicism! When I went to find out how frequently people were bitten and killed by these snakes, I found what I gave you here. It is pretty piss poor data. I would love to have better information!

If you have better information on the frequency of bites and the morbidity and mortality rates, please share it.

Thanks for your comment.

hard to say hard to say?

By jack swanson (not verified) on 20 Nov 2012 #permalink

Evolution smution- his holiness the flying spaghetti monster designed them both on his etch a sketch in the time that was before time- known by pastafarians as "before lunch".

You only mention sea snakes around Australia & Indonesia but many years ago I ran into one while snorkeling off the Canaries. Fortunately it did not bite me, hence I am still alive & kicking!

Do you think the two sea snake species are closely enough related that we should be talking parallel rather than convergent evolution? I'm not sure where the dividing line is/should be.

By Jim Thomerson (not verified) on 20 Nov 2012 #permalink

There is a very important question that needs to be considered here: what definition of "species" is the author using? There are so many different definitions of what makes one species different from another. Some might say that if the two snakes can mate efficiently and produce viable fertile offspring, then they are the same species. Others might confuse difference in allele frequency as different species when they really are the same--(this is like saying that African Americans and Asians are different species because one is typically tall and the other short).

That being said, there isn't really much evidence here of convergent evolution. Heck...this could be an example of divergent evolution in action! Either way, the species definition is crucial in exploring if this is even a difference in species; and if there is not difference in species, there is no convergent evolution here.

Matthew, good question.

Prior to this research, all the snakes in this sample would have been attributed to one species. With this analysis, other previously identified species are now known to intervene in the phylogeny between two groups of the original species. That is the key point.

It may be that the species concept would be challenged by examining the larger group of sea snakes. It could be that they grade into each other, as the socially and linguistically constructed "races" of humans you mention do. While that may be true of some sea snake "species" I don't think this is generally considered to be the case.

Having said that, imagine that all the sea snakes that are members of this larger group (in the phylogenetic diagram above) were considered to be simply different subspecies or populations. It would still be the case that what was once considered a single monophyletic group isn't.

Humans are the convergence of pig, dolphin, chimpanzee, and ape.

I woke up this morning and found a porsche in my parking lot. I discovered a storm was responsible for assembling this beast. It started off with a little steel and lightning and then.... Lo and behold. One of the many miracles of evolution.

So you *willingly* posted this to Reddit?

By Pitchguest (not verified) on 20 Nov 2012 #permalink

Well ... it's just ... don't you think you're making it too easy for them?

I mean, it is quite badly written. No offense.

By Pitchguest (not verified) on 20 Nov 2012 #permalink

Pitchguest, I disagree. The post is well written, informative, and the research is very interesting.

Further proof evolution is a lie.

Evolution shouldn't even be a debate anymore. But convergence has me on the fence. A donky + horse= ass. But asses can't produce offspring. It's strange to me that given this trait of these similar species, that it doesn't hold true with others. Then again; just because that example doesn't work, others won't. Need more science to convince me.

scary snakes. I won't wish to spend a vacation in Australia but I'll not go near to the beach.

I refreshed my memory on parallel and convergent evolution. It is somewhat of a continuum, and dropping the term parallel evolution has been cogently suggested. What convergent evolution means is evolution of similar characters, not the result of close relationship. Streamline body form in a tuna and a porpoise is a nice example of convergent evolution in very distantly related organisms. Similarities due to convergent evolution are not evidence of close relationship. Here is a case where biologists have been fooled by convergent evolution into thinking two fairly distantly related snakes were the same species. Hooray for DNA!

By Jim Thomerson (not verified) on 21 Nov 2012 #permalink

A recent red tide kill in the gulf of mexico , washed ashore a very small snake as this beaded sea snake . Interesting i have just confirmed that they are hear in the united states . possible natural immiagration were there areas are much larger than thought to be existed . perplexing , however a friend that introduced this theorm introduced me to the white pelican often associated with africa . You tube also pointed out other snakes such as annacondas . I have seen one in the water that connects to the gulf of mexico . However with modern transportation customs and immigration people must consider having ships clean from objects that may live aboard such as the european ship rat . that also has been living along the gulf of mexico . noccrutrnal but also a facinating find to say the least . thank you greg and by the way were is my wife .