Sea snakes and burrowing eels

Sea snakes are fascinating animals. There were a number of dead ones pickled in jars in the old Northcote Science Building of the University of Hong Kong. They had been gathered from the nets of the fishing fleet or the fishery research vessels. It was difficult to make out the form and original coloration of the pickled specimens. The most interesting, of which there were photographs, was one with a tiny head, thin neck but wider body. I now realise it was the Slender Sea Snake, Hydrophis, now Microcephalophis, gracilis, and could well have been the same preserved specimen used as an illustration in Hong Kong Amphibians and Reptiles. The timing is right since it was the only specimen caught in Hong Kong waters—in 1963 in Deep Bay.

The preserved specimen of the Slender Sea Snake used in
Hong Kong Reptiles and Amphibians. I have added the
red ellipse to show the very small head and thin neck

But why the small head and thin neck in many species of sea snake? The old idea, which turned out to be a ‘just-so’ story, was included on the short section on sea snakes (there was not a single photograph) in Schmidt & Inger’s 1957 book, Living Reptiles of the World:

It is remarkable that some of the longest of the sea snakes have a small head and slender neck and anterior half of the body, with a bulky abdominal portion that is very much larger in diameter. This curious body form, which recalls that of the extinct marine plesiosaurs, seems to be associated with the mechanical requirements for striking at prey in the water. Without any fixed fulcrum from which to launch its stroke, the free-swimming venomous snake makes use of the inertia of the heavy abdomen, while the great resistance of the water is made less by the slenderness of the head and neck…

Later, Harold and Helen Voris of the Field Museum in Chicago examined data on what species of fish the various sea snakes prey. They found that a large number of species eat eels. Subsequently it was found that the snakes with a small head feed on burrowing eels and gobies, and those with a very small head feed virtually exclusively on burrowing eels. Divers have also reported seeing some species of sea snake with their heads in a burrow. Therefore, it has become clear that the small-head-thin-neck type of sea snake is adapted to entering the long burrows of eels and emerging with a meal.

Along with their various physiological adaptations to life at sea, which I will not go into further here,  and the specialized feeding habits—including species which feed exclusively on fish eggs—a significant feature of sea snakes is the number of species within a relatively small geographical area. A recent paper* relates feeding on burrowing eels to the rapid increase in the number of species. The pursuit of a previously untapped food source has been well established in other animals as a trait that would be favourably selected and, therefore, act as a driver of speciation.

From*. I have added a red ellipse to highlight the differences in the width of the head and neck

There are more than 60 species of sea snake. A rapid speciation, accounting for 60% of known species—the fastest known amongst reptiles—occurred from about 7.5 million years ago. The change in body form associated with feeding on burrowing eels appears to have occurred in six or seven of the lineages determined by genomic analysis. In other words, there is strong evidence of convergent evolution.

The authors conclude:

Our study has revealed that trophic specialization has had a strong influence on body morphology in sea snakes, and this relationship is predominantly driven by the convergent evolution of microcephalic burrowing eel specialists. Dietary specialization appears to invoke strong selective pressures that manifest as predictable and rapid morphological changes. Future studies are needed to examine the genetic and developmental mechanisms underlying these dramatic body shape changes and address their role in speciation. 

Then I started to think about implications for how the small-headed sea snakes feed. Let’s assume the diameter of the neck equals the diameter of the eel’s burrow. If the snake swallows the eel (the diameter of the burrow) while in the burrow its neck will be twice the diameter of the burrow and it could get stuck. Therefore, is it perhaps more likely that the snake pulls the eel out of the burrow before swallowing it? But, if alive, the eel would stand a chance of escaping, so is that why the venom of these sea snakes is so powerful? To ensure that the eel is dead before being pulled from its burrow and to enable a quick kill by a snake that must swim to the surface in order to breathe? Alternatively, perhaps the snake only enters burrows that are wider than its neck. Then it could, perhaps ingest its prey without getting stuck. But a large eel in a large hole might then be too big to be swallowed by the small head (the maximum width of prey has been found to 1.5 times that of the neck)? Divers or remote cameras watching how these small-headed sea snakes feed may be the only ways of providing an answer.

The dangers of a snake swallowing its prey with its neck in a confined space was brought home to us on Boxing Day 1966 in Hong Kong. We were walking along Conduit Path a short distance from the top of University Drive when we found a dead rat snake with its head in a small hole in the bank. A good pull was needed to extract the snake. It was in process of swallowing a toad and the swollen throat had jammed the snake plus toad in the entrance to the hole. We could only assume that the oxygen within the small hole had run out and the snake had asphyxiated itself as it tried to retreat. Unwisely, with hindsight, we left the snake plus toad on the bank of earth, intending to pick it up on the way back from our walk and preserve it in the lab. But it had gone; some scavenger had made off with it.

*Sherratt E, Rasmussen AR, Sanders KL. 2018. Trophic specialization drives morphological evolution in sea snakes. Royal Society Open Science 5, 172141. http://dx.doi.org/10.1098/rsos.172141 

Karsen SJ, Lau M W-N, Bogadek A. 1998. Hong Kong Amphibians and Reptiles. Second Edition. Hong Kong: Provisional Urban Council.

Peaker M, Peaker SJ. 1968. Death of a snake while swallowing prey. British Journal of Herpetology 4, 38-39. 

Schmidt KP, Inger RF. 1957. Living Reptiles of the World. London: Hamish Hamilton.

Voris HK, Voris HH. 1983. Feeding strategies in marine snakes: an analysis of evolutionary, morphological, behavioral and ecological relationships. American Zoologist 23, 411-425.