Sea snakes that can sense light on their tails

Night divers first discovered that olive sea snakes (Aipysurus laevis) withdraw their vulnerable tails in response to light. Sea snakes live their entire lives at sea, swimming with paddle-shaped tails and resting under coral or rocky overhangs. The elongate body of sea snakes means that their tail-paddle is a large distance from the head, and so a light-sensing ability in the skin of the tail (tail photoaxis) is thought to help snakes stay hidden from sharks and other predators.

During my PhD research, we tested for light-senistive tails in eight species of sea snakes, but found that only three Aipysurus species have tail phototaxis. Judging by their phylogenetic position, we hypothesize that this unique ability probably evolved in the ancestor of just six closely-related Australian species. This is interesting because it means that out of the ~60 species of sea snake, less than 10% have tail phototaxis. Why hasn’t this trait evolved in other species?

There may be phsyiological and/or genetic constraints that make tail phototaxis a rare trait in vertebrates. Only a few vertebrates have evolved tail phototaxis including aquatic amphibians (e.g. Cryptobranchus, Necturus), hagfish and juvenile lamprey. And in the ~10,000 species of reptiles only six Aipysurus sea snakes appear to have evolved it.

To profile what genes are active in the skin of sea snakes, we used RNA-sequencing or a ‘whole transcriptome approach’ in the skin and other organs of sea snakes. We discovered some phototransduction and retinal recycling genes that are involved in converting light into information in the nervous system. One promising candidate gene for a light-sensitivity in the skin is ‘melanopsin’, this protein is used in a range of genetic pathways that are linked to sensing overall light levels around us: it is used by some animals, including humans, for regulating sleep cycles and in frogs to change their skin colour as a camouflage. Further study will be needed to target these genes before we can really understand the genetic pathways involved in this fascinating behaviour.

This is a phylogenetic tree that shows the relationships among species of sea snakes (note that some branches are condenced). The distribution of tail phototaxis is represented by colours: red branches are species that showed phototactic responses to localized white light on the tail but not the midbody, blue branches are species that were unresponsive to localized white light on both the tail and the midbody, and untested species are shown in grey. The red dot shows one of the most parisimonious inferences of when tail phototaxis evolved. The two black asterisks show key species (or lineages) that need to be tested to confirm our hypothesis on the evolution of tail phototaxis. The red asterisk shos the only previously studied species.

This is a phylogenetic tree that shows the relationships among species of sea snakes (note that some branches are condenced). The distribution of tail phototaxis is represented by colours: red branches are species that showed phototactic responses to localized white light on the tail but not the midbody, blue branches are species that were unresponsive to localized white light on both the tail and the midbody, and untested species are shown in grey. The red dot shows one of the most parisimonious inferences of when tail phototaxis evolved. The two black asterisks show key species (or lineages) that need to be tested to confirm our hypothesis on the evolution of tail phototaxis. The red asterisk shos the only previously studied species.

Read more in our publication in Molecular Ecology.

Please contact me if you’d like a copy of this paper.

Crowe-Riddell, JM, Simões, BR, Partridge, JP, Schwerdt, JG, Breen, J, Ludington, A, Gower, DG and Sanders, KL (2019) ‘Phototactic tails: Evolution and molecular basis of a novel sensory trait in sea snakes’, Molecular Ecology, 28, 2013–2028. doi: 10.1111/mec.15022.

 

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