Sea snakes have to change to keep seeing

Sea snakes have been evolving ever since they entered the marine environment 15 million years ago to survive in changing light conditions, according to a new study.

They have had to continually modify genetically to allow them to continue to see prey and predators deep below the surface, says a team from the UK, Australia, Denmark, Bangladesh and Canada in a paper in the journal Current Biology.

And in an unexpected twist, the researchers suggest that diving sea snakes share their adaptive properties not with other snakes or marine mammals, but with some fruit-eating primates.

“Our study also shows that snake and mammal vision has evolved very differently in the transition from land to sea,” says lead author Bruno Simões from the University of Plymouth, UK.  

“Sea snakes have retained or expanded their colour vision compared to their terrestrial relatives, whereas pinnipeds and cetaceans underwent a further reduction in the dimensions of their colour vision.”

The researchers say that despite being descended from highly visual lizards, snakes have limited (often two-tone) colour vision, attributed to the dim-light lifestyle of their early snake ancestors.

However, the living species of front-fanged and venomous elapids are ecologically very diverse, with around 300 terrestrial species (such as cobras, coral snakes and taipans) and 63 fully marine sea snakes.

To try and establish how this diversity occurred, they analysed various species of terrestrial and sea snakes from sources including fieldwork in Asia and Australia and historical museum collections.

They investigated the evolution of spectral sensitivity in elapids by analysing their opsin genes (which produce visual pigments that are responsible for sensitivity to ultra-violet and visible light), retinal photoreceptors and eye lenses.

The results showed that sea snakes underwent rapid adaptive diversification of their visual pigments when compared with their terrestrial and amphibious relatives.

In one specific example, a particular lineage of sea snake expanded its UV-Blue sensitivity. Sea snakes forage on the sea floor in depths exceeding 80 metres, yet must swim to the surface to breathe at least once every few hours. This expanded UV-Blue sensitivity helps them see in the variable light conditions of the ocean water column.

Also, most vertebrates have pairs of chromosomes resulting in two copies of the same genes. In some fruit-eating primates, the two copies might be slightly different (alleles) resulting in visual pigments with different spectral properties, expanding their colour vision. This suggests, the researchers say that some sea snakes used the same mechanism to expand their underwater vision with both UV sensitive and blue-sensitive alleles.

An olive sea snake surfacing to breathe off the coast of Western Australia. Credit: Bruno Simões, University of Plymouth