A marine snake has evolved to see colour after losing the ability millions of years ago. It is one of only two reptiles known to have regained this type of colour vision.
Throughout evolutionary history, snakes mainly dwelt in the dimly lit undergrowth of forest habitats where they were exposed to a very limited range of colours. So, they mostly evolved to drop two of their five types of colour-seeing genes, known as opsins.
Over the last 25 million years though, more than 60 species of snakes have made themselves at home in brighter, more colourful marine environments. One of these is the venomous, blue-banded sea snake (Hydrophis cyanocinctus), which slithers around the shallow waters of tropical Australia and Asia.
Prior research had already suggested some specimens of H. cyanocinctus snakes could have the colour-seeing opsin called SWS1, but this wasn’t widespread, and which individuals have it was down to more of a “genetic lottery”, says Isaac Rossetto at the University of Adelaide in Australia.
He and his colleagues pored over the complete genomic data from five species of snakes in the Elapidae family – including the blue-banded sea snake, cobras and mambas – comparing them to see whether they consistently had this SWS1 colour gene. What they found surprised them. In what Rossetto calls an “extremely rare” turn of events, they found that H. cyanocinctus snakes have evolved to bring back four copies of this ancestral SWS1 gene.
Although scientists don’t yet know when this gene was regained and then duplicated, this suggests the snake’s ability to see colour is not just an evolutionary fluke.
“We know and appreciate colour vision, but there are almost no other snakes within all 4000 snakes that have it as good as us,” he says. “We now think that there is a group of very interesting marine snakes that do.”
Like the original, ancestral SWS1 opsin, two of these copies sense ultraviolet light, which is found in abundance at the surface of seas and oceans. The other two copies have instead evolved to sense blue light, the primary wavelength that reaches the depths, where sunshine is more attenuated and scattered. This nifty adaptation “makes sense,” says Rossetto, as these are both environments the snakes have now adapted to.
Ultimately this suggests blue-banded sea snakes are probably better than all of their snake relatives when it comes to seeing colour on prey or predators, and in their environment. Before this discovery, the only known case of this colour-vision gene among reptiles was the Helicops angulatus water snake.
Yohey Terai at the Graduate University for Advanced Studies, SOKENDAI in Japan says the discovery is exciting, but just because the genes are present doesn’t mean they are being used to see colour the way we might assume. “Genome assembly often contains errors,” he adds.
To be sure that this species of sea snake sees colour like we do, Terai says researchers need to actually test the serpents’ colour vision in experiments.