Some animals have markings resembling a second eye on their hindquarters to trick predators into attacking the wrong end. Australian sea snakes have instead developed a light detector on their tail that can perform the eye's most basic function, helping them move to safety. A small subgroup of sea snakes have been found to be the only reptiles with the trait.

Decades ago, scuba divers on night dives noticed that when they shone their torches on olive sea snakes' (Aipysurus laevis) paddle-like tails the snakes would move their tails away. A 1990 study confirmed the effect was real, with the sea snakes being able to detect light with their tails. However, Dr Kate Sanders of the University of Adelaide told IFLScience no follow-up work had been done since.

“The olive sea snake was the only reptile, out of more than 10,000 reptile species, that was known to respond to light on the skin in this way,” PhD student Jenna Crowe-Riddell said in a statement. 

Sanders and Crowe-Riddell have addressed this with a paper in Molecular Ecology. Besides confirming A. laevis' capacity, they tested a variety of other sea snakes and found two other members of the Aipysurus genus have similar light-detecting patches, while five more distantly related sea snakes don't.

The pair, along with several co-authors, found the detection relies on melanopsin, a light-sensitive protein, while several genes enable the processing of light into information relayed to the snakes' nervous systems.

The snakes evade their predators by hiding under coral outcrops. “Because sea snakes have long bodies, the tail-paddle is a large distance from the head, so benefits from having a light-sense ability of its own,” Crowe-Riddell said. Without it, they might not realize when their tail was dangerously exposed.

Melanopsin is far from unique to sea snakes. Humans use it for regulating sleep cycles, while it assists some amphibians' camouflage. The melanopsin detects light falling on certain frogs' bodies, which then adjust their color to suit. However, Sanders told IFLScience that, aside from the most primitive vertebrate, hagfish and lampreys, Aipysurus appears to be the only vertebrate that moves in response to light detected other than through the eyes.

The authors think the light-detecting tails evolved just once, but expect it to have survived in all six species that descend from this common ancestor. Sanders said it is not clear if this reflected a chance event, or if something about the Aipysurus ecological niche makes tail hiding particularly useful.

Disappointingly, Sanders said, there is no sign of the light-detecting tails evolving further in any of the species studied, so there's no reason to expect there might one day be sea snakes with functioning eyes on each end of their body.