Sea snakes, best known from shallow tropical waters, have been seen swimming at 250 meters in the deep-sea ‘twilight zone.

Credit: Image credit: INPEX-operated Ichthys LNG Project

“Sea snakes were thought to only dive between a maximum of 50 to 100 metres because they need to regularly swim to the sea surface to breathe air, so we were very surprised to find them so deep,” says Dr Jenna Crowe-Riddell, lead author of the study.

Footage of a sea snake swimming at 245 metres deep has been given to University of Adelaide by INPEX Australia, operating in the Browse Basin off the Kimberley coast. Both snakes appeared to belong to the same species.

“We have known for a long time that sea snakes can cope with diving sickness known as ‘the bends’ using gas exchange through their skin,” says Dr Crowe-Riddell. “But I never suspected that this ability allows sea snakes to dive to deep-sea habitats.”

Oceanic depths between 200 and 1000 metres are sometimes called the ‘twilight zone’ because only a small amount of light reaches that depth.

“In some of the footage the snake is looking for food by poking its head into burrows in the sandy sea floor, but we don’t know what type of fish they’re eating or how they sense them in the dark,” she says.

The snakes were filmed in 2014 and 2017 using a remotely operated vehicle or ‘ROV’.