Sea Snakes Are Taking Refuge in the Twilight Zone and No One Knows Why

Scientists have found that sea snakes are living in the twilight zone of the ocean.

The reptiles, including coral-reef snakes, and the yellow-lipped sea krait, mostly live across the tropical waters of the western Pacific and Indian Ocean, but their populations in reefs across the Indo-Pacific have been declining.

Australian scientists set out to discover what could be causing this. Before their study, Sec. Marine Ecosystem Ecology, surveys of the species occurred only on shallow reefs, despite many living in the depths of the ocean.

Scientists analyzed underwater footage from Ashmore Reef—a previous sea-snake hotspot in northwestern Australia that has seen rapid declines of the species over the last two decades—to assess factors that could have contributed to their decline.

The scientists also used video surveys to explore what the species were doing far below the ocean in the mesophotic zone—164 ft to 492 feet below the surface.

Conrad Speed, co-author, marine ecologist and postdoctoral researcher at the Australian Institute of Marine Science, told Newsweek that this is where they found one of their most exciting discoveries.

"One of the most exciting discoveries was the abundance and diversity of sea snakes living at these depths, which had previously been undocumented," said Speed.

"This is of particular importance because five of the species—-Dubois, spotted, King's, Western turtle-headed, and the short-nosed sea snake—had not been observed for a decade or more in the shallow waters around Ashmore Reef," he said.

In the study, scientists said this deep-water habitat may be providing the sea snakes with a "possible refuge," from other species such as sharks.

Speed said that sharks living in shallow-water reefs, such as tiger sharks, are known to prey on sea snakes, but this hypothesis will need more research.

"We have seen an increase in shark numbers at Ashmore Reef over the past decade, possibly due to improved enforcement of the Marine Park.

"It's possible that the population recovery of sharks that are known to predate on sea snakes has influenced the decline in sea-snake numbers in the shallows, although other factors are also likely to have influenced this decline. Additional data need to be collected to confirm this hypothesis," Speed said.

Despite the rediscovery of sea-snake species lying deep below the surface of the ocean, the study says that their disappearance from Ashmore Reef remains concerning.

From the data analyzed, scientists said that the species has declined from the reef by 90 percent since 2000.

What remains a mystery is how the sea snakes travel back up the surface to breathe.

"Many species of sea snake are known to spend time in these deep waters, although the exact importance to their populations is unknown. Like their ancestors on land, sea snakes are air-breathers and therefore need to surface from time to time to breathe," Speed said.

"How much time they spend at depth remains a mystery, although our initial results suggest they may seek some refuge in the deeper waters."

The initial findings that the snakes live in the mesophotic zone can help guide further research into the species, and their decline in certain areas.

"Our findings suggest that some species of sea snake may routinely use the mesophotic zone," said Speed. "This is important information that can guide the design of future surveys, which would ideally include shallow and deep waters, to provide a more complete representation of the habitat used by sea snakes."