Why Do Sea Snakes Keep Approaching Divers?

For years, scuba divers have reported unusual, unprovoked attacks from sea snakes. This behavior baffled scientists because land snakes prefer to avoid humans, rather than confront them. Why would their marine cousins be any different? Now, a study published in Scientific Reports last week reveals that the snakes may not be trying to attack humans at all.

"Apparent 'attacks' on divers by sea snakes are mostly due to males searching for females, and getting confused," study author and Macquarie University Department of Biological Sciences Professor Rick Shine tells Treehugger in an email. 

Snake Attacks

The snakes most often reported to "attack" divers are the highly venomous olive sea snakes (Aipysurus laevis). They are the most common sea snakes along Australia’s northern coast and nearby islands, Oceana explains. Their name comes from the yellow-green tint of their skin, and they can grow to be more than six feet long. This can make it particularly frightening for the divers who encounter them on tropical coral reefs.

"Snakes swim directly towards divers, sometimes wrapping coils around the diver's limbs and biting," the study authors write.

Shine says, however, that the snakes do not bite very often, meaning the encounters are rarely fatal. But still, “approaches are very common—and dangerous because a diver might panic."

The researchers wanted to understand the strange encounters for two reasons. First, they made very little sense from the snakes’ point of view.

"[W]hy would a free-ranging snake approach and bite a person that has not harassed it, is too large to be a prey item, and could readily be evaded in the complex three-dimensional world of a coral reef?" they asked. 

Second, understanding what motivated the attacks could help divers know how best to respond.

Mistaken Identity

To investigate the mystery, the researchers turned to a data set collected nearly 30 years ago. As a PhD student, study author Tim Lynch made a total of 188 scuba dives in the Great Barrier Reef between May of 1994 and July of 1995, according to the study and a Nature press release. During these dives, which lasted approximately 30 minutes, he would record the number of sea snakes that approached him and the details of these encounters. Every time a snake approached, he would move to the seafloor and remain still until the snake left him alone. 

That data remained unpublished until the coronavirus pandemic gave Shine, who was aware of the research, some free time. "I contacted [Lynch] and suggested we work together on publishing it," Shine tells Treehugger. 

Analyzing Lynch’s experience led the study authors to conclude that the attacks were a case of what they call "mistaken identity." They write, "For example, a reproductively active male, highly aroused, mistakes the diver for another snake (a female or a rival male)."

They drew this conclusion for several reasons.

• Sex: Male snakes were much more likely to approach divers than female snakes.
• Timing: Most of the approaches took place during the snakes' mating season, and males were more likely to approach during this time. For females, the season made no difference when it came to approaching divers. Further, Lynch recorded 13 instances when he was "charged" by a snake. All of these took place during the mating season. For males, the charges occurred after the snake had either given chase to a female or gotten into a fight with a male rival. For females, the charges occurred mostly after being chased by males.
• Behavior: Three male snakes coiled around the diver’s fin, which is something they only do during courtship. 

While it might seem strange for a snake to mistake a diver for a potential mate, the study authors argue that sea snake evolution makes it possible. Land snakes typically locate females with the help of pheromones deposited along the ground, but this type of location is more difficult underwater, where the females do not move along a solid surface and the chemicals they release are not water-soluble, meaning it would be more difficult for males to locate them from a distance. 

Further, while olive sea snakes have better vision than some other underwater snakes, they do not see as well as land snakes, and the light-scattering quality of water makes it even more difficult for them to spot females. The Turtle-headed sea snake has also been observed courting the wrong species, including human divers.

Protection Advice

The explanation provided by Lynch, Shine, and their co-author Ross Alford answers the question of what divers should do if they find a sea snake rapidly swimming their way. "Stay calm, let the snake check you out," Shine advises. "It will soon realize that you’re not a female snake, and go on its way."

But while this research focuses on how humans can protect themselves from sea snakes, sea snakes also need protection from human activity. While olive sea snakes are considered a species of least concern by the International Union for Conservation of Nature Red List, their population is decreasing. 

One main threat to the species is being accidentally caught by bottom trawling fishers. Because the snakes tend to leave the reef at night to hunt prey along the ocean floor, Oceana explains, they are more likely to be accidentally captured with bottom-dwelling fish. 

They are also reliant on the coral reef ecosystems where they make their home, meaning any threat to coral is also a threat to sea snakes. "To save them, we need to protect coral reef ecosystems from threats such as coral bleaching," Shine says. "So addressing climate change would be a good start."