Why does the only ‘planktonic tetrapod’ dive? Determinants of diving behaviour in a marine ectotherm

• We study diving behaviour and discuss reasons for diving in the pelagic sea snake.
• This snake dives constantly and, underwater, spends its time floating passively.
• It reduces energetic expenditure further by targeting cooler water layers.
• This species could dive to escape costly sea surface turbulence and predation.
• These results are useful to understand similar diving patterns in marine turtles.

Marine tetrapods represent powerful models for studying how evolutionary transitions shape the life history traits, physiology and behaviour of animals. Whereas adaptations to diving are driven primarily by the need to secure underwater food, diving in the pelagic sea snake, Hydrophis (Pelamis) platurus, seems less foraging-dependent. Indeed, this reptile captures fish while floating at the sea surface. Despite knowledge about its surface ecology, its underwater behaviour remains mostly unknown. Yet, study of this life phase is a key to understanding how and why this snake dives. In this study, we reanalysed published data on its diving behaviour. The pelagic sea snake spends 95% of its time underwater, where it can dive to 50 m and stay for 3.5 h without breathing. Dives are S-shaped, with a long phase of gradual ascent during which the snake is neutrally buoyant. Snake lungs deflate slowly during this phase at a rate that increases with water temperature, and thus metabolism. Dive duration is linked to inferred lung volume at the start of the dive, suggesting aerobic diving. We propose that pelagic sea snakes dive for multiple reasons, but primarily to avoid sea surface turbulence. Underwater, they can reduce metabolism by targeting cooler water layers. By hovering in the water column, they reduce energy expenditure and escape both surface and bottom predators, while more easily locating their own prey from underneath. Some marine turtles occasionally display S-shaped dives, but the role of such dives remains under debate. We suggest the study of sea snakes can help interpret diving behaviour in other lineages of marine reptiles.