Why does the pelagic sea snake dive?


Note: It is the end of the year and this blog is going to be in hibernation for a short time. So this post may be the last for several weeks. But it will be active again in the near future.

Species that forage exclusively at the sea’s surface but spend much of the rest of their time submerged could offer a rare opportunity to shed light on the evolution of diving behavior that is independent from foraging. The viviparous Yellow-bellied Sea Snake, Hydrophis (formerly Pelamis) platurus (Hydrophiinae), also known as the Pelagic Sea Snake, provides this opportunity.

In a recent paper in Animal Behaviour, Cook and Brischoux (2014) note the Pelagic Sea Snake drifts passively with surface and subsurface currents, spending its entire life cycle at sea. The result is a wide distribution covering the entire tropical Indo-Pacific basin, one of the largest distributions of any squamate reptile. Another remarkable feature of the Pelagic Sea Snake’s unusual life history, is it spends most of its day-to-day life floating in the water column 20 to 50 m deep. Submergence time is interrupted by surfacing, which can be brief to breathe or longer to forage. The foraging strategy of the Pelagic Sea Snake is remarkable for a marine tetrapod, it ambushes larval fish that are concentrated under debris on oceanic labile features such as slicks or drift lines, doing ‘float-and-wait’ foraging at the oceanic surface.  

Hydrophis platurus is the only marine tetrapod foraging specifically at the ocean surface, but spending a considerable proportion of its time budget submerged. An activity pattern offering a unique opportunity to study diving independently from foraging.

The authors found the Pelagic Sea Snake spends 95% of its time underwater, where it can dive to 50 m and stay for 3.5 hours 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.

The pelagic sea snakes dive for multiple reasons, but the primary reason seems to be to avoid sea surface turbulence. Underwater, they can reduce metabolism by targeting cooler water layers. And by hovering in the water column, they reduce energy expenditure and escape both surface and bottom predators. At the same time they can more easily locating their own prey from underneath.

A detailed analysis of the diving behavior of H. platurus shows how this exclusively marine species of tetrapod manages its dive cycle and the influence environmental parameters have upon its diving and surfacing behaviors. This has opened the door to a better understanding of the adaptations developed by this species.

Interestingly, there is an important parallel in behavior between H. platurus and several species of marine turtles. Adaptations in both these lineages of reptiles reflect a response to pressures of the marine environment experienced during the evolutionary transition from terrestrial to oceanic life. Unfortunately, the behavior of sea snakes at sea is still inadequately known compared to that of marine turtles despite being a highly diversified group comprising four families and ca. 90 species.

The authors suggest the study of sea snakes can help interpret diving behavior in other lineages of marine reptiles.

Citation
Cook TR & Brischoux F (2014). Why does the only ‘planktonic tetrapod’dive? Determinants of diving behaviour in a marine ectotherm. Animal Behaviour, 98, 113-123.