Yellow-bellied sea snakes rely on freshwater to stay hydrated, which is quite a challenge for a species that ventures far from land, drifting on the Indo-Pacific ocean gyres and currents that extend from southern Africa to the Pacific coast of Central America. Sea snakes possess salt glands, and it was previously thought that these structures enabled them to drink sea water and excrete the excess salt. But yellow-bellied sea snakes become dehydrated during periods of drought, demonstrating lower body water content and poor body condition than after periods of rainfall. Therefore, biologists realized the animals may not be drinking seawater to remain hydrated.

To verify this idea, Harvey Lillywhite and collaborators studied yellow-bellied sea snakes found in the coastal waters off Guanacaste province, Costa Rica, after a period of drought in 2017. Snakes were caught with a net and then transported to a laboratory, where they were weighed before being placed in a container holding freshwater. The following day, the snakes were re-weighed and then released. The researchers continued to catch snakes for 8 days, with heavy rain occurring between Day 2 and Day 3 and continuing intermittently for the rest of the study. Most snakes (80%) caught on Day 1 and 2 drank freshwater soon after it was provided to them and gained weight overnight. Once the rains began, the proportion of snakes drinking water and gaining weight in the lab decreased to only 13% by Day 8. The biologists believe that rainfall created a freshwater layer on the ocean’s surface, from which yellow-bellied sea snakes were able to ‘binge drink’ so they no longer needed to drink water when it was provided in the lab.

The study findings help us understand the species’ biology, but also identify a future challenge for sea snakes as climate change is expected to alter rainfall patterns and result in longer and more intense droughts in their tropical habitats. Will the species’ drifting behaviour, and potentially their distribution, change as an adaptation to less freshwater from rainfall being available during their oceanic travels? Only time and further studies will help us understand if the yellow-bellied sea snake can survive a changing environment.

Further reading:

Lillywhite, H.B., C.M. Sheehy III, M.R. Sandfoss, MR, J. Crowe-Riddell & A. Grech. 2019. Drinking by sea snakes from oceanic freshwater lenses at first rainfall ending seasonal drought. PLoS ONE 14: e0212099. https://doi.org/10.1371/journal.pone.0212099.
(Title includes a line from “Rime of the Ancient Mariner” by Samuel Taylor Coleridge.)