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Snakes On A Boat! How Do Sea Snakes Stay Hydrated During Long Ocean Trips?

Lillywhite HB, Sheehy CM, III, Sandfoss MR, Crowe-Riddell J, Grech A (2019) Drinking by sea snakes from oceanic freshwater lenses at first rainfall ending seasonal drought. PLoS ONE 14(2): e0212099. https://doi.org/10.1371/journal. pone.0212099

Sea snakes are awesome animals. Evolved from terrestrial ancestors, modern sea snakes have developed a number of adaptations to survive in the harsh environments of our tropical oceans. They have flattened tails and smooth bodies to allow for easier swimming, extra long lungs for efficient gas exchange, and nose flaps that prevent water from leaking into their lungs while submerged. They also happen to include some of the most venomous snake species on Earth, a legacy from their Australasian ancestry (don’t worry though, sea snakes are ironically docile for the massive punch their venom glands can pack!)

The Yellow-bellied Sea Snake, Hydrophis platurus, in Costa Rica. by Aloaiza

Though they live a largely aquatic life, sea snakes are air-breathers and are often seen on or near land where they may be hunting or breeding in shallow reefs, or have become stranded on the beach. One species, however, is known for its lengthy trips into the open ocean.

The Yellow-bellied Sea Snake (or Hydrophis platurus as it’s recently been dubbed) is known to spend months at sea, hunting for fish in drifting debris rafts that float across our oceans. In many ways, the open ocean is like a desert, and the apparent willingness of Yellow-bellied Sea Snakes to enter this rough environment has baffled scientists. Because sea snakes have specialized glands in their heads that allow for salt excretion, it has always been assumed that they simply drink seawater if they can’t find a terrestrial freshwater source.

But, Yellow-bellied and other sea snakes have been shown to dehydrate when at sea for long periods, becoming weak and losing weight. This shouldn’t be the case if they can refresh with a gulp of seawater.

Some scientists have suggested that sea snakes might get their drinking water from a short-lived freshwater “lens” that forms on top of the saltier ocean water shortly after intense rain storms. For a uniquely pelagic snake like the Yellow-bellied, drinking from a temporary lens in the middle of the ocean might be the only way to get freshwater.

Cast away seaweeds and other debris attract fish looking for shelter and make great open ocean hunting grounds for sea snakes. by John Martin Davies

To investigate, researchers from the University of Florida and James Cook University traveled to Costa Rica at the end of the annual dry season to find Yellow-bellied Sea Snakes. Then they offered them a drink!

Lillywhite’s team captured just under 100 snakes from the Gulfo de Papagayo off Costa Rica, where the Yellow-bellied Sea Snake population is thought to be healthy and, crucially for the scientists, don’t move around much. To assess the condition of the snakes, the researchers weighed freshly-caught specimens, then placed them in containers with fresh water overnight, and reweighed them in the morning. If the snake had taken a drink, it would be reflected in a change in their weight.

Before the seasonal rains began, Lillywhite’s team found that 80% of snakes increased in weight during their stay in the lab, indicating that they took a drink within a short time of being captured. When the rain started, only 13% of captured snakes took the offered drink.

Range of the Yellow-bellied seasnake, by Darekk2 with data from the International Union for Conservation of Nature (IUCN) 2010. Pelamis platura. In: IUCN 2015. The IUCN Red List of Threatened Species. Version 2015.2. http://www.iucnredlist.org. Downloaded on 19 July 2015.

This apparent difference in thirst suggests that the snakes are in fact drinking freshwater at the lens formed during wet season rains. This is big news for herpetologists, as sea snake behaviour is still poorly understood. It’s also important information for the on-going management of sea snakes and opens up questions about the future of Yellow-bellied Sea Snakes under climate change. Do they just hope they happen upon a rainstorm before dehydrating completely, or do they have a way of finding those narrow freshwater lenses? Do they follow the rains? And why didn’t that remaining 20% of snakes captured in Lillywhite’s experiment take a drink despite the fact that they were clearly dehydrated? Are sea snakes so accustomed to being dehydrated that they don’t feel thirst, and how will this affect their search for fresh water in a warming ocean?

Stay tuned!



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