Planes on a snake? Scientists discover crabs hitching a ride on sea snakes

In 2012, scientists working off the Pacific Coast of Costa Rica found dozens of sea snakes with crabs and other marine animals living on them. It was the first time any motile animal had been documented living on sea snakes.
The crabs were thought to be oceanic rafting crabs in the genus Planes, which are known associates of sea turtles, but they could not be confidently identified at the time. The crabs were preserved and curated at the Florida Museum of Natural History for future study.
More than a decade later, scientists sequenced DNA from these specimens and determined that the crabs were not Planes but instead belonged to three other species from the same family.

Like all other snakes, sea snakes shed their skin as they grow. But because they live near the surface of the open ocean, they don’t have the benefit of removing their old skin by slithering against a hard surface. Instead, they wriggle into a pretzel and use the friction generated by their scales to get the job done.

That’s why, according to marine herpetologist Joseph Pfaller, “When you find a sea snake’s skin, it’s usually in a knot.”

In 2012, Pfaller discovered that sea snakes might shed a lot more than just their skin when it’s time to molt. He’d just started his doctoral degree at the University of Florida and joined a research expedition led by UF professor Harvey Lillywhite to study yellow-bellied sea snakes (Hydrophis platurus) off the coast of Costa Rica.

“On almost the first snake we caught, this little crab fell off.”

Most herpetologists, he said, might have been too excited about the snake to notice the crab. But Pfaller had previously studied sea turtles and the many organisms that live on them, called epibionts. On the long list of sea turtle epibionts are crabs in the genus Planes, which live in the protective space between the turtle’s tail and the posterior portion of its shell.