Key points
- 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.
- Planes, the only genus in this family known to live on other animals, was inexplicably absent. Equally as strange, no representatives from the numerous other crab families that lived in the area were found.
- The scientists suspect they may be observing a behavior that predisposed this family of crabs for rafting and eventually to life on other animals.
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.
Pfaller wondered whether the crab he’d noticed had been a resident on the snake before it was evicted. But barnacles and other stationary animals were basically the only things known to live on sea snakes. None other than Charles Darwin documented this when he wrote about an unknown type of barnacle a colleague sent him that had been found growing on a sea snake near Borneo. In 1975, scientists reported finding two types of normally stationary, tube-dwelling worms attached to a Dubois’ sea snake (Aipysurus duboisii). The snake also carried a moss animal, a type of sedentary colonial organism, all of which was unusual enough that the scientists suggested there was something odd about the snake’s behavior that had attracted such unlikely freeloaders. No reports of crabs or anything else with legs had ever been reported hanging out on a sea snake.
But the more sea snakes Pfaller collected, 371 in all, the more crabs and other hitchhikers he found. Snakes hosted the occasional crab, shrimp, snail and even a small clingfish. With so long and conspicuous a list of creatures, the only explanation for the total absence of any mention of them in the scientific literature seems to be that they were simply overlooked. Given the striking appearance of yellow-bellied sea snakes, their strange distribution and the unique behavioral adaptations they’ve evolved for living in the sea, it’s understandable that they’d be more attention grabbing than the many hangers on that come with them.
The crabs Pfaller had found were mostly megalopae, a term for the developmental stage in crabs that exists between that of the larva and the adult. These stages of a crab’s lifecycle can look radically different from each other. Comparatively little information exists on the anatomical diversity of megalopae, which meant Pfaller couldn’t identify them at the time. But he did find a few juvenile crabs as well, and among them was an individual of the genus Planes.
“I knew Planes from my turtle work, so I thought the megalopae might be the larval stage of Planes,” he said.
Pfaller preserved and deposited the sea snake epibionts at the Florida Museum of Natural History. But after the expedition, he shifted work to his dissertation research on sea turtles, and the dozens of megalopae he’d collected went unidentified.
A decade later, Dr. Pfaller was working as a courtesy faculty member at the University of Florida and happened to mention the old sea snake project to crab expert Robert Lasley, then a postdoctoral researcher at the Florida Museum. Before he could finish describing the results, Lasley interrupted.
“‘Wait! Wait! Wait! Are you saying you have planes on a snake?’” Pfaller recounted. Lasley was alluding to the cult classic “Snakes on a Plane” starring Samuel L. Jackson. The play on words was too good to pass up. “That’s when we decided, alright, no matter what happens, we have to do this.”
To accurately determine the identity of the crabs, they’d need to sequence DNA from the specimens Pfaller collected back in 2012. DNA sequencing is the sort of delicate but routine work that budding scientists can cut their teeth on. A new undergraduate student at UF, Soma Elefánti, had just joined the invertebrate zoology lab Lasley worked in, and Lasley used the crab project as an opportunity to show him the ropes.
But when Soma had finished the work and they’d identified the crabs, they were flummoxed.
“Honestly, the thought that we could name this study planes on a snake was enough to get me motivated, but when we got the results, it was evident that something weird and interesting was going on here,” said Lasley, who is now an assistant professor and curator of Invertebrate Zoology at the University of Guam.
There are two crab species in the genus Planes, and both are oceanic rafters. In addition to sea turtles, they’ve been found living on seaweed, driftwood, feathers, tarballs, plastic, explosive mines, buoys, pumice and all other manner of flotsam with a suitable surface area.
Given their propensity for making a home out of anything that floats and the fact that they’d found a juvenile Planes on one of the sea snakes, it made sense to expect that at least some — if not all — of the cryptic megalopae would also be Planes. But, as it turned out, none of them were.
“They were crab species that are not rafters,” Pfaller said. “In fact, the adults don’t even live close to the open ocean and don’t raft. And they’re all in the Grapsidae family. There are also dozens of other crab species from several other families living along the coast of Costa Rica, but we only found species from one family – Grapsidae.”
The three species were the Sally lightfoot crab (Grapsus grapsus), whose adults live on and around rocks along shorelines; the racer mangrove crab (Goniopsis pulchra), which lives exactly where you’d expect; and the mottled shore crab (Pachygrapsus socius), which is equally at home in a variety of environments between the high- and low-tide line.
So why, then, were the megalopae of these crabs hanging out on sea snakes in open water when the adults they ultimately turn into live on the shore?
The answer to that question requires a short review of the crab lifecycle.
Most crabs, even those that live on land, release their larvae into the water. Crab larvae, which Lasley described as swimming balls with beaks, spend a lot of time looking around for something to eat and — as often as not — being eaten. Those that survive morph into megalopae. During this stage in their lifecycle, crabs go off in search of the environment they’ll live in as adults. Typically, this means a developing crab floating around in the open ocean will find somewhere on the seafloor or along the shore to call home.
“They can swim well, but these things are also built to climb or settle onto something,” Lasley said.
Crabs that live on or near land but are born at sea must make their journey to shore at this stage. Ocean currents do much of the work for them. As a result, the megalopae of certain crab species show up around drift lines, areas where two downwelling currents run into each other.
A lot of other marine organisms and other floating objects also accumulate and meet in this liminal space. Megalopae making their way shoreward might stand a better chance at arriving if they latch onto a piece of debris and wait for it to get washed up on the beach. This may be even more effective if there’s a river nearby, because it provides more debris, and during high tide, much of that debris gets swept back into the river with the inrush of seawater.
It’s also possible that some megalopae just like resting on objects to take a break from swimming, whether or not this gets them any closer to their destination. Scientists don’t know enough about crab behavior at this developmental stage to be sure.
What is certain, though, is that sea snakes also favor slick lines. They’re one of the best places to find food.
“So, likely what happens is the crabs are coming out of the plankton, grabbing a hold of something, and possibly hoping that they get pulled into a river or cast on shore. The snakes are just there, and the crabs don’t know the difference,” Pfaller said.
This misapprehension is unfortunate for the crabs because yellow-bellied sea snakes avoid getting too close to shore. This makes it unlikely that any crabs that stay attached to them will survive to adulthood.
“It’s not a good idea. These crabs have made a poor decision,” Lasley said.
That was one mystery hypothetically solved, but it still didn’t explain why crabs in the family Grapsidae were the only ones that exhibited this behavior. Fiddler crabs, ghost crabs and others also live along Costa Rica’s shores, but none of them seemed inclined to crawl aboard sea snakes. It also didn’t explain why crabs in the genus Planes, which are all obligate rafters, weren’t on the manifest.
Planes crabs are also in the family Grapsidae. At first, this might make it seem even more strange that their megalopae don’t show up on sea snakes. However, sea snakes don’t actually make good habitat for crabs. Unlike sea turtles, they don’t have cozy pockets for crabs to hide away in or a broad shell on which grows algae and various marine animals that Planes crabs eat. And, of course, sea snakes shed their skin periodically. Turtles do this too, but their skin breaks apart into small flakes — similar to the way human skin sheds — rather than the long, continuous sheets that snakes produce.
In short, the authors reason that Planes crabs know better than to crawl up onto sea snakes. Other grapsid crabs that are just trying to get to land aren’t, however, as judicious.
Why, then, were other non-grapsid crabs not found on sea snakes? The authors aren’t sure, but they think it might be an instance of seeing evolution in action at the family level.
“Clearly, something about being a grapsid compels you to climb,” Lasley said. The inclination to move toward the ocean’s surface and hoist themselves up onto floating objects might explain how Planes initially evolved to live on turtles.
The authors published the results of their study in the journal Ecology and Evolution.
Gustav Paulay, of the Florida Museum of Natural History, is also a co-author of the study.
Support for the study was provided in part by the National Science Foundation (grant no. GECCO 1457769).
Sources: Robert Lasley, lasleyr@triton.uog.edu;
Joseph Pfaller, jpfaller@ufl.edu;
Soma Elefánti, elefantisoma@ufl.edu
Media contact: Jerald Pinson, jpinson@flmnh.ufl.edu, 352-294-0452