In 2012, off the coast of Costa Rica, a tiny crab fell off a sea snake and changed the way scientists think about ocean hitchhikers. Joseph Pfaller was just starting his doctoral degree at the University of Florida, aboard an expedition led by professor Harvey Lillywhite to study yellow-bellied sea snakes near the surface of the open ocean. "On almost the first snake we caught, this little crab fell off," Pfaller remembered. Most herpetologists — scientists who study reptiles — might have been too busy admiring the snake to notice a stowaway crab. But Pfaller had previously studied sea turtles and the many creatures that live on them, and he saw something nobody else had.
Until that moment, scientists believed only stationary animals like barnacles ever lived on sea snakes. As far back as the 1800s, Charles Darwin himself wrote about an unknown barnacle found growing on a sea snake near Borneo. In 1975, researchers reported finding tube-dwelling worms and a moss animal attached to a Dubois' sea snake — creatures so unusual that scientists wondered what strange behavior had attracted them. But no living, moving, crawling creature with legs had ever been documented hitching a ride on a sea snake. Then Pfaller kept collecting.
Over the course of the study, Pfaller gathered 371 sea snakes, and the more he gathered, the more hitchhikers he found. Snakes turned out to host crabs, shrimp, snails and even a small clingfish. Most of the crabs were megalopae — the stage in a crab's life between larva and adult, which can look wildly different from the full-grown animal. Because so little is known about megalopae, Pfaller couldn't identify them at first. But he also found a few juvenile crabs, including one from the genus Planes, the same group of crabs he remembered from his turtle research, which live in the protective space between a turtle's tail and shell.
Pfaller stored the specimens at the Florida Museum of Natural History and moved on to studying sea turtles, leaving dozens of mysterious crab larvae unidentified. A decade later, now a courtesy faculty member at the University of Florida, he mentioned the old project to crab expert Robert Lasley. Before Pfaller could even finish describing his results, Lasley interrupted: "Wait! Wait! Wait! Are you saying you have Planes on a snake?" The discovery, published in the journal Ecology and Evolution under the title "Planes on a Snake? On the Identities of Crab Larvae Rafting on Sea Snakes," rewrites what we know about life in the open ocean.
Given how striking yellow-bellied sea snakes are, it's easy to see how their many hangers-on got overlooked for so long. That a small crab got noticed at all came down to one scientist's sharp eye — and a willingness to look past the snake itself.