This species of barnacle can glide around its turtle hosts
Barnacles aren’t exactly known for their athleticism, sticking in place for much of their lives. But turtle-riding barnacles are restless travelers.
As adults, turtle barnacles (Chelonibia testudinaria) can move about 1.4 millimeters per week through turtle shells, researchers report Oct. 6 at Proceedings of the Royal Society B. Previous sightings of barnacles glued to green sea turtles suggested that the creatures were somehow mobile, propelled either by external forces or by their own actions. But this is the first experimental confirmation that they are embarking on self-driving rides.
Barnacles begin life as free-swimming larvae, eventually settling and adhering to rocks, ship hulls, or even the skin of marine mammals (SN: 09/27/16). Some species have been known to spin on their base or even scooch a little when pushed by a close neighbor. But once settled, they live and grow, eating food particles drifting from what has long been thought to be their permanent address.
Now it turns out that some may need forwarding addresses. Benny KK Chan, a marine ecologist at Academia Sinica in Taipei, Taiwan, decided to test C. testudinaria‘s mobility experimentally when one of his students managed to transfer crab turtle barnacles onto an acrylic plate. The team tracked 15 translocated barnacles with time-series photographs for a year.
Chan’s team also collaborated with researchers in Spain to track the movement of barnacles on the shells of five captive loggerhead sea turtles for a few months and with citizen scientist divers who collected photos of wild green sea turtles in Taiwan. The team recorded the positions of green sea turtle barnacles for 16 weeks.
Turtle-riding barnacles moved up to 54 millimeters – just less than the length of an adult human’s thumb – during this time. Lab barnacles also moved, leaving streaks of pale cement in crescent-shaped layered patterns. “We were amazed,” Chan says.
How barnacles move remains a mystery, but researchers believe the crustaceans can partially dissolve their own cement and lift their soft base slightly from the surface. “Then the barnacle can secrete a new layer of cement and probably surf on the cement,” Chan says.
The barnacles mostly traveled against all currents, showing that they weren’t simply moving under the pressure of flowing water. They also didn’t get close, suggesting the barnacles are looking for better places to filter food out of the water rather than mating opportunities.
“It’s rock solid proof of something that is otherwise anecdotal,” says marine biologist Henrik Glenner of the University of Bergen in Norway, who was not involved in the study.
Barnacles typically exemplify biological competition for space and resources, because after settling they must compete there for the rest of their lives, says Glenner. But being mobile upsets this dynamic.
And that raises new questions. Glenner wonders if barnacles in crowded intertidal environments might also be able to move. And Tara Essock-Burns, a marine ecologist at the University of Hawaii at Manoa, wants to know more about cement itself and its flexible properties. “It is possible that turtle barnacle cement has a very different biochemistry than other barnacles that permanently adhere to [surfaces],” she says. This is precisely what Chan and his team plan to investigate next.
“There’s a reason Darwin was so captivated by barnacles,” says Essock-Burns. “They never cease to amaze us.”