Hitchhiking Barnacles

When you’re a sessile, or sedentary creature, choosing your habitat or home may not be the easiest thing. Plus, you only get one chance.

Take the mighty barnacle. These extraordinary crustaceans (you can learn about their feats here or here) begin their lives as swimming larvae. But they eventually settle down on a surface, be it rock, substrate, or other living organism. Sharks, sponges, turtles, other crustaceans, and even large cetaceans can all host barnacles.

Once they reach their adult state, they are cemented to their host surface for life. So how do barnacles know where to settle when they make this lifelong commitment?

Spanish scientists were curious about how this worked and chose a barnacle and host to work with. For 30 years, striped dolphins (Stenella coeruleoalba) have been washing ashore, or stranding, along the Mediterranean coast of Spain. In all, the researchers collected data from 242 dolphins. The coronulid barnacle, Xenobalanus globicipitis, only settles on cetaceans, including the striped dolphin, so the team had a perfect commensal relationship to study. (These barnacles have a symbiotic relationship with the dolphins—they don’t affect the cetaceans, but they gain a habitat and get to travel to nutrient-rich environments.)

The team discovered that the barnacles were almost exclusively found in a strip on the “trailing” or back edge of the dorsal fin, flippers, and tails of the dolphins. In fact, the barnacles seem to prefer dolphin tails, where a higher density, occurrence, and abundance of barnacles occurs. And for good reason, too. The researchers determined that tail-dwelling barnacles had larger shells than those on the other fins. Bigger can be better in barnacles, the scientists report in a study published today in PLoS ONE. “Maximum shell diameter (MSD) has been shown to have a strong relationship with body-size and reproductive state of barnacles.”

So, as David Byrne might ask, how did they get there? The authors suggest that the barnacles may chemically recognize the dolphin skin and passively find a location through the ‘vortex’ that forms as water flows over and around the dolphins’ fins. Barnacles selected the edge of fins and attached facing away from water flow, possibly benefiting from a suitable environment to filter nutrients for food and to protect developing larvae.

Nice house hunting, barnacles!

Image: Barnacles on dolphin's tail. From video here by Tim Hammond.