Barnacle Exuvia Adrift

The exuvia, or cast-off exoskeleton, of a barnacle.

When I spotted this white speck, about the size of an aspirin tablet, floating among the kelp fronds, I assumed that it was a small swimming creature. The image seemed to show some kind of swimming barnacle. Mature barnacles have cirri, the tentacles of the scoop-like feeding organ at lower left in the image, but they are immobile. Larval barnacles swim freely, but look like bugs. They don't have cirri. After searching the web a bit, I found that barnacles are close relatives of crabs, and like them have exoskeletons. (They wear their exoskeletons inside the hard shells that they secrete, gluing them to rocks or other substrate.) Like crabs, barnacles shed their exoskeletons, including the coating of the cirri, as they grow. Cast-off exoskeletons of barnacles, as well as those of crabs and lobsters, are called exuvia or casts. This is the exuvia of a growing young barnacle, floating through the ocean, like a child's outgrown outfit floating through rummage sales.