Barnacles and Relatives: Thecostraca

Most barnacles and their relatives can move about freely while they are larvae. Some ascothoracids still have the ability to swim as adults, attaching themselves to objects only temporarily when they are ready to feed. They attach themselves by the front of their heads using glue that comes from special glands in their antennules.

At low tide, barnacles on rocky shores survive out of water by sealing the plates in their shells to keep themselves moist and protected from high temperatures. The larvae are attracted to chemicals produced by the adults and usually settle in large numbers where there are other living or dead barnacles.

Many barnacles and their relatives are hermaphrodites (her-MAE-fro-daits), where individuals have both male and female reproductive organs. In species with both males and females, the male is often much smaller and permanently attached to the female like a parasite. These males seldom feed and lack internal organs, other than those directly involved with reproduction. However, most barnacles require both males and females to reproduce.

The eggs of barnacles and their relatives are brooded inside the chamber in the carapace or mantle. The eggs hatch into free-swimming nauplius (NAH-plee-us) larvae. Nauplius larvae use antennae and mouthparts—the only appendages they have—for swimming. They molt, or shed their external skeletons (exoskeletons), several times before reaching the next larval stage. This stage does not eat, and its body is completely surrounded by a folded carapace.