The common rock barnacle is an arthropod. It appears in two forms: at the larval stage, it is mobile and swims in open water. At the adult stage, it is immobile and attached to a rock.
In fact, the adult barnacle is nothing other than a modified “shrimp” that lives attached by its back to a rock and that has made itself a calcareous shell for protection (we say that it is sessile, or lacking in self-locomotion). Despite its six double limbs, at the adult stage it cannot move around (only the larvae are mobile). Its legs (which are called cirri) look like fine feathers and are used to filter food from the water. The barnacle’s shell is made up of four, six or eight calcareous plates, depending on the species.
Semibalanus balanoides, the most common species in the Gulf of St. Lawrence, has six plates. Its shell, in the shape of a small truncated cone, resembles a miniature volcano. The opening above the shell is blocked by two other calcareous plates, which open and close with the help of small muscles. These two plates make up what is called the operculum.
The calcareous shell is greyish white but may also contain some yellow, orange or pink. If the operculum is opened, the flesh inside is white or pinkish white. The adult barnacle has no eye.
In all, there are six barnacle species in the Gulf of St. Lawrence.