Giant Acorn Barnacle

Giant Acorn Barnacle (Balanus nubilus) as seen in our Sandy Shores exhibit.

The Giant Acorn Barnacle is the largest of the white barnacles. It resembles a shrimp standing on its head with surrounding plates that form a shell (carapace). The off-white side plates of the acorn barnacle are heavy and form a conical structure with a flat top. These barnacles are most easily identified by their size and the bright yellow to orange inside lip of the shell. Acorn barnacles live on hard substrate surfaces, such as rocks and pier pilings, down to 300 feet (91 m) deep. Its plates are designed to withstand strong currents and pounding waves. Intertidally, they grow close to each other, and deeper they may actually grow on top of each other. If exposed to the air, the barnacle can close its plates tightly and retain water until the tide returns. In order to eat, the barnacle extends its cirri (appendages that would be similar to legs if it were a walking animal) and sweeps the water for floating food. Whelk snails eat barnacles by forcing the valves open or by using their radula to drill into the barnacle shell. Crabs feed on barnacles. Native Americans of the Northwest once roasted them on open fires and ate them. Today humans do not usually eat barnacles.


Range & Habitat

Acorn barnacles live on hard substrate surfaces, such as rocks and pier pilings, down to 300 feet (91m) deep. Its plates are designed to withstand strong currents and pounding waves. Intertidally they grow close to each other, and deeper they may actually grow on top of each other. They can be found Southern Alaska to San Quentin, Baja California.


Conservation Status

Common. The abundance of barnacles can only be estimated. On a rock-covered 1,000-yard stretch of shore, there might be 1,000 million acorn barnacles, and they could produce one million million larvae in one year. Like most planktonic larvae, the vast majority are eaten by other animals before they settle.

Back