The Giant Acorn Barnacle is the largest of the white barnacles. These barnacles are most easily identified by their size and the bright yellow to orange inside lip of the carapace. The off-white side plates of the acorn barnacle are heavy and form a conical structure with a flat top. Its plates are designed to withstand strong currents and pounding waves. If exposed to the air, the barnacle can close its plates tightly and retain water until the tide returns.
When eating, the barnacle extends its cirri and sweeps the water for suspended food.
Whelk snails and crabs are the barnacle’s major predators. Once, Native Americans of the Pacific Northwest also harvested them for food. They are no longer consumed by people.
Acorn Barnacles live on hard substrate surfaces, such as rocks and pier pilings, down to 300 feet (91m) deep. They can be found from southern Alaska to Baja California.
Common. The abundance of barnacles can only be estimated. On a rock-covered 1,000-yard stretch of shore, there might be as many as 1,000 million Giant Acorn Barnacles, and they could produce hundreds of millions of larvae in one year.