Thatched Barnacle (Semibalanus cariosus)

Description: Thatched barnacles are typically white in color, but can sometimes look a bit more gray, brown, or green. The thatched barnacle is named for the rough edges on its outer wall with ribbing that runs vertically. This texture is seen more prominently in younger individuals; it wears away in older thatched barnacles that have been smoothened by waves or close contact with other barnacles. Similar to other species like the acorn barnacle, the thatched barnacle can vary in size and shape. When it has room to grow, this barnacle’s outer shell is volcano-shaped, but in crowded conditions it grows to be taller and skinnier. On average, thatched barnacles reach about 5-6 centimeters across.

Habitat: This species is found living on rocks in the intertidal zone from Alaska to southern California.

Diet: They feed by filtering plankton out of the water with their feathery feet called cirri, which emerge from the barnacle’s trap doors while underwater. 

Tide Pool Tidbits:

• Thatched barnacles can live to be up to 15 years old!

• If you look inside a barnacle’s outer shell and don’t see inner plates inside, that means the animal is no longer living inside. If the barnacle’s outer shell seems to be closed by some inner plates, that means it is still alive.

• While other barnacle species may have some ribbing on their outer shells, no other barnacle has the thatched finger-like projections of the thatched barnacle.

Reference: Central Coast Biodiversity