Pacific Acorn Barnacle (Balanus glandula)

Description: Pacific acorn barnacles are a species of little white barnacles, only reaching about 0.8 cm across. Many people don’t realize barnacles are living creatures; they’re actually a crustacean with the white shells that we see being their outer skeleton. When it has a lot of space, the Pacific acorn barnacle will grow its shell to be volcano-shaped. However, when living in more crowded conditions amongst many Pacific acorn barnacles, it will grow to be much more tall and thin (2 cm tall).

Habitat: Ranging from Alaska’s Aleutian Islands down to Mexico, this animal is most commonly found in the mid to high intertidal zone.

Diet: This barnacle is able to get oxygen both from the air and from the water; they use their feet called cirri to absorb oxygen. The Pacific acorn barnacle is a filter feeder that uses its cirri to grab plankton from the water to eat.

Tide Pool Tidbits:

• In the densest of conditions, people can find up to 70,000 Pacific acorn barnacles per square meter!

• Not only do Pacific acorn barnacles compete for space amongst themselves, but also with other sessile organisms like mussels and sea anemones. 

• Over half of the identified barnacle species in the world are types of acorn barnacles (900 of over 1400 species)

•  Predators of this barnacle (and other barnacle species) include ochre sea stars, barnacle-eating nudibranchs, and snails like dogwinkles that can drill holes through a barnacle’s shell with their mouths. 

References: Monterey Bay Aquarium, Puget Sound Museum of Natural History