Pacific Acorn Barnacle (Balanus glandula)

Description: Pacific acorn barnacles are a species of little white barnacles, only reaching about 0.8 centimeters across. Many people don’t realize barnacles are living creatures; they’re actually a crustacean living inside the white shells that we see. 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 centimeters tall, maximum). This barnacle is able to get oxygen both from the air and from the water; they use their feet called cirri to absorb oxygen from the water. The cirri are a unique structure that look like long feathers, and they only come out from the “trap door” formed by the barnacle’s plates while underwater. Having such long feet is essential for a barnacle to get food because these creatures are sessile, meaning they cannot move. Plankton larvae float through the water until they are mature enough to cement to an object, whether that be a rock, a snail’s shell, a boat, or even a whale! From there, they form their outer shell and live inside, essentially upside down on their heads since they use their feet to grab things. 

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

Diet: In addition to oxygen absorption, the Pacific acorn barnacle 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 anemones. 

• Barnacles use their trap doors for many things, such as hiding from predators or retaining moisture when exposed to air during low tides.

•  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: Central Coast Biodiversity, University of Puget Sound