The steeply conical shell is made up of 6 wall plates and a flat bottom. There are 2 opercular plates, each with a beak at the tip that is often purple in color. When the opercula open, there are 6 pairs of feathery-looking cirri that come out to grab the food and exchange gases.
Habitat
Rocky areas or other hard surfaces from the low intertidal to subtidal depths of 300 feet, usually in areas with a high current or waves
Diet
Filter and suspension feeder of plankton
Life History
Acorn barnacles do not have separate sexes. Every mature individual produces both eggs and sperm. Self-fertilization is possible but usually cross-fertilization occurs. Each barnacle has a strong and stretchable penis that is discarded after mating, but a new one will form. Eggs are brooded in the mantle cavity for 4 months and hatch as nauplii. A single brood can contain thousands of nauplius larvae. Nauplii are released and feed on plankton, undergoing several molts as they grow. There’s a total of 7 larval stages, and the last one is called a cypris, a non-feeding stage whose job is to find a suitable spot to live out the adult stage of its life. The cypris attaches to the substrate, using glue from cement glands in its first antennae. It glues its head to a hard surface, such as a rock, the shell of another animal, a piling, etc., and builds the adult shell. Some of the swimming legs become the cirri.
Individuals can reach a maximum diameter of nearly 3 inches wide and 5 inches high. They can live for over 25 years.
IUCN Status
Not Evaluated
Ecosystem & Cultural Importance
Predators of the giant acorn barnacle include the ochre star, the green ribbon worm, and predatory sea snails.
Sometimes these barnacles will grow in large clusters that then create habitat. The empty shells of a dead barnacle provide a hole or nesting site to many other species, like rock crabs, red octopuses, or the grunt sculpin, who is specially adapted and uses its appearance to camouflage as a barnacle.
These barnacles contain the largest individual muscle fibers of any animal.
Once the barnacle has built its shell, it will remain here for the rest of its life. To feed, the opercula open and the cirripedia extend and unfurl. Since these used to be the swimming legs, barnacles catch their food with their legs. They also use their cirri for gas exchange. When there’s slow flowing water around them, they might continuously extend their cirri, beating the cirri to help to keep the water flowing so they can feed. When the water is moving faster, they don’t beat their cirri as much.
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