Giant Acorn Barnacle

Mostly subtidal rocks; some very low intertidal or may be found on pilings. Common in areas of current or waves.

Geographical Range: Southern Alaska to San Qintin, Baja California, Mexico.

Depth Range: Low intertidal to 90 m

This large, mostly subtidal barnacle has 6 wall plates and the rostrum overlaps the plates on both sides of it. The tips of the terga are drawn out into a beak. The base of the shell is calcified. The exterior of the barnacle may be heavily eroded. Young individuals may have low ridges on the plates but the ridges are usually eroded away in older individuals. The scuta have no longitudinal striations. Often 5 or more cm in diameter, and may exceed 8 cm. This species also shows bright yellow or orange color when it opens up.

How to Distinguish from Similar Species: This is the largest barnacle likely to be encountered in this area, and perhaps in the world. Most other species here are less than 5 cm diameter. Of barnacles with beaklike terga, Semibalanus cariosus has thatchlike projections on the wall plates. The subtidal Balanus balanus and B. rostratus are rarely greater than 3 cm diameter and the walls of large individuals are much less eroded. B. balanus has septate tubes in the wall plates and B. rostratus has shiny overlaps between the plates. In California it can be distinguished from the large Balanus aquila because it has beaked terga and no longitudinal striations on its scuta.

Biology/Natural History: The plates of this species may be heavily eroded by the boring sponge Cliona. Often grow on one another, forming clusters. Barnacles are hermaphroditic, and fertilization is internal. They brood their eggs for several weeks before releasing them as nauplii. The nauplius molts to a cypris, which feeds in the plankton then settles and metamorphoses into an adult. Predators of this species include Pisaster ochraceous. The empty shells are refuges for Cancer oregonensis and Octopus rubescens. This species contains very large muscle fibers which have been used in the study of muscles.