California Barnacle

Megabalanus californicus is a large acorn barnacle with a diameter of up to 30 mm (1.2 in). The steep-sided shell is formed of six plates finely striped vertically with reddish-purple and white. There are relatively wide, reddish radii between the plates where they fuse. The mantle, visible through the wide aperture, is margined with red, orange, yellow and blue. This barnacle could be confused with Megabalanus coccopoma, but that species is paler, has a smaller aperture and narrower radii between the plates.

Megabalanus californicus occurs in the Eastern Pacific Ocean in the subtidal and intertidal zones of rocky shores. Its range extends from northern California southwards to the Gulf of California. It usually grows in groups and is most common on pilings and buoys. In a year following an El Niño event, it is often found to have increased its distribution northwards.