Acorn Barnacle         Balanus glandula

The most common of the local barnacles, the Acorn barnacle inhabits rocks, cobbles, piling and other hard surfaces in the mid to high intertidal zone from  Alaska to southern California.  The smooth sided, volcano-shaped, white to light tan house contains a shrimp-like crustacean which is permanently attached to the hard surface.  When underwater the beak-like rectangular cap opens, allowing the legs to sweep the water column for food such as plankton and bits of detritus.  They are fed upon by a variety of animals from flatworms and snails to sea stars and fish all of which have a variety of strategies for penetrating the hardened defense.

See video of Acorn Barnacle.

The Acorn barnacle breeds during winter and spring producing up to some 30,000 larvae in a large individual.  Breeding is facilitated by a very long copulatory organ that allows the male to fertilize the female. The embryos  are retained within the shell till they reach the larval stage.  After several transformations they settle in April as rice-sized juveniles that attach to any hard surface from the high intertidal level downward.  However their survival depends on how much time they can spend covered by water feeding and how long they are exposed to predators who must be underwater to feed.  As a result the juveniles settling in the highest level of the intertidal zone die from drying while the lowest ones are eaten.  The survivors can grow to 3/4 of an inch wide and 1/2 inch high, living for eight to ten years.

The smaller brown barnacle  in the photograph is a different species called the Little Brown barnacle (Chthamalus dalli).