Acorn Barnacle
Balanus glandula
Physical Description
- Large barnacles, almost an inch in diameter.
- Have a cone-shaped shell, white or gray in coloration.
- Have no stalk but cement shell directly to hard surface.
- Cemented in place, with a valved shell that opens to allow their modified legs to come out and filter for particles in the water as it passes them.
- Opening of valved shell (operculum) is diamond-shaped.
Range
- Common throughout north Atlantic and north Pacific oceans.
- Aleutian Islands to Bahia de San Quentin, Baja California, Mexico.
Habitat
- Live in the intertidal zone (area between low and high tide zone).
- Can live out of the water at low tide because they can fully close their shell.
- Common on pier pilings, rocks, and even on other animals.
- Live exclusively in marine water, not fresh or estuarine areas.
Reproduction
- Although almost all other crustaceans have separate sexes, barnacles are hermaphrodites (each animal has both male and female organs).
- They have to mate with another barnacle to have offspring, but they are stationary, so they have a very long, extendable penis.
- Acorn barnacles have the longest penis relative to body size of any animal!
- They must mate with animals within a three inch radius of them.
- After every mating season, the penis dissolves and grows back the next year.
- Newborn barnacles extend as one-eyed larvae that feed on plankton and molt several times until they look like miniature shrimp.
Diet
- Filter-feeder, eats tiny particles and plankton out of the water with their modified legs.
Predators
- Snails that drill holes through barnacle shells.
- Ochre sea stars and other starfish.
- Dog whelks and grazing limpets.
Interesting Facts
- Barnacles are crustaceans, like crabs or lobsters, even though adult barnacles are permanently affixed to one spot.
- Acorn barnacle larvae can settle in very high densities, up to 70,000 in one square meter (10.76 square feet).
Sources:MARINe; UC Santa Cruz; Oceana; University of Puget Sound
Photo: Kevin Lee