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


  • Common throughout north Atlantic and north Pacific oceans
  • Aleutian Islands to Bahia de San Quentin, Baja California, Mexico


  • 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


  • 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


  • Filter-feeder, eats tiny particles and plankton out of the water with their modified legs


  • 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

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