Acorn Barnacle
Balanus glandula

Physical Description

  • Large barnacles, almost an inch in diameter
  • Cone-shaped shell, white or gray
  • 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


  • 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 barnacles 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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