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Marine Animal Encyclopedia

Acorn Barnacle Semibalanus balanoides

Like all adult barnacles, the adult acorn barnacle remains fixed in one place once it has anchored itself to a site. The free-swimming juveniles pass through several larval stages before molting into a form that can detect both other acorn barnacles and suitable anchoring sites. Once a larva fixes itself to a rock, using cement produced by glands in its antennae, it molts again. It then secretes six gray calcareous plates, forming a protective cone that looks rather like a miniature volcano. Four smaller, movable plates at the top of the cone open, allowing the acorn barnacle to feed. It does this when the tide is in by waving its modified legs, called cirri, in the water to filter out food. When the tide is out, the plates are closed to prevent the barnacle from drying out. Acorn barnacles are hermaphrodites that possess both male and female sexual organs, but they function as either a male or a female. They do not shed their eggs and sperm into the water; instead they use extendable penises to transfer sperm to receptive neighbors.

Charles Darwin and Barnacles

Before the British naturalist Charles Darwin (1809–1882) proposed his revolutionary theory of evolution in The Origin of Species (1859), he spent eight years studying barnacles. Realizing the impact his ideas on evolution would have on existing scientific and religious thinking, he delayed writing and instead produced four monographs on the classification and biology of barnacles. This work earned him the Royal Society’s Royal Medal in 1853, validating his reputation as a biologist.

Acorn Barnaclezoom image
  • Subphylum Crustacea
  • Length Up to in (1.5 cm) diameter
  • Habitat Intertidal zone of rocky shores
  • Distribution Northwest and northeast Atlantic, Pacific coast of North America
Acorn Barnacle habitat mapzoom image