A sessile crustacean (yes — barnacles are crustaceans, not mollusks) that builds a calcified plate-armor home cemented to rocks. Tiny feathery legs extend through the top to filter plankton.

Also called: Common Acorn Barnacle, Rock Barnacle

Native status: Native; circumpolar northern

§ Quick facts

SizeShell diameter 1–2 cm (rarely to 3 cm)
Weight~1 g (shell + animal)
Lifespan2–6 years
OrderSessilia
FamilyBalanidae

§ Encyclopedia

Semibalanus balanoides is a common and widespread boreo-arctic species of acorn barnacle. It is common on rocks and other substrates in the intertidal zone of north-western Europe and both coasts of North America.

Source: Wikipedia — Semibalanus balanoides Licensed under CC BY-SA 4.0

§ Identification

Field marks. Small whitish volcano-shaped shell with six interlocking plates; central diamond-shaped operculum that opens during feeding.

Similar species. Other Balanid barnacle species (subtle plate differences). Goose Barnacle (different shape, attached by long stalk).

Voice. N/A

§ Diet & behavior

Diet. Phytoplankton, diatoms, microscopic organisms filtered from water with feathery cirri (modified legs).

Behavior. Cemented head-down to intertidal rocks. Larvae spend weeks searching for ideal attachment spot, testing surfaces with chemical cues; once cemented, the barnacle is anchored for life. Cement is among the most powerful natural adhesives known — being studied for medical and industrial applications. Compete for rock space; territorial battles consist of crushing neighbors with shell-plates.

§ Range

Atlantic coast from Arctic to Cape Hatteras; Pacific coast

§ Habitat

Intertidal rocks, pilings, docks, vessel hulls

§ Threats

Coastal pollution, ocean acidification weakening shells, vessel hull cleaning.

§ Conservation status

According to the IUCN Red List, the acorn barnacle is currently classified as Least Concern.

§ What helps

Support clean coasts. Reduce ocean acidification (climate action). Don't scrape barnacles from natural rocks.

Support our work More ways to help

§ Further reading

§ Sources cited

  • Smithsonian Marine Station
  • Marine Biological Laboratory
  • iNaturalist