FaunaHub

Barnacle (infraclass Cirripedia)

Marine invertebrateCrustaceanFilter feeder

Northern acorn barnacles (Semibalanus balanoides) encrusting a rock.

Acorn barnacles (Semibalanus balanoides), Norway.

Image: Ryan Hodnett, CC BY-SA 4.0, via Wikimedia Commons.

Overview

Barnacles are marine animals that look like little volcano-shaped shells cemented to rocks, shells, piers, and even whales — but they are not molluscs. Barnacles are crustaceans, relatives of crabs, shrimp, and lobsters, and one of the few crustacean groups that live permanently fixed in one place. The reference shown here is the northern acorn barnacle (Semibalanus balanoides).

An adult barnacle lives head-down inside its protective plates and reaches out with feathery legs to comb food from the water — a surprising lifestyle for an animal in the same broad group as fast-moving shrimp.

Note: there are many barnacle types (acorn barnacles, goose barnacles, and others), so details vary by group. Treat general statements accordingly and verify against authoritative sources.

Habitat & Range

Barnacles live in seas worldwide, most familiarly packed across rocks in the intertidal zone, where they endure pounding waves and being exposed to air at low tide. Others attach to floating objects, ship hulls, drifting debris, and the bodies of animals such as whales and sea turtles.

Diet

Barnacles are filter feeders. When covered by water, an acorn barnacle opens the plates at the top of its shell and sweeps out feathery, leg-like appendages called cirri to rake plankton and fine particles from the water, then withdraws them to pass food to the mouth.

Behavior

A barnacle begins life as a tiny swimming larva, drifting in the plankton. When ready, it settles head-first onto a surface and cements itself in place for life, then builds its hard shell. Because adults cannot move to find mates, many barnacles have unusually long reproductive structures to reach neighbours, and most are hermaphrodites.

Human Interaction & Conservation

Barnacles are a classic subject in marine biology and a major form of “biofouling” — their build-up on ship hulls increases drag and maintenance costs, which has driven a lot of research into how they stick so firmly. Some goose barnacles are also harvested as food in certain regions. Consult authoritative sources for specifics.

More photos of the barnacle

An acorn barnacle with its protective plates (operculum) visible.

Acorn barnacle showing its operculum plates.

Image: Bobjgalindo, CC BY-SA 4.0, via Wikimedia Commons.

Frequently Asked Questions — Barnacle

Is a barnacle a shellfish like a clam?
No. Although a barnacle looks like a little shell, it is a crustacean — related to crabs, shrimp, and lobsters — not a mollusc like a clam. Its larval stages and feeding legs (cirri) give away its crustacean identity.
How do barnacles eat without moving?
Adult barnacles are filter feeders. When underwater, they open the top of their shell and extend feathery, leg-like cirri that sweep plankton and particles from the water, then pull them back in to feed. They never need to leave their spot.
How do barnacles attach so firmly?
A barnacle larva settles head-down and cements itself permanently to a surface using a strong natural glue, then grows its hard plates around itself. This adhesive is remarkably tough, which is one reason barnacles are studied and why they are hard to remove from ships.
Do barnacles live on whales?
Some do. Certain barnacles specialise in attaching to whales, sea turtles, and other animals, hitching a ride and filter-feeding as their host swims. They don't feed on the host — they simply use it as a moving home.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.