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Biography of a Barnacle

January 12, 2007
Jocie Ingram

Common Acorn Barnacle
Common Acorn Barnacle
photo © Dave Ingram

Like most people who have grown up on the coast, I've had run-ins with barnacles. Barefoot, I remember gingerly stepping over them in an effort to reach my shoes. Sometimes I made it unscathed, but at other times I received small stinging cuts.

Encased in their hard, grey, volcano-like shells, it is hard to believe that these creatures are alive. The common acorn barnacle (Balanus glandula), can survive on upper beaches despite exposure to terrestrial conditions like long hours of scorching sun. Muscle controlled plates close tightly to shut out the elements. Yet as soon as a wave washes over a barnacle, it comes alive. The plates open, and a soft living creature is revealed. Six feathery plumes, called cirri, comb the water for microscopic plankton. The food collected on the plume is transferred to the mouth.

Barnacles have two distinct phases of life: a larval stage and an adult stage. Baby barnacles are tiny, free swimming larvae that float around eating plankton. At this stage, they resemble the larvae of their relatives, the Crustaceans, which include creatures like crab, shrimp and lobster. The first larval stage, called a nauplius, is bristly with a single eye, and three pairs of appendages.

The larva undergoes a series of moults, and eventually changes into a less mobile form called a cypris. The cypris explores surfaces that may be suitable for its adult stage. Having found the right spot, the cypris fastens itself down head first, using glue secreted by glands near the head. Further secretions create a small volcano of overlapping calcareous plates. The barnacle will spend the rest of its life (1-7 years) in its new home, glued to one spot. Though the larva is transformed, the six feet are retained, and these become the feathery feeders (cirri) that supply the barnacle with food.

The oddness of the barnacle was aptly described by American naturalist Louis Agassiz who said, "the barnacle is a shrimp-like animal standing on its head in a limestone house kicking food into its mouth".

The strange life of barnacles does not end there. Barnacles are hermaphrodites, possessing both male and female parts. They cannot, however self-reproduce. In order for sperm to be transferred from one barnacle to another, the barnacle has an extendible penis that can reach its neighbour. After fertilization, eggs are brooded until conditions are right, at which time they hatch into free-swimming larvae, starting the cycle over again.

Thatched Barnacle
Thatched Barnacle
photo © Dave Ingram

There are about one thousand species of barnacles in the world, in many forms and sizes. The acorn, or "volcano" shaped barnacles are the most familiar. The common acorn barnacle (Balanus glandula) is the most ubiquitous species. Another species found quite high up on the beach is the small acorn barnacle (Chthamalus dalli). Looking directly down at this barnacle, the lines between the plates form a cross. This distinguishes it from the common acorn barnacle, which has a wavy line. Subtidally there are larger species of barnacles, such as the giant barnacle (Balanus nubilus), and the thatched barnacle (Semibalanus cariosus). The thatched barnacle has distinctive ridges, a bit like lava flowing down the volcano's sides.

Gooseneck Barnacle
Gooseneck Barnacle
photo © Dave Ingram

Some barnacles, called pelagic barnacles, attach by a fleshy stalk to pieces of floating driftwood. On the west coast of Vancouver Island, pelagic goose-neck barnacles (Lepas anatifera) are often found washed ashore. Another species, the goose-neck barnacle (Pollicipes polymerus), attaches by a short, tough stalk to surf pounded rocks. When water runs off the rocks, these barnacles face that direction and feed by casting their cirri like a net into the water.

Barnacles play an important role in marine ecology. Though it may seem that nothing would eat a barnacle, acorn barnacles are a favourite food of snails. Predatory snails can drill a hole through the outer plate of a barnacle. After a barnacle has died, its empty shell provides shelter for numerous creatures.

The "cement" that binds barnacles to rocks is very adhesive. Scientists are interested in "barnacle glue" and the application it could have for human medicine. Research is ongoing to discover the secrets of the glue that never cracks and can withstand extremes of heat and cold.

Barnacles can be found along any rocky shore and are also common on floating docks and pilings. After storms, interesting barnacles that are normally subtidal may wash ashore. Care should be taken when exploring our beaches, which are teeming with fragile, living things. Even barnacles have a life!

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