Acorn barnacle

Natural history

Once attached, it will metamorphose into a juvenile barnacle, which is a miniature version of the adult. Then each builds its own fortress—a cone-shaped calcareous shell with a trapdoor in the ceiling. When water covers a barnacle, the trapdoor opens and the barnacle's feathery appendages, called cirri (thoracic limbs), emerge to sweep the water for plankton and detritus. When the tide is out, a barnacle closes its trapdoor to conserve moisture. The barnacle spends the rest of its life in this position—head down and feet up.

Conservation

The barnacle is a successful group with abundant and diverse populations. Scientists have identified about 1,445 living species, of which 900 are acorn barnacles. Their abundance can create serious and expensive fouling problems on ship bottoms, buoys and pilings. In less than two years, 10 tons of barnacles can become attached to a tanker. Barnacles encrusted on ships can cause enough drag to increase fuel consumption by 40 percent.

Today, barnacles range farther and farther because their larvae catch free rides in the ballast water of ships. These invasions of exotic species can damage local ecosystems.

Cool facts

• The Balanus barnacle is hermaphroditic (it has both female and male sex organs), so it cross-fertilizes with its next-door neighbors. Living in tight groups comes in handy when it's time for the stationary barnacles to fertilize each other’s eggs internally.
• The barnacle broods fertilized eggs within its shell.
• The barnacle has no gills—gases are exchanged through cirri (feathery appendages) and body walls.
• Cement glands within the antennae produce the brown glue that fastens a barnacle to a hard surface. Acids and alkalis don’t dissolve this powerful glue that can hold the base of the shell to a surface long after the barnacle is dead.