Acorn barnacles, related to shrimp, hide their identity in snail-like shells. But they begin life as free swimming larvae. When the time comes to settle, the larvae "glue" their heads to hard surfaces, such as pilings, wharfs, ships, rocks or other hard-shelled animals.
Once attached, they change into juvenile barnacles, minatures of the adults. Then each builds its own fortress—a cone-shaped limestone shell with a trap door in the ceiling. When water covers a barnacle, the trap door opens, and the barnacle's feathery legs emerge to sweep the water for plankton and detritus. When the tide is out, barnacles close their trap doors to conserve moisture. Barnacles spend the rest of their lives in this position—head down and feet up.
Barnacles are successful creatures 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.
Established adult barnacles secrete compounds that attract larvae to populated areas. Living in tight groups comes in handy when it's time for the stationary barnacles to fertilize each other's eggs internally.
Barnacles are hermaphroditic (they have both female and male sex organs), so they cross-fertilize with their next-door neighbors.
Barnacles brood fertilized eggs within their shells.
Barnacles have no gills—gases are exchanged through cirri (feathery legs) and body walls.
Cement glands within the antennae produce the brown glue that fastens a barnacle to a hard surface. Acids and alkalis do not dissolve this incredibly strong glue that can hold the base of the shell to a surface long after the barnacle is dead. Dentists, interested in the adhesive power of this glue, have been trying to determine its properties.