BarnaclesThe barnacle has famously been described as “nothing more than a little shrimp-like animal standing on its head in a limestone house and kicking food into its mouth.” It “kicks” food into its mouth with its long, feathery “feet” which stick out through the shell opening. As the creature pulls

back its thoracic limbs (“cirri”) its comblike mouth apparatus scrapes food particles off the legs.

The "barnacle zone" is the highest of the intertidal zones, where barnacles are least vulnerable to sea stars, sunflower stars, barnacle nudibranch, and flatworms. Barnacles can live six weeks out of water and up to three years with brief submergence once or twice a month.

Although they may look like mollusks with their shell-like covering, barnacles are actually crustaceans. They look like tiny shrimp in their larval stage, where they swim as members of zooplankton in the ocean. When they are ready to settle down, they search for a suitable site, pulling themselves along by the adhesive tips of their antennae. Biologists have observed barnacles in the laboratory taking as long as an hour to pick a location. In nature, barnacles may take days to find a suitable spot, investigating one area, then allowing the currents to carry them to another.

After selecting a spot, the barnacle secures itself by 2 of its antennae. (Its glue is so strong, the barnacle's cone base is left behind long after the creature has died.) Now the larva is ready to grow into an adult and build its tough housing.

The barnacle secretes the calcium-hard plates which encase its body. These white cones have six nearly fitted plates that form a “house” around the crustacean. Four more plates form a "door" which the barnacle can open or close, depending on the tide. When the tide goes out, the barnacle closes shop to conserve moisture. As the tide comes in, a muscle opens up these four plates, and the feathery legs of the barnacle sift the water for food. All six pairs of these feather-like feeding appendages, called cirri (cirripedia is Latin for “curl-footed”), are jointed and set with sensory hairs which brush through the water collecting plankton for the barnacle to eat. The legs also are equipped for gas exchange.

The more the barnacle eats, the more it grows. Not only does the growing barnacle have to deal with its surrounding house, it is also constrained by its own shell (made from chitin, the same material as crab shells). Like crabs, barnacles shed their shell when it gets too small. But since they never leave their plated homes, they must enlarge their current one. There is probably a chemical secretion that dissolves the inner layers while new material is added to the outside.

Most barnacles are hermaphrodites. While they have both male and female sex organs, they must be fertilized by a neighbor. A functioning male uses a retractable tube that can reach outside the shell as far as several inches to fertilize a nearby barnacle. A newly molted barnacle is most likely to be a functioning female.

Newborn barnacles emerge from their parent shells as one-eyed larvae. They feed on plankton voraciously about 6 months, molting 5 times and growing into non-feeding, shrimp-like larvae. These larvae settle to the bottom, and begin feeling around for a new home, beginning the cycle all over again.

Rather than a silly critter living on its head, I think of a barnacle as Sir Acorn, wearing a coat of armor, living in a castle, spending its whole life benefitting the health of the marine kingdom.

The blue mussel is a competitor for space, possibly outgrowing and smothering barnacles. Excessive growth of the algae rockweed can also overpower a colony of barnacles.