Barnacles are crustaceans which, as “youngsters,” attach themselves to solid objects like rocks, pilings, and the hulls of ships; or on turtles, crabs, horseshoe crabs, sea snakes, whales, mollusks, and other barnacles. (Beer cans will do too.) Most inshore barnacles are acorn or rock barnacles, the shape of which, not surprisingly, somewhat resembles an acorn. Most offshore barnacles are goose (or “gooseneck”) barnacles, which have a long, flexible stalk, and limey plates more in the shape of a wigwam.
The 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 is hermaphroditic. That is, each creature has both male and female organs. However, barnacle eggs must be cross-fertilized by a nearby neighbor, who extends a long slender sperm tube “next door.” The resulting eggs are nourished inside the barnacle until they hatch into sea-ready larvae, which grow, molt, and go through metamorphasis.
The exterior of the barnacle is a calcareous shell. (Though there are some barnacles without shells. These barnacles live as parasites on coral, crabs, sea stars, sponges, and other creatures.) Typically the barnacle has a small head, a six-segmented thorax, and a five-segmented abdomen. From the thorax extend six pairs of appendages. At the end of the abdomen is the caudal furca. The stalkless acorn barnacles have flat bases. Strong suckers cement the creature to its host; that is to say, the object to which it is attached.
Incidentally, if you get a cut by stepping on, or handling, a barnacle, clean the cut well and apply an antibiotic. These wounds can easily become infected.
For a barnacle living in the intertidal zone (that area of the beach over which the tide goes in and out), there is the risk of exposure to the air. To protect itself from the several hours of daily exposure, the barnacle clases up its plates.
Some barnacles can survive long peroids out of the water. For example, Balanoides balanoides can go six weeks out of the water, and Cthamalus stellatus has been known to live for three years with only brief submergence one or two days a month.
There are many species of barnacles, ranging in size from less than a quarter of an inch, to barnacles which are nine inches tall, including the aforementioned Acorn Barnacle; the Ivory Barnacle (Balanus eburneus); the Fragile Barnacle (Chthamalus fragilis, also known as the “Little Gray Barnacle”); the Bay Barnacle (Banaus improvisus); and B. subalbidus (Octolasmic muelleri, one of the commensal species, this one living on the gills of certain crabs); and the Goose Barnacle (Lepas anatifera.)
Others include the Star Barnacle (Chthamalus stellatus), and several barnacles with red or purple on the shell. The largest colorful barnacle found on the Southeastern part of the U.S. coast is Megabalanus antillensis), a pinkish, 2-inch beauty. The other colored barnacles of that area include Balanus trigonus, B. venustus, and B. amphitrite.
When Charles Darwin attempted in 1850 to categorize barnacles, he complained of “this confounded variation,” and after two years' more study of barnacles' taxonomic tangle he stated, “I hate a Barnacle as no man ever did before.” (See note below.)
The oyster flatworm, Stylochus ellipticus, is a predator of barnacles. The opportunistic worm approaches a feeding barnacle, quickly inserting its pharanyx through the open valves. The barnacle shuts its valves, but the worm has “a foot in the door,” so to speak. It begins to eat away at the barnacle. Gradually the barnacle weakens, gapes, and the whole oyster flatworm slips inside, continuing the meal.