Barnacles are a type of arthropod that belong to the subclass Cirripedia in the subphylum Crustacea. They are exclusively marine and tend to live in shallow and tidal waters, typically in erosive settings. Barnacles are sessile (nonmobile) and most are suspension feeders, but those in infraclass Rhizocephala are highly specialized parasites on crustaceans. They have four nektonic (active swimming) larval stages. Barnacles are encrusters, attaching themselves temporarily to a hard substrate or a symbiont such as a whale, a sea snake, or another crustacean like a crab or a lobster. The most common among them, "acorn barnacles" (Sessilia), are sessile where they grow their shells directly onto the substrate. Pedunculate barnacles (goose barnacles and others) attach themselves by means of a stalk.
Barnacles feed through feather-like appendages called cirri. As the cirri rapidly extend and retract through the opening at the top of the barnacle, they comb tiny food particles from the water column and pass them to their mouth parts inside their protective plates. Barnacles secrete a fast-curing cement that is among the most powerful natural glues known, with a tensile strength of 5,000 pounds per square inch and an adhesive strength of 22-60 pounds per square inch. This glue is so strong that researchers are trying to figure out how it can be used commercially. Barnacles like places with lots of activity, like underwater volcanoes and intertidal zones, where they reside on sturdy objects like rocks, pilings, and buoys. Moving objects like boat and ship hulls and whales are particularly vulnerable to the pesky critters. Large barnacle colonies cause ships to drag and burn more fuel, leading to significant economic and environmental costs.
