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Cirripedia, the barnacles, make up an infraclass of arthropods (although they are sometimes considered a class or subclass) with about 1000 species.  They are exclusively marine organisms, well-represented in the fossil record back to the Cambrian (500 million years ago), very diverse and abundant, and found in just about every marine environment, from shallow and tidal waters to deep sea abyss. 

As adults, most barnacles are sessile suspension feeders; they pump the current with six biramous thoracic appendages, a feature they are named for: Cirripedia is Latin for curled foot.  The most highly modified of the arthropods, barnacles generally secrete a calcareous carapace; in fact Linnaeas originally classified the barnacles as molluscs for this reason.  

Most adult barnacles filter feed from hard substrates, upon which they glue their shells directly or attach from a stalk, or form burrows in mollusc shells or coral skeletons with a scute across the opening.  Some species are parasites, such as barnacles in order Rhizocephala which live on other crustaceans, especially decapods.  The adult phase of parasitic barnacles usually has a derived morphology specialized for its lifestyle, and far simpler than that of free-living barnacles.  These parasites are essentially an unsegmented sac-like body with no appendages, no carapace, and thin rhizomes for extending into the body of their host to feed.  While these adult stages are so diverse as to share almost no features with free-living adult species, barnacles are united by the morphologies of their larval stages, especially the nauplus stage.

Barnacles are one of the better-known marine invertebrates, since they are water-foulers, attaching to ships and other structures where they can cause damage.  Some species are eaten as a delicacy, such as gooseneck barnacles, e.g. Pollicipes cornucopia harvested for consumption especially for the Spanish market, where they are called percebes) and acorn barnacles, such as the giants Austromegabalanus psittacus (called picoroco in Chilean cuisine) and Balanus nubilus, the world’s largest barnacle at up to 3 inches across which is endemic to the Pacific coast of North America and traditionally eaten by Native Americans.

(Kozloff 1990; Newman and Abbott 1980; The Oregon Coast Aquarium 2014; Wikipedia 2013)


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