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.
Sacculina is a genus of barnacles that is a parasitic castrator of crabs. They belong to a group called Rhizocephala. The adults bear no resemblance to the barnacles that cover ships and piers; they are recognised as barnacles because their larval forms are like other members of the barnacle class Cirripedia. The prevalence of this crustacean parasite in its crab host can be as high as 50%.
A barnacle is a type of arthropod constituting the infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have two nektonic larval stages. Around 1,220 barnacle species are currently known. The name "Cirripedia" is Latin, meaning "curl-footed".