Barnacles are crustaceans that are related to crabs and lobsters. They are exclusively marine and tend to live in shallow and tidal waters, typically in erosive settings. 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 type of barnacle is the acorn barnacle, which is sessile and grows its shells directly onto the substrate. Pedunculate barnacles, such as goose barnacles, attach themselves by means of a stalk.

Barnacles feed through feather-like appendages called cirri, which they use to comb the water for microscopic organisms. They secrete hard calcium plates that completely encase them, forming a white cone made up of six calcium plates that forms a circle around the crustacean. Most barnacles are hermaphrodites, meaning that they have both male and female sex organs, and they must be fertilized by a neighbor to create baby barnacles.

Barnacles can also be parasites, such as the rhizocephalan barnacle, which is an internal parasite in other crustaceans. The sacculinidae barnacles are parasites that interfere with the crab’s genitalia and reproduction and leave them castrated for life. When a young barnacle latches onto a female crab, it grows into a sac-like structure on its underside near its abdomen, taking the place where the crab typically stores its eggs. Over time, the parasite destroys the crab’s genitalia and renders it infertile for life. When a female barnacle latches onto a male crab, it alters the male’s hormonal balance, sterilizes it, and over time, makes it secrete female crab hormones.