Barnacles are marine invertebrates classified within the subphylum Crustacea, related to crabs and lobsters. These animals are notable for their unique adult lifestyle, as they are largely sessile, meaning they remain permanently attached to a surface. Though their hard shells resemble mollusks, their biology classifies them as crustaceans. Over 2,100 barnacle species have been identified, inhabiting various marine environments across the globe.
Barnacle Anatomy and Development
A barnacle’s adult form is characterized by its protective external structure, composed of calcified plates. Most barnacles are enclosed by a carapace of six hard calcareous plates, with an operculum (lid) of four additional plates that can open or close. Inside, the animal lies on its stomach, extending its appendages. These appendages include six pairs of feathery thoracic limbs, called cirri, used for feeding. Some barnacles, like acorn barnacles, cement their shells directly to a substrate, while others, such as goose barnacles, attach by a stalk.
The life cycle of a barnacle begins with a microscopic, free-swimming larva called a nauplius. This planktonic stage feeds and develops through several molts. The nauplius then transforms into a cyprid larva, a non-feeding stage adapted to locate a suitable surface for attachment. Once a cyprid finds a suitable spot, it secretes a protein-based cement, anchoring itself. Following this attachment, the cyprid undergoes a metamorphosis, developing into the sessile adult barnacle.
Life on the Seafloor and Beyond
Barnacles exhibit a wide range of habitats, attaching to virtually any solid surface in marine and brackish waters. They are commonly found in rocky intertidal zones, adhering to rocks and jetties, and frequently colonize human-made structures like boat hulls and dock pilings. Barnacles also attach to marine animals, including whales, sea turtles, and other crustaceans. Their ability to adhere to diverse substrates is attributed to a strong, water-insoluble cement, continuously secreted.
Once attached, barnacles employ their cirri for filter feeding. These appendages extend into the water to capture plankton and detritus. The cirri create water currents, drawing food particles towards the mouth. Barnacles consume any small organic particles they can collect. This feeding method allows them to acquire nutrients while remaining stationary.
Barnacles in the Ecosystem
Barnacles impact marine ecosystems and human activities, particularly through biofouling. Their attachment to marine structures and vessel hulls increases hydrodynamic drag, which can significantly raise fuel consumption for ships, sometimes by up to 40%. This increased fuel usage leads to higher operational costs and elevated carbon emissions, posing an economic and environmental challenge for the shipping industry. Annual costs associated with combating biofouling can amount to billions of dollars globally.
Despite their nuisance in some contexts, barnacles play various roles within the marine food web. They serve as a food source for several marine animals, including whelks, sea stars, fish, and birds. In some cultures, certain barnacle species, such as goose barnacles, are considered a delicacy and are harvested for human consumption.
Barnacles also engage in symbiotic relationships with other marine organisms. A common example is their commensal relationship with whales, where barnacles attach to the whale’s skin. In this interaction, the barnacles benefit from transportation to nutrient-rich waters and access to a continuous food supply, while the whale is not harmed. Some barnacle species can also be parasitic, living within or on other crustaceans and altering their host’s behavior or appearance.