# What Are Barnacles? > Barnacles look like small volcanoes fixed to rocks, piers and ship hulls, but the animal inside is a crustacean related to crabs and lobsters. Most familiar acorn barnacles spend adult life cemented headfirst to a hard surface. They open plates at the... Canonical URL: https://www.argo.net/what-are-barnacles/ Byline: ARGO.net Editorial Team Published: 2026-08-29T12:07:28+00:00 Categories: Explainer, Nature ![Detailed view of barnacles clinging to rocks with a seashell on a sunny shore](https://www.argo.net/wp-content/uploads/2026/08/barnacles_on_rocks.jpg) Barnacles look like small volcanoes fixed to rocks, piers and ship hulls, but the animal inside is a crustacean related to crabs and lobsters. Most familiar acorn barnacles spend adult life cemented headfirst to a hard surface. They open plates at the top of the shell and sweep feathered limbs through the water to collect food. NOAA's profile of [barnacles](https://oceanservice.noaa.gov/facts/barnacles.html) describes more than 1,400 species in waterways around the world. Their ability to attach permanently in wet, turbulent places makes them successful inhabitants of the shore and costly passengers on vessels. The fixed shell hides an active marine animal with a complex life cycle. Larvae swim and choose a surface, adults feed with moving limbs and colonies reproduce by exchanging sperm between neighbors. The transition from plankton to a permanent home links barnacles with currents, available hard habitat and the vessels that can carry them between distant ports. ## Barnacles begin life as swimming larvae Adult barnacles release larvae that live in the plankton. Early **nauplius larvae** swim and feed as they grow through several stages. Their appearance reveals the crustacean connection more clearly than the shell of an adult. The later cyprid stage does not feed. It carries energy reserves while searching for a suitable place to settle. Chemical cues, surface texture and the presence of other barnacles can influence the choice. A poor decision is permanent for species whose adults remain fixed. A cyprid explores with specialized appendages before attaching. It then transforms into a juvenile barnacle and begins building protective plates. The Smithsonian's species account for an introduced barnacle describes the same progression from feeding nauplius through a nonfeeding [cyprid stage](https://invasions.si.edu/nemesis/species_summary/89628) to a settled filter feeder. **Settlement choice** determines the adult's access to flowing water and nearby mates. Larvae can reject unsuitable surfaces before cementing, but their swimming period is limited by stored energy. Dense adult patches provide a cue that a location has supported barnacles before. Currents influence both dispersal and return. Larvae carried too far from suitable hard surfaces may fail to settle, while retention near a rocky shore can replenish a local population. Temperature and food affect development time, changing how long a larva remains exposed to transport. ## Natural cement holds in moving water Attaching beneath the waterline presents a chemical problem because ordinary glues struggle on wet, contaminated surfaces. Barnacles prepare the contact area and release a protein-rich adhesive that cures into a strong bond. The attachment must endure waves and the force of water moving past a ship. **Barnacle cement** has attracted researchers interested in medical or industrial adhesives. A useful surgical glue, for example, would need to bond in a wet environment. Translating the biological mechanism requires understanding how the animal controls fluid, proteins and surface chemistry at the attachment site. The hard shell above the cement grows with the animal. In an acorn barnacle, overlapping calcium carbonate plates form a low cone. Plates at the opening act as a door, closing when conditions threaten the soft body. Attachment does not make the animal invulnerable. Predators can drill or pry into shells and intense heat or drying can kill an intertidal barnacle. Clustering in crevices reduces some exposure while increasing competition for space. **Adhesive proteins** operate at the boundary between the animal and the surface. Researchers investigate how those proteins displace water and harden, yet barnacle cement is a complex biological material rather than a single ready-made industrial compound. ## Feathered cirri collect food When covered by water, a barnacle extends jointed appendages called **cirri**. Fine bristles intercept plankton and suspended particles. The limbs retract through the opening to bring food toward the mouth, then sweep out again. Flow changes feeding behavior. Water movement delivers new particles, but very strong turbulence can make extension risky. Different species adjust the form and motion of their cirri to the conditions they usually experience. Low tide requires another response. Acorn barnacles close their opercular plates and trap moisture inside the shell. Closing the plates lets many species occupy rocks that spend part of each day exposed to air. The vertical band where a species lives reflects its tolerance of drying as well as competition and predators. **Suspension feeding** connects the fixed animal to the moving plankton community. Food availability changes with tides and season, so growth can vary even among barnacles only a short distance apart. Turbulence that delivers particles can also force the cirri to shorten their sweep. ## Ships pay a cost for attached colonies Barnacles are part of a broader community of organisms that settle on submerged structures, a process called **biofouling**. Colonies roughen a hull and increase friction with the water. The engine must supply more power to maintain speed, raising fuel use and emissions. NOAA cites a U.S. Navy estimate in which heavy barnacle growth can increase ship weight and drag by as much as 60 percent, with fuel consumption rising by as much as 40 percent. The figures describe heavy growth rather than every fouled vessel and the actual penalty depends on hull condition and operation. Cleaning and protective coatings limit buildup. Methods must balance effectiveness with the risk that coating chemicals or removed organisms enter surrounding water. Biofouling also transports species when vessels move between ports, so hull management has an ecological role beyond fuel economy. The [Smithsonian Environmental Research Center](https://serc.si.edu/node/38613) has used barnacles collected from commercial ships to study dispersal. Genetic comparisons can reveal connections among ports and help explain how attached organisms cross natural geographic barriers. **Hull inspections** look beyond visible adult shells. Sea chests, intake grates and other recessed areas can shelter fouling communities from ordinary cleaning. Management standards focus on the whole wetted surface because a small protected population can release larvae at the next port. ## Whales and shorelines provide living habitat Some barnacle species attach to whales or other marine animals. The relationship varies, but barnacles on gray whales usually gain transport through food-rich water without feeding on the whale. Their patches contribute to the mottled appearance of the skin. Rocky shores contain dense barnacle zones because hard surfaces and moving water meet there. Space is scarce and larvae settle among adults. Their shells create small crevices that can shelter other organisms, adding physical structure to the intertidal community. Most barnacles are hermaphrodites, with both male and female reproductive organs, yet they generally exchange sperm with a neighbor. A long reproductive organ reaches outside the shell because adults cannot move together. Fertilized eggs develop before larvae enter the plankton and repeat the cycle. **Barnacles** combine two very different lives: a mobile larva selects a home and a fixed adult feeds from whatever water passes. Their cement, protective plates and cirri solve the central problems of remaining attached, avoiding desiccation and gathering food from a constantly moving environment. **Species differences** explain why one description cannot cover every barnacle. Stalked gooseneck barnacles extend from flexible peduncles, while acorn barnacles sit directly on their base. Parasitic relatives have highly modified bodies. The familiar white cone represents one successful branch of a much broader crustacean group. Ecologists can read barnacle zones as records of physical stress and biological interaction. The upper limit may reflect drying, while predators and competition influence the lower boundary. Changes in recruitment from year to year can indicate shifts in currents or larval supply without implying that adult barnacles moved. Long records can separate one poor settlement season from a sustained ecological change. **Related reading:** [bivalves](https://www.argo.net/what-is-a-bivalve/) and [pelagic and benthic zones](https://www.argo.net/pelagic-vs-benthic-zone-what-is-the-difference/). **Explore this topic:** [What Animals Live in Coral Reefs?](https://www.argo.net/what-animals-live-in-coral-reefs/) and [What Is Bioluminescence?](https://www.argo.net/what-is-bioluminescence/).