The feeling of revulsion many people experience when encountering barnacles is a common reaction to an organism that defies typical expectations of marine life. Barnacles are highly specialized crustaceans, making them relatives of crabs and shrimp, though they appear nothing like their mobile cousins. Their unusual appearance and life habits result from a successful evolutionary path toward a sessile, or stationary, existence. This unique biology, from permanent fixation to unusual feeding and reproduction methods, is the source of the public’s strong, negative perception.
From Free Swimmer to Fixed Shell
Barnacles begin their lives as tiny, free-swimming nauplii larvae, drifting through ocean currents for several weeks. This stage is followed by the cyprid larva, a non-feeding stage dedicated solely to finding a suitable hard surface for permanent attachment. The cyprid uses specialized antennules to explore and test potential settlement sites before making a final commitment.
Once a location is selected, the larva cements itself head-first to the substrate and undergoes metamorphosis into its adult form. This permanent fixture is achieved by secreting a strong, protein-based substance known as barnacle cement. This adhesive is one of the strongest natural glues, allowing the barnacle to bond securely to underwater surfaces, including rocks, wood, and metal.
The adult barnacle then constructs a protective shell around its body, typically composed of six hard calcareous plates. Within this armor, the animal is essentially an upside-down crustacean, fixed in place with its jointed legs facing upward. This armored state makes barnacles appear less like a living creature and more like an unappealing, crusty infestation.
The Shocking Mechanisms of Feeding and Reproduction
The appearance of the adult barnacle changes dramatically once it is submerged and begins to feed, revealing the animal inside the shell. When feeding, the barnacle slides open the plates at the top of its shell and rhythmically extends feathery, leg-like appendages called cirri. These twelve thoracic limbs are covered in fine hairs and function like a net, sweeping through the water to capture plankton and detritus.
The rhythmic extension and retraction of the cirri draws a current of water and food particles into the shell. This filter-feeding action allows the barnacle to survive its stationary life, but the sight of these pale, actively moving appendages emerging from the hard shell is often described as unsettling or alien. The barnacle can also generate its own current by rapidly beating its cirri, pulling food toward itself even in still water.
The barnacle’s sessile lifestyle necessitated an unusual method for sexual reproduction. Most barnacles are simultaneous hermaphrodites, possessing both male and female reproductive organs, though they rarely self-fertilize. Since they cannot leave their shells to mate, they engage in a process called pseudocopulation with their neighbors.
To accomplish this, barnacles developed an extremely long and flexible copulatory organ that is extended out of the shell to fertilize a nearby receptive individual. This organ can be eight times the barnacle’s body length, giving the barnacle the largest proportional reproductive organ in the animal kingdom. This anatomical adaptation is necessary for the transfer of sperm between fixed neighbors.
Why Barnacles Are the Ultimate Hitchhikers
The barnacle’s ability to settle and permanently adhere to virtually any submerged surface is the foundation of its status as a pervasive pest. Barnacles are major contributors to biofouling, the accumulation of organisms on wetted surfaces. Once a single larva settles, it often attracts more of its kind, leading to the formation of dense, hard colonies that colonize marine structures and animals.
On man-made structures, barnacle biofouling creates significant economic and environmental problems for the global shipping industry. The rough texture of barnacle colonies on a ship’s hull increases hydrodynamic drag, requiring the vessel to use substantially more engine power to maintain speed. A vessel with over ten percent hull coverage may require up to 36% more shaft power, translating to increased fuel consumption and carbon emissions.
For marine life, barnacles attach themselves to whales, sea turtles, and large fish. While host animals often tolerate the presence of these hitchhikers, the weight and drag of large colonies can be taxing. The scale and ubiquity of barnacle colonization, from the bottom of a boat to the back of a whale, reinforces the idea of them as a persistent, hard-to-remove marine infestation.