Barnacles are marine crustaceans, distantly related to crabs and lobsters, that have adapted to a unique, stationary lifestyle. While they may appear like mollusks due to their hard outer casing, these creatures are found firmly attached to various surfaces in marine environments, from rocks and ship hulls to the bodies of whales. Their cone-shaped or stalked appearance belies a complex biological structure hidden within.
The Outer Shell: A Protective Fortress
The barnacle’s robust outer shell serves as a defense mechanism. This protective casing is primarily composed of calcium carbonate, the same mineral found in limestone and other seashells. This inorganic material provides significant hardness and rigidity to the barnacle’s external structure.
Reinforcing the calcium carbonate are various organic materials, including chitin and proteins. The shell consists of multiple plates, often six in acorn barnacles, which overlap or interlock to form a sealed, conical structure. These plates grow in layers, adapting to the barnacle’s increasing size. This intricate design protects the soft-bodied animal inside from predators, the drying effects of exposure during low tide, and the powerful forces of waves and currents. Two of these plates function as a “door” or operculum, which the barnacle can open to feed and close for protection.
Inside the Shell: The Barnacle’s Body
Within the hard outer shell, a barnacle houses its soft-bodied components, positioned essentially “head down.” The body is broadly divided into a head and thorax, with little to no abdomen. Its feathery feeding appendages, known as cirri, are a primary feature of its internal anatomy. These are modified thoracic limbs, six pairs, that extend out through the shell’s opening.
The cirri beat rhythmically, creating currents that draw microscopic food particles, such as plankton and detritus, from the water into the barnacle’s shell. Sensory hairs on these cirri help detect food and water movement. Once captured, the food is moved towards the mouthparts. Barnacles also possess a digestive system, allowing them to process the collected food. Barnacles are hermaphroditic, possessing both male and female reproductive organs, with ovaries located in the base and testes towards the back of the head.
The Superglue Secret: How Barnacles Attach
Barnacles attach themselves to surfaces underwater. This is achieved through a specialized adhesive, often referred to as “barnacle superglue.” This adhesive is produced by specialized cement glands, located at the base of their antennae in adult barnacles. The substance secreted is a multi-protein complex, primarily proteinaceous, making up over 90% of its composition, with trace amounts of carbohydrates, ash, and lipids.
This protein-based cement is fast-curing and water-resistant, allowing barnacles to bond to a wide variety of surfaces in harsh marine environments. The proteins within the cement self-assemble into stable nanofiber networks, contributing to the adhesive’s strength and underwater adhesion. The adhesive achieves its strength through a combination of mechanisms, including hydrogen bonding, electrostatic forces, and hydrophobic interactions, allowing it to remain attached even under significant shear forces. This natural underwater adhesive is so effective that researchers are studying its properties for potential commercial applications, including in medical and dental fields.