Barnacles are marine creatures found clinging to various surfaces in the ocean. While they might resemble small mollusks due to their hard shells, barnacles are actually crustaceans, like crabs and lobsters. Adult barnacles are unique among crustaceans because they are sessile, meaning they permanently attach themselves to a substrate and remain fixed for their entire adult lives. They are widespread in marine environments, inhabiting intertidal rocks, ship hulls, and even the skin of whales.
The Outer Shell and Its Purpose
A barnacle’s hard, protective shell is composed of multiple calcareous plates. In typical acorn barnacles, six plates form a cone-like structure encircling the animal, with four additional plates forming an operculum, or “door,” at the top. These plates fit together, creating a robust outer layer. Stalked barnacles, in contrast, have their body encased in plates but are elevated on a fleshy stalk that attaches to the substrate.
The shell’s purpose is protection. It shields the barnacle from predators like whelks and starfish, desiccation when exposed to air during low tide, and powerful wave action. Closing its operculum allows the barnacle to seal itself off, conserving moisture and defending against harsh conditions.
The Living Body Within
Beneath the barnacle’s outer shell lies its soft body. Prominent internal features include feathery appendages called cirri, which are modified thoracic limbs. Barnacles extend these six pairs of cirri through the shell’s opening, sweeping them through water to capture food particles. These cirri act like a net, filtering plankton and detritus from the water, which are then brought to the mouth.
The barnacle possesses a digestive system to process filtered food, along with a nervous system and a basic circulatory system. Respiration, the process of breathing, occurs through the mantle, a fleshy membrane lining the inside of the shell. While cirri circulate water, the mantle’s surface directly facilitates gas exchange, allowing the barnacle to take in oxygen from the surrounding water.
Life Cycle and Reproduction
Barnacles exhibit a reproductive strategy where most species are hermaphroditic, meaning each individual possesses both male and female reproductive organs. Despite this, cross-fertilization with a neighboring barnacle is common. To achieve this, barnacles have developed a long, extensible penis, which can reach out to fertilize individuals several inches away. After fertilization, eggs mature within the mantle cavity.
Barnacles undergo larval stages before reaching adulthood. Fertilized eggs hatch into free-swimming nauplius larvae, tiny, one-eyed organisms that float as plankton. After several molts, the nauplius transforms into the cyprid larva, a non-feeding, shrimp-like stage. The cyprid larva’s role is to locate a suitable surface for permanent attachment, a decision determining its adult habitat. Once a site is chosen, the cyprid undergoes metamorphosis, settling head-first and transforming into a sessile adult.
Sticky Secrets: Attachment and Habitat
The ability of barnacles to adhere to surfaces is facilitated by a specialized adhesive. During the cyprid larval stage, a cement gland in its antennules produces a strong, fast-setting protein-based glue. This adhesive allows the cyprid to anchor itself to a chosen substrate. The bond created by barnacle cement is durable and withstands harsh marine conditions, including strong ocean currents and waves.
Barnacles inhabit marine environments and attach to any solid surface. They are found on intertidal rocks, pilings, and buoys in coastal areas. Beyond stationary objects, barnacles attach to moving surfaces, including boat and ship hulls, a phenomenon known as biofouling, which can increase drag and fuel consumption. They also form symbiotic relationships by attaching to larger marine animals like whales and sea turtles. Their strong attachment allows them to thrive in these varied and challenging habitats.