Barnacles are crustaceans, related to crabs and shrimp, yet they exhibit a dramatic lifestyle shift. The answer to whether a barnacle moves depends entirely on its life stage. While the mature adult is famous for its immovable, fixed existence, the young barnacle is a highly mobile, free-swimming creature. This transition from a mobile planktonic larva to a permanently attached, sessile adult is central to the barnacle’s survival.
The Sessile Adult State
The mature barnacle, often seen encrusting rocks, boats, and whales, is defined by its sessile lifestyle, meaning it is fixed in one place for its entire adult life. This immobility is facilitated by its unique body plan: an arthropod standing on its head and encased in a protective shell. The shell of an acorn barnacle is a conical structure made of several calcareous plates cemented directly to the substrate.
These hard, mineralized plates form a robust wall, which protects the soft-bodied animal inside from predators and harsh intertidal conditions. The animal’s body is permanently housed within this structure, preventing whole-body locomotion. The base, affixed to the surface, is either a solid calcareous plate or a membranous structure, permanently locking the barnacle in place. This fixed state offers protection and stability in high-energy environments, but sacrifices mobility.
The Mobile Larval Stages
Before achieving this fixed state, the barnacle spends a period of weeks to months as a mobile, planktonic organism. The life cycle begins with the Nauplius larva, a tiny, single-eyed form that is the primary dispersal stage. The Nauplius swims freely in the water column using three pairs of specialized cephalic appendages, allowing the young organism to find new feeding grounds and spread the species widely.
After several molts, the Nauplius transforms into the Cyprid larva, a non-feeding form dedicated to finding a permanent home. The Cyprid is enclosed in a bivalved carapace and swims efficiently using six pairs of thoracic appendages. This mobile stage actively explores potential surfaces, using modified antennules to “walk” and inspect substrates before committing to settlement. The Cyprid’s movement is a directed search for a favorable site, often attracted by chemical cues from established adult barnacles.
Movement for Survival (Feeding and Respiration)
While the adult barnacle cannot move its entire body, it possesses a specialized, rapid internal movement that is fundamental to its survival. This movement involves the cirri, which are six pairs of feathery, jointed thoracic legs that protrude through a small opening at the top of the shell. The cirri are rhythmically extended and withdrawn into the shell cavity, acting as a net to capture food particles.
This rhythmic “beating” of the cirri sweeps the surrounding water, trapping plankton and detritus for filter feeding. The movement is also critical for gas exchange, as the cirri function as respiratory structures, circulating water in and out of the mantle cavity to facilitate breathing. Barnacles exhibit different types of cirral activity, such as a “normal beat” or an “extension” to capture larger prey in moving water. This internal appendage movement is the only deliberate motion the sessile adult performs, linking its survival to the flow of water.
Permanent Attachment and Cementation
The period of mobility ends abruptly with the irreversible process of permanent attachment, or cementation, transitioning the Cyprid larva into the fixed adult form. Upon locating a suitable substrate, the Cyprid uses specialized cement glands to secrete a powerful, fast-curing bio-adhesive. This glue is a complex, multi-component proteinaceous cement that enables the barnacle to adhere robustly underwater.
The cement gland is retained from the larval stage, and the Cyprid uses it to affix itself head-first to the chosen spot. This proteinaceous cement hardens quickly, creating an extremely strong bond that withstands the forces of ocean currents and waves. After the initial bond is formed, the larva undergoes a radical metamorphosis, losing its swimming appendages and developing the calcareous plates that characterize the sessile adult.