Barnacles are indeed alive, a fascinating group of marine organisms often found clinging to rocks, ship hulls, or even whales. Despite their stationary appearance, these creatures engage in dynamic biological processes. Understanding their unique biology reveals their active feeding, reproduction, and specialized attachment mechanisms.
Barnacles: Are They Animals?
Barnacles are animals, classified as crustaceans within the arthropod phylum. This places them in the same group as crabs, lobsters, and shrimp. While they might resemble mollusks or inanimate growths due to their hard, shell-like exterior and sessile lifestyle, their internal anatomy and developmental stages establish their animal lineage.
An adult barnacle possesses a hard calcareous shell composed of six plates forming a cone, topped by four more plates that act as a movable “door” or operculum. Inside this casing, the barnacle lies on its back, with its body divided into a head and thorax, featuring minimal abdomen. This body plan, along with shared larval stages with other crustaceans, distinguishes them from plants or rocks they might resemble.
The Active Life of a Barnacle
Despite their rooted adult existence, barnacles lead an active life, particularly in how they acquire food and reproduce. Most barnacles are filter feeders, extending feathery appendages called cirri through their shell’s opening. These six pairs of thoracic limbs sweep through the water, combing it for microscopic organisms, plankton, and detritus. When sensing a threat or when the tide recedes, they retract their cirri and close their shell.
Barnacle reproduction involves most species being hermaphroditic, meaning each individual possesses both male and female reproductive organs. This adaptation is advantageous for sessile animals, increasing the likelihood of successful reproduction by allowing any nearby barnacle to be a mate. Cross-fertilization is common, often involving one barnacle extending a long penis, relative to its body size, to reach and fertilize a neighboring individual.
Barnacles begin life as free-swimming larvae, undergoing two stages: the nauplius and the cyprid. The nauplius is a tiny, one-eyed larva that feeds on plankton and molts through six stages over about two weeks. This stage then transforms into the cyprid larva, a non-feeding, developed stage specialized for finding a suitable place to settle. The cyprid stage is crucial for dispersal and can last from days to weeks as it explores surfaces.
How Barnacles Stay Put
Barnacles remain permanently affixed to surfaces due to specialized biological mechanisms. After its free-swimming larval phase, the cyprid larva locates a suitable substrate. It uses modified antennules to explore surfaces, assessing environmental cues such as texture, chemistry, and the presence of other barnacles. Once an ideal spot is identified, the cyprid attaches head-first.
Permanent attachment is achieved through the secretion of a strong, natural adhesive. This “barnacle glue” is produced by specialized cement glands, which develop from structures in the cyprid larva. The adhesive is not a simple chemical but a complex, protein-based material forming a dense, insoluble matrix. Before the main proteins set, barnacles secrete lipids and other compounds that clear away water and contaminants from the surface, ensuring a robust bond. This underwater bonding mechanism allows adult barnacles to withstand harsh marine environments and remain fixed for their entire sessile life.