Barnacles are common marine crustaceans often seen clinging to rocks, ship hulls, and even whales. Their hard, cone-like shells lead many to mistake them for mollusks, but they are arthropods related to crabs and shrimp. These creatures are known for their stationary adult life, permanently fixed to a single spot. Less known is the complex and mobile journey a barnacle undertakes to reach this point, which begins with a fertilized egg and involves multiple free-swimming stages.
Barnacle Reproduction and Fertilization
Most barnacle species are hermaphrodites, meaning each individual possesses both male and female reproductive organs. While self-fertilization is possible, they engage in cross-fertilization with neighbors to maintain genetic diversity within their dense populations. To overcome their fixed positions, barnacles have developed a long and retractable penis that can extend many times the barnacle’s body length, allowing it to reach adjacent individuals. After the sperm is delivered, the receiving barnacle’s eggs are fertilized internally.
Once fertilization occurs, the eggs are not immediately released. Instead, they are brooded within the parent’s mantle cavity, a protected space inside the shell. This period of internal brooding shields the developing embryos from the harsh marine environment and predators, ensuring they have a higher chance of survival before being released.
The Barnacle Egg Life Cycle
Fertilized eggs are held within the mantle cavity in clusters called ovigerous lamellae. A single barnacle can brood thousands of eggs at once, which develop for weeks or months depending on the species and water temperature. When development is complete, the eggs hatch, and the parent releases the first larval stage, a free-swimming larva known as a nauplius, into the water column.
The nauplius larva is tiny, possessing a single eye, a head, and three pairs of limbs used for swimming and feeding. As a component of the zooplankton, it drifts with ocean currents and feeds on phytoplankton to fuel its growth. During this phase, which can last for several weeks, the nauplius will molt multiple times as it increases in size. This mobility allows barnacle populations to disperse over wide areas.
After several molts, the nauplius larva transforms into the second larval stage, the cyprid. This stage is named for its resemblance to a type of crustacean called a cypris and has a bivalved carapace similar to a tiny clam. The emergence of the cyprid marks the end of the feeding phase of the barnacle’s early life; its next task is to find a permanent home.
From Larva to Settlement
The cyprid larva is a non-feeding, settlement-focused organism. Its purpose during this phase, which may last from a few days to a couple of weeks, is to locate a suitable surface for its adult life. The cyprid actively explores potential locations using a pair of modified antennules that function as sensory organs, allowing it to “walk” across surfaces and test for specific textures and chemical signals.
Cyprids are highly selective and respond to cues that indicate a safe and advantageous environment. They are often attracted to chemicals released by adult barnacles of the same species, a behavior that leads to the dense clustering seen in barnacle populations. They can also detect biofilms created by bacteria, which signal a stable surface.
Once an appropriate spot is found, the cyprid initiates the attachment process. It secretes an adhesive cement from glands at the base of its antennules, gluing itself head-first to the chosen surface. Following this permanent attachment, the cyprid undergoes its final metamorphosis, transforming from a mobile larva into a juvenile barnacle that develops the familiar six calcite plates of the adult shell.