Barnacles are marine crustaceans recognizable by their hard, calcified shells and fixed, sedentary adult lifestyle. They often cling to hard surfaces, including coastal rocks, pilings, and the bodies of sea creatures, leading many people to view them simply as pests. The question of whether barnacles are merely hitchhikers or true parasites requires a closer look at the diverse relationships they form. Their ecological role varies widely, ranging from harmless associations to profoundly destructive ones.
Defining Commensalism and Parasitism
Scientists categorize long-term ecological interactions based on the effects on each participating species. Commensalism occurs when one species benefits from the interaction while the other is neither helped nor harmed. Commensal organisms gain advantages like transportation or access to food without affecting the host’s fitness. Conversely, parasitism is an interaction where the parasite benefits at the expense of the host. The host is harmed as the parasite extracts resources, often causing disease, tissue damage, or reproductive failure.
The Typical Barnacle: A Case of Commensalism
The majority of barnacle species that the public commonly encounters, such as acorn barnacles fixed to docks and intertidal zones, are free-living filter feeders. These cirripedes are known for their sedentary adult phase, where they secrete a powerful cement to attach to a substrate for life. Once fixed, they use feathery, leg-like appendages called cirri to sweep the water column, capturing plankton and detritus for food.
When these barnacles attach to large, mobile animals such as whales or sea turtles, the relationship shifts to commensalism. The barnacles gain a stable habitat that transports them through nutrient-rich waters. This continuous travel ensures a steady supply of food particles.
The large marine host is generally considered unaffected by the presence of these organisms. While a massive accumulation of barnacles may increase drag, the effect is negligible for such large animals. Some whale barnacle species, like Coronula diadema, evolve specialized shell structures that embed deeply into the host’s skin for a secure grip. This arrangement allows the barnacle to benefit from transportation and feeding without significantly damaging the host, confirming the commensal classification.
The Exception: Truly Parasitic Barnacles
The answer lies with a highly specialized group known as the Rhizocephala, or root-head barnacles. These barnacles, represented by the genus Sacculina, bear almost no resemblance to their shelled relatives, having undergone radical morphological simplification. The adult form lacks the familiar calcified plates, mouth, gut, or limbs of a typical barnacle.
The female Sacculina begins as a microscopic larva that seeks out a decapod crustacean host, usually a crab. The larva injects a mass of cells, the vermigon, into the crab’s hemolymph. Once inside, the vermigon develops into the interna, a complex, root-like network that spreads throughout the crab’s body cavity, reaching the central nervous system.
This internal root system absorbs nutrients directly from the host’s tissues, effectively starving the crab. The parasite’s reproductive body, called the externa, emerges as a bulbous sac on the crab’s abdomen. The most profound damage is caused by parasitic castration, where the barnacle destroys the host’s reproductive organs and hormonally manipulates the crab. Infected male crabs are feminized, developing a wider abdomen to accommodate the parasite and exhibiting nurturing behaviors typically reserved for females.