The intriguing creatures known as barnacles often provoke questions about their lifestyle, particularly whether they are parasites. These marine invertebrates are commonly found clinging to various surfaces in the ocean. Understanding their typical behaviors and the specific definition of parasitism helps clarify this common query, revealing that while most barnacles are not parasitic, some specialized species indeed are.
Defining Parasitism
Parasitism describes a biological interaction where one organism, the parasite, lives on or inside another organism, the host, benefiting at the host’s expense. The parasite derives nutrients, shelter, or other resources from the host, typically causing some degree of harm or reduced fitness. This relationship is long-term and distinct from predation, where a predator usually kills its prey. Tapeworms living in an animal’s intestines or fleas feeding on a mammal’s blood are common examples of parasitic relationships.
Barnacles: More Than Meets the Eye
Most barnacles are crustaceans, distantly related to crabs and lobsters. As adults, they are sessile, meaning they are permanently attached to a hard surface such as rocks, marine vessels, or other marine animals. They secure themselves using a strong, natural cement produced from glands at the base of their antennae.
These cirri rhythmically sweep through the water, capturing sustenance and bringing it to the barnacle’s mouth. This feeding method means they obtain their nutrition directly from the surrounding water, not from the organism or surface they attach to. Therefore, most barnacles are considered epibionts or engage in commensalism; they live on another organism or surface without causing harm or taking nutrients from it. While their presence might cause minor hydrodynamic drag on a host, they generally do not negatively impact the host’s health or survival.
When Barnacles Break the Mold
While most barnacles lead a non-parasitic existence, a small, specialized group known as Rhizocephala represents a significant exception. These barnacles are parasitic, primarily targeting other crustaceans, especially crabs. Their adult morphology is dramatically reduced, lacking shells, limbs, or even a digestive system.
A well-known example is Sacculina, which infects crabs. A female Sacculina larva injects cells into the crab’s body, which then develops into a root-like network called the “interna” that spreads throughout the crab, absorbing nutrients from the host’s hemolymph, or blood, and often destroying its gonads, leading to parasitic castration. An external sac, the “externa,” then emerges from the crab’s abdomen. The Sacculina can even manipulate the host’s behavior, causing infected male crabs to develop female-like characteristics and care for the parasite’s externa. This extreme adaptation clearly fits the definition of parasitism, as the barnacle lives inside the host, derives all its nutrition from it, and significantly harms its physiology and behavior.