Barnacles are a common sight on various marine surfaces, from rocks and boat hulls to the shells of larger sea creatures like crabs. These seemingly simple organisms engage in a fascinating biological interaction with their hosts.
What Exactly Is a Barnacle?
Barnacles are marine crustaceans belonging to the subclass Cirripedia, making them relatives of crabs and lobsters. Unlike their mobile cousins, adult barnacles are sessile, fixed in one place. Their bodies are encased in a protective shell composed of multiple hard, calcareous plates.
To feed, barnacles extend feathery appendages called cirri through an opening in their shell, sweeping the water for microscopic food particles such as plankton. The life cycle of a barnacle involves two free-swimming larval stages: the nauplius and the cyprid. The nauplius larva hatches from the egg and undergoes several molts before transforming into the cyprid, the stage responsible for finding a suitable place to settle.
How Barnacles Attach to Crabs
The cyprid larval stage is central to barnacle attachment. This non-feeding larva, equipped with modified antennules, actively explores surfaces for a suitable settlement site. Once an appropriate spot is identified, the cyprid attaches head-first using its antennules and secretes a powerful, protein-based cement. This adhesive is among the strongest natural glues known and allows for permanent attachment.
A crab’s hard outer shell provides an attractive and stable substrate for barnacles. The constant movement of the crab through the water column can increase the barnacle’s access to food currents, a significant advantage for a filter feeder. While many barnacles attach to the crab’s exterior in a relationship known as commensalism, some specialized barnacles, such as those in the genus Sacculina, exhibit a parasitic relationship, integrating into the crab’s internal body.
The Impact on Crab Health and Life
The effects of barnacle attachment on crabs vary significantly depending on the barnacle species. For many common, externally visible barnacles, the relationship is largely commensal, meaning the barnacle benefits without causing substantial harm. While these barnacles add weight and can create some drag, hindering the crab’s movement or making feeding more difficult, the impact is often minimal. A natural mechanism for crabs to shed these external hitchhikers is molting, where the crab sheds its old exoskeleton, along with any attached barnacles, and grows a new one.
In contrast, parasitic barnacles, like those of the genus Sacculina, have a far more profound and detrimental impact. These barnacles do not remain on the exterior; instead, a larval Sacculina burrows into the crab’s body and develops a root-like network that spreads throughout the host, absorbing nutrients. This internal invasion often leads to the castration of the crab, preventing reproduction. Parasitized male crabs can even develop female characteristics and behaviors, such as a broader abdomen and performing mating dances.
Barnacles Are Not Part of the Crab
A common misunderstanding is that barnacles are an inherent part of a crab’s anatomy or a type of growth. Barnacles are distinct organisms with their own life cycles. They utilize the crab’s shell as a hard surface for attachment, similar to how they attach to rocks, pilings, or boat hulls.
This distinction is important for understanding marine ecosystems. While some barnacles can affect a crab’s well-being, particularly parasitic species, the visible barnacles on a crab’s shell are independent animals seeking a stable place to live and feed.