Barnacles are common on marine organisms, including crabs. These creatures often adhere to the crab’s exoskeleton, raising questions about their impact.
What Are Crab Barnacles?
Barnacles are crustaceans, distantly related to crabs, known for their sessile adult stage. While some attach to rocks or boats, those on crabs often belong to Rhizocephala, like Sacculina. Unlike typical barnacles with hard shells, Sacculina appears as a sac-like growth on the crab’s underside. This external sac is the parasite’s reproductive organ; its main body forms a root-like network inside the crab. These internal roots spread throughout the host’s tissues, absorbing nutrients directly from the crab’s hemolymph.
How Barnacles Attach to Crabs
Barnacle attachment begins with a microscopic larval stage. For parasitic rhizocephalan barnacles, a nauplius larva hatches and develops into an infective cyprid larva. This cyprid actively seeks a crab host, often attaching to a bristle.
Upon finding a host, the cyprid injects cells into the crab’s body through a tiny opening. These cells form the invasive root-like system that spreads throughout the crab’s internal organs.
For non-parasitic, commensal barnacles, the cyprid larva secretes a strong adhesive, attaching to the crab’s carapace. Attachment is largely irreversible for both types.
The Impact on Crab Health
Barnacles can have varied impacts on a crab’s health. Commensal barnacles, using the crab for transport, cause minimal harm, though many could increase drag. In contrast, parasitic rhizocephalan barnacles, like Sacculina, significantly influence their hosts.
These parasites castrate crabs, preventing molting and diverting energy from the crab’s reproduction and growth to the barnacle’s external reproductive sac. The infected crab’s reproductive organs atrophy, and it becomes sterile.
The parasitic barnacle can also manipulate the crab’s behavior, causing both male and female crabs to exhibit brooding behaviors. An infected male crab might fan and clean the external sac, protecting the parasite’s offspring. This manipulation, coupled with physiological drain, compromises the crab’s fitness and survival. The crab’s immune system often fails to eliminate the parasitic network, allowing the barnacle to continue its influence.
Crab Defenses Against Barnacles
Crabs possess several mechanisms to defend against barnacle infestations. The most effective defense against external barnacles, whether commensal or parasitic, is molting. During molting, the crab sheds its entire exoskeleton, including any attached barnacles, removing external burdens.
Crabs also groom, using legs and claws to scrape off debris and smaller attached organisms. While this can remove newly settled barnacles, it is ineffective against larger, firmly attached individuals or internal parasitic structures. The crab’s immune system mounts a response, but highly adapted parasitic barnacles like Sacculina have evolved ways to evade or suppress it, ensuring their survival within the host.