Barnacles are crustaceans often seen clinging to rocks, ships, and marine life. Crabs, also crustaceans, frequently share these habitats. While both are arthropods, their interactions range from benign to detrimental.
Types of Barnacle-Crab Interactions
The relationship between barnacles and crabs varies. Many barnacle species exhibit commensalism, benefiting without harming the crab. For instance, barnacles like Balanus or Octolasmis attach to a crab’s shell, using it for transport to new feeding grounds.
These external barnacles are filter feeders, capturing food particles as the crab moves. The crab experiences few adverse effects, and the barnacles are often shed when the crab molts.
In contrast, certain barnacle species are parasites, harmful to crabs. These parasitic barnacles belong to Rhizocephala, with Sacculina being a well-known genus.
Unlike their sessile, shelled relatives, adult rhizocephalan barnacles bear little resemblance to typical barnacles; however, their larval forms confirm their classification.
Once a female Sacculina larva finds a crab host, it burrows into the crab’s body, developing an internal, root-like network. This structure absorbs nutrients directly from the crab, establishing a parasitic relationship.
Specific Harmful Effects of Parasitic Barnacles
Parasitic barnacles, particularly Sacculina, inflict physiological and behavioral changes on their crab hosts. One impact is parasitic castration, where the barnacle prevents the crab from reproducing.
The parasite’s internal root system grows into the crab’s gonads, destroying them and rendering both male and female crabs infertile. This redirects the crab’s reproductive energy to produce the parasite’s own offspring.
Beyond sterilization, Sacculina manipulates the crab’s physiology and behavior. Infected crabs experience an energy drain as the parasite siphons nutrients, leading to reduced growth and impaired overall health.
Male crabs infected by Sacculina can undergo feminization, developing broader, flattened abdomens that resemble those of female crabs.
This hormonal manipulation can cause male crabs to exhibit behaviors associated with female crabs, such as caring for the parasite’s external brood sac.
The parasite’s external reproductive sac, known as the “externa,” emerges from the crab’s abdomen, where the crab would normally carry its own eggs.
Crab Molting and Barnacle Management
Crabs naturally defend against external attachments through their molting process, also known as ecdysis. As crabs grow, they periodically shed their rigid outer exoskeleton.
This shedding allows them to remove any organisms, including commensal barnacles, that have attached to their old shell. Once the old shell is cast off, the crab emerges with a new, soft exoskeleton that quickly hardens, effectively removing external attachments.
However, this natural process offers no respite from the internal infestation of parasitic barnacles like Sacculina. The root-like network of these parasites is embedded within the crab’s soft tissues and organs, not merely attached to the outer shell.
Therefore, when an infected crab molts, the parasitic barnacle’s internal structure remains firmly within the host. This means the crab cannot shed the parasite, and the infection persists throughout its life, continuing to drain resources and manipulate its host.
Infected crabs may be unable to molt normally, further hindering their growth and ability to shed other external attachments.