Barnacles and crabs are both crustaceans, but their relationship is not always one of simple coexistence; it is often one of highly specialized parasitism. While the most familiar barnacles are sessile filter feeders, a small group of barnacles have evolved to become internal parasites of crabs and other decapods. These specialized parasites exert a profound, systemic influence on the crab’s biology, physiology, and behavior. The effects range from subtle internal starvation to the dramatic manipulation of the host’s reproductive life and gender characteristics.
Identifying the Specialized Parasite
The most dramatic effects on crabs are caused by barnacles belonging to the infraclass Rhizocephala, which includes the genus Sacculina. These parasitic barnacles look nothing like the familiar hard-shelled acorn barnacles typically seen on rocks or boats. Adult rhizocephalans have lost nearly all the distinguishing features of a crustacean, lacking segmentation, appendages, and even a digestive system.
The parasite’s highly reduced body plan is an adaptation for its unique lifestyle. The only external part of the adult female is the externa, a sac-like structure attached to the crab’s abdomen that contains the parasite’s reproductive organs.
Internal Invasion and Nutrient Theft
Infection begins when a female larval stage of the rhizocephalan settles on a crab and develops into a form known as a kentrogon. This stage possesses a hollow, retractable stylet used to pierce the crab’s cuticle, often at a joint. The parasite then injects a mass of undifferentiated cells into the crab’s body cavity, or hemocoel.
This cellular mass travels through the crab’s circulatory system and migrates to the ventral abdominal sinus. Once established, it develops into the parasite’s internal structure, called the interna. The interna is a complex network of root-like filaments that spreads throughout the crab’s body, infiltrating tissues and organs, including the hepatopancreas and even the nervous system. This root system functions as a biological siphon.
Manipulation of Host Biology and Behavior
Once the interna is fully developed, the parasite forms the externa, a visible, bulbous reproductive sac that emerges from the crab’s abdomen. The presence of the parasite alters the host’s endocrinology. One significant consequence is parasitic castration, where the parasite suppresses the crab’s reproductive system and causes the gonads to atrophy.
The parasite also chemically manipulates the host’s molting cycle. Infected crabs stop molting, which prevents the crab from shedding the parasite’s external sac. This cessation of molting ensures the long-term survival and reproductive success of the barnacle.
Feminization of male crabs occurs when the parasite alters the male crab’s hormonal balance, causing it to develop female secondary sexual characteristics, such as a broadened abdomen. The male crab is also behaviorally manipulated to adopt a female’s “nursing” behavior. The infected male will protect, clean, and ventilate the parasite’s egg sac.
Non-Parasitic Shell Attachments
In contrast to the systemic damage caused by rhizocephalans, other species of barnacles that attach to a crab’s shell are considered non-parasitic commensals or foulers. These barnacles, such as the common acorn barnacles, are merely using the crab as a mobile surface for attachment. Their effect is purely mechanical and external, not biological or internal.
These fouling organisms can increase the crab’s weight and hydrodynamic drag. If the fouling is heavy, it can interfere with the movement of appendages or obstruct sensory organs. However, because the crab periodically molts, it sheds its entire exoskeleton, including all attached barnacles. The non-parasitic barnacles do not steal nutrients, alter the crab’s hormones, or manipulate its behavior.