Small, cone-shaped growths are commonly found firmly attached to the shell of a freshly caught lobster. These marine hitchhikers are barnacles, which are themselves a type of crustacean, often mistaken for external disease or sea clams. This arrangement is generally described as commensalism, a form of symbiosis where one organism benefits while the other is mostly unaffected, though minor detriments can occur. Observing these growths raises questions about the lobster’s health and the quality of its meat for consumers.
Identifying the Barnacle Hitchhiker
These white, volcano-shaped organisms found on a lobster’s shell belong to a group of arthropods called Cirripedia, making them cousins to the lobster, crab, and shrimp. Barnacles are crustaceans that have adopted a unique, sessile, or stationary, adult lifestyle. The species most often seen on lobsters are typically acorn barnacles, which form a hard, multi-plated shell directly on the surface, or occasionally goose barnacles, which attach via a flexible stalk.
The barnacle’s outer shell is composed of six calcium plates that form a cone, with a four-plated “door” at the top that can open and close. When submerged, the barnacle extends its feathery, jointed legs, called cirri, through this opening to filter plankton from the water column for food. On a lobster, these stationary filter-feeders will settle on any hard, exposed surface, including the carapace, claws, and legs.
The Relationship: How Barnacles Attach and Survive
The barnacle’s survival on a lobster is entirely dependent on its ability to secure permanent residency on a moving surface. Once a barnacle larva, known as a cyprid, finds a suitable spot, it attaches head-first using a powerful, fast-curing protein-based cement secreted from specialized cement glands. This bio-adhesive is one of the strongest natural glues known, necessary to withstand the constant turbulence of the ocean.
The ultimate threat to a barnacle’s fixed existence is the host’s molting cycle. As lobsters grow, they periodically shed their entire hard outer shell, or exoskeleton, a process called ecdysis. This shedding removes every attached organism, including the barnacles, forcing new larvae to choose a long-term home.
Consequently, barnacles tend to accumulate on older, larger lobsters that molt less frequently, sometimes only once a year. The slow molting rate of larger lobsters offers a relatively stable period of attachment. This biological timing explains why heavily encrusted lobsters are generally mature adults who have not shed their shell in a long time.
Impact on Lobster Health and Quality
The presence of barnacles on a lobster is generally considered to have a minimal direct impact on the host’s overall health. The barnacles are ectoparasites or commensals, meaning they live on the outside of the lobster and filter their own food from the water, not drawing nutrients from the host’s tissues. However, a very heavy growth of barnacles can transition the relationship from benign commensalism to one of mild detriment.
A dense colony of barnacles creates hydrodynamic drag, forcing the lobster to expend more energy to move, hunt, and escape predators, which can be a subtle drain on its resources. If barnacles settle on the joints or knuckles of the claws or legs, they can also mechanically impede the lobster’s movement, making it more difficult to forage or defend itself. While these effects can impair an individual lobster, barnacles do not penetrate the shell or cause infectious disease.
For the consumer, the most important consideration is that the barnacles are strictly surface-level organisms. They adhere only to the external shell and have no effect on the safety, flavor, or texture of the meat inside. A lobster with a barnacle-covered shell simply indicates an older individual that has not shed its shell recently.