The relationship between a shark and the remora fish, often called a suckerfish, is a classic example of symbiosis in the marine world, where two species live in close association. This interaction involves the remora attaching to the shark using a specialized suction disc, derived from its dorsal fin. The central question is whether this arrangement is truly mutualistic, meaning both species gain a measurable advantage, or merely commensalistic, benefiting the remora while leaving the shark largely unaffected or even slightly harmed.
The Remora’s Advantage
Transportation and Protection
The remora gains a substantial advantage by attaching itself to a shark or other large marine animal. This attachment provides efficient, long-distance transportation without expending the remora’s energy reserves. The host’s movement also facilitates ram ventilation for breathing, requiring less energy than swimming independently. The constant presence of the formidable shark provides protection, deterring potential predators.
Diet
The remora’s diet is subsidized by the shark, as it feeds on scraps of prey dropped during a kill, sometimes called “predatory fallout.” Remoras also consume parasites and dead skin cells from the shark’s surface.
The Shark’s Perspective on Association
The prevailing scientific view often classifies the remora-shark relationship as commensal, suggesting the shark is largely neutral to the arrangement. However, a limited mutualistic benefit exists, centered on the remora’s feeding habits on the shark’s skin. Remoras consume external parasites, such as parasitic copepods, which can irritate the shark and potentially affect its health. This cleaning behavior is considered the primary positive contribution to the host shark.
The remora may also graze on dead or damaged tissue surrounding minor wounds, providing a form of natural wound management. Some studies suggest that certain shark species may alter their behavior, slowing down to allow remoras to attach. Despite this cleaning service, the benefit provided to a large, healthy shark is generally minor, and the shark is not reliant on the remora for its well-being.
The Cost of Association
While the shark gains minor cleaning benefits, the association carries potential negative consequences, primarily related to hydrodynamics. The presence of remoras disrupts the smooth flow of water over the shark’s body, increasing hydrodynamic drag. This forces the shark to expend more energy to maintain its swimming speed. The energy cost is proportional to the number and size of the attached remoras and the speed of movement.
Though the suction disc is non-damaging, repeated attachment and detachment can cause minor skin irritation or abrasion. In rare cases, this may create a point of entry for secondary infections. Furthermore, remoras occasionally interfere with the host’s feeding activities by snatching food from the shark’s mouth. Overall, the shark pays a measurable price in increased energy consumption and minor irritation for the remora’s presence.