Remoras, often called suckerfish, are frequently observed attached to the bodies of sharks. This well-known association prompts curiosity about the nature of their interaction. This article explores the biological classifications of such relationships and examines the roles each animal plays.
Defining Symbiotic Relationships
Relationships between different species that live in close association are broadly categorized as symbiotic. There are several types of symbiotic interactions, each defined by how the participating organisms affect one another. Mutualism benefits both species, sometimes to the extent that their survival depends on the interaction.
Commensalism is a long-term interaction where one species obtains benefits, while the other is neither helped nor harmed. The benefiting species, or commensal, may gain nutrients, shelter, support, or locomotion, with the host remaining largely unaffected. In contrast, parasitism describes a relationship where one organism, the parasite, benefits at the expense of the other, the host, often causing harm.
The Remora’s Strategy
Remoras possess a modified dorsal fin on their head that functions as a suction disc, allowing them to attach to larger marine animals. This oval-shaped organ features slat-like structures that create a vacuum, enabling the remora to adhere firmly to surfaces like a shark’s skin. The disc also includes structures that enhance grip and prevent slippage.
By attaching to sharks, remoras gain several advantages. They receive free transportation across vast oceanic distances, conserving energy they would otherwise expend swimming. This hitchhiking strategy also provides remoras with access to food scraps that fall from the shark’s meals, making foraging more efficient. Additionally, the presence of a large predator like a shark offers remoras protection from their own predators.
The Shark’s Involvement
The shark’s role in this association is often debated. It has been proposed that remoras provide a cleaning service to sharks by consuming parasites and dead skin. This cleaning behavior could potentially contribute to the shark’s overall well-being.
However, scientific evidence indicates that sharks are largely indifferent or unaffected by the presence of remoras. While some accounts suggest sharks may slow down to allow remoras to attach, this behavior is not consistently observed across all shark species. Some shark species, such as sandbar and lemon sharks, have been documented acting aggressively towards remoras, and in rare instances, consuming them. The attachment of remoras can also create a minor increase in hydrodynamic drag for the shark.
Assessing the Relationship
Based on the definitions of symbiotic relationships and observed behaviors, the relationship between remoras and sharks is most accurately classified as commensalism. The remora clearly benefits by securing transportation, obtaining food from the shark’s leftovers, and gaining protection from predators.
Conversely, the shark does not appear to experience any substantial benefit or harm from the remora’s presence. While some older theories suggested remoras clean sharks of parasites, more recent scientific consensus indicates that the primary food source for remoras is typically food scraps, fecal matter, or sloughed-off skin, rather than a consistent and significant removal of harmful parasites that would substantially benefit the shark. The minor drag imposed by a remora’s attachment is generally not considered detrimental to the shark’s health or swimming ability. Therefore, the shark remains largely unaffected, reinforcing the classification of this interaction as commensal.