In nature, no organism exists in isolation. The life of one species is woven with the lives of many others, creating a complex web of interactions. These relationships, ranging from fleeting encounters to lifelong partnerships, shape the structure and function of entire ecosystems. These close physical associations between different species take on many forms, and the variety of these partnerships reflects the diverse strategies life has evolved to survive and reproduce.
The “Sharing a Table” Relationship
Commensalism describes a specific type of long-term interaction between two different species. The term itself comes from the Latin ‘commensalis’, which means “sharing a table.” This origin provides an intuitive way to understand the dynamic, first formally described by paleontologist Pierre-Joseph van Beneden in 1876. He initially used it to describe how some animals follow predators to eat the leftover scraps from a kill.
In this relationship, one organism benefits while the other is not significantly helped or harmed, which is represented as a “+/0” interaction. The species that gains the benefit is the commensal, and the unaffected partner is the host. While the commensal species may show significant adaptations suited to the relationship, the host organism is essentially unchanged by the interaction.
The benefit for the commensal can come in various forms, including:
- Access to food
- Shelter from the elements
- Protection from predators
- Transportation
Examples of Commensalism
A classic example of commensalism occurs between remoras and sharks. The remora has a modified dorsal fin that acts as a suction cup on its head. This allows it to attach to a shark, gaining transportation through the ocean without expending its own energy. The remora feeds on scraps of food dropped by the shark during its meals, gaining both a ride and a source of nutrition while the shark is largely unaffected by its small passenger.
Another well-known case is the relationship between whales and barnacles. Barnacles begin their lives as free-swimming larvae and, upon finding a suitable host like a humpback whale, they permanently attach themselves to its skin. As filter feeders, the barnacles benefit by being transported through plankton-rich waters, which provides a constant food supply. A whale can carry up to 1,000 pounds of barnacles without any significant negative impact on its health or mobility.
In terrestrial ecosystems, tree frogs often use plants for protection. A frog might take shelter under the large leaves of a plant to hide from predators or to shield itself from heavy rain. In this interaction, the frog receives a clear benefit—shelter—while the plant is not harmed or helped by the frog’s temporary presence. The plant’s structure provides a safe haven, but its own biological processes like photosynthesis and growth are not impacted.
How Commensalism Compares to Other Relationships
Commensalism is one of several types of symbiotic, or long-term, interactions between species. Its unique “+/0” dynamic becomes clearer when compared to other relationships.
A different kind of interaction is mutualism, where both species involved receive a benefit. This is a “+/+” relationship. A common example is the connection between bees and flowers; the bee gets nectar for food, and in the process, it pollinates the flower, allowing the plant to reproduce.
At the other end of the spectrum is parasitism, a “+/-” relationship where one organism benefits at the expense of the other. A tick feeding on a dog is a clear instance of parasitism. The tick receives a blood meal, but the dog is harmed through blood loss and the potential transmission of diseases. Unlike commensalism, where the host is neutral, a host in a parasitic relationship is negatively affected.