Commensalism describes a relationship between two organisms where one benefits and the other experiences no significant harm or help. The term is applied to a wide range of interactions, from those providing transportation to those offering shelter or food. This interaction is one of several types of close relationships, known as symbioses, that shape the natural world.
Commensalism in Marine Environments
The world’s oceans are home to many examples of commensal relationships, often involving access to food or transportation. One well-known case is the interaction between sharks and remora fish. Remoras have a modified dorsal fin that acts as a suction cup, allowing them to attach to the bodies of larger marine animals like sharks. This attachment provides the remora with transportation and allows it to feed on the scraps left over from the shark’s meals. The shark, in this relationship, is largely unaffected, as the small size of the remora does not impede its movement or take a meaningful amount of its food.
A different marine example involves whales and barnacles. Certain species of barnacles attach themselves to the tough skin of whales, providing the barnacles with a stable surface to live on. As the whale moves through the ocean, the barnacles gain access to plankton-rich waters, which they filter for food. For the whale, the presence of barnacles is typically a neutral event, as the weight and drag caused by the barnacles are generally considered negligible for an animal of such immense size.
Commensalism on Land
On land, commensalism is frequently observed in the interactions between large animals and smaller organisms. The relationship between cattle egrets and grazing livestock, such as cattle or horses, is a classic illustration. These birds are often seen foraging in fields close to the large mammals, capitalizing on an easy food source. The livestock, as they walk and graze, disturb insects and other small invertebrates hiding in the vegetation, flushing them out into the open.
This activity makes the insects easy targets for the egrets, which would otherwise have to expend more energy to find them. The livestock, on the other hand, are generally indifferent to the presence of the egrets. The birds do not interfere with their grazing, and their proximity causes no harm.
Another form of terrestrial commensalism involves epiphytic plants, such as certain species of orchids and bromeliads, that grow on the branches of large trees, particularly in dense rainforests. These plants require sunlight to survive, which can be scarce on the shaded forest floor. By growing on the limbs of tall trees, epiphytes secure a position high in the canopy where sunlight is abundant.
These plants are not parasitic; they do not draw nutrients or water from the tree itself. Instead, they absorb moisture from the rain and nutrients from the air and accumulating organic debris. The tree simply serves as a physical platform, providing the epiphyte with access to light without being impacted.
Commensalism for Shelter and Transport
Many commensal relationships are based on one organism providing shelter or transportation to another. Hermit crabs offer a clear example of commensalism for shelter. These crabs lack a hard shell of their own and protect their soft abdomens by inhabiting the empty shells of deceased sea snails. The crab receives a durable, mobile home.
This type of commensalism is sometimes called metabiosis, where one organism uses something produced by another, now-deceased, organism. The snail that originally created the shell is long dead and thus completely unaffected by the crab’s tenancy. As the hermit crab grows, it must find progressively larger shells.
Transportation is another benefit seen in commensalism, particularly in seed dispersal. The burdock plant, for instance, produces seeds contained within burrs covered in stiff hooks. When a mammal, such as a bear or deer, brushes past the plant, these burrs cling to its fur. The animal then carries the seeds far from the parent plant before they eventually fall off.
This process allows the burdock to disperse its seeds over a wide geographic area, increasing the chances of germination in a suitable location with less competition. For the most part, the animal is not harmed by the burrs, which are a temporary inconvenience that does not cause injury.
How Commensalism Differs from Other Symbiotic Relationships
Understanding commensalism requires distinguishing it from other types of symbiosis, mainly mutualism and parasitism. In mutualism, both species involved in the relationship benefit. A well-known example is the interaction between bees and flowers; the bee gets nectar for food, and in the process, it pollinates the flower, enabling it to reproduce.
Parasitism, conversely, involves one organism benefiting at the expense of the other. A tick feeding on a dog is a clear example of parasitism. The tick gains a blood meal, which it needs to survive and reproduce, while the dog is harmed through blood loss and potential disease transmission.
Commensalism is defined by the neutral outcome for one of the participants. While one organism benefits, the other is neither helped, as in mutualism, nor harmed, as in parasitism.