Commensalism is a relationship between two organisms where one benefits and the other is neither helped nor harmed. A classic example is barnacles hitching a ride on whales: the barnacles get carried to food-rich waters, while the whale is unaffected. But commensalism shows up across nearly every ecosystem, from tropical rainforests to your own skin. Here are the most well-known examples and what makes each one work.
Barnacles and Whales
This is the textbook case. Barnacles are small, shelled sea creatures that cement themselves onto whale skin. The whale’s movement through the ocean carries barnacles into plankton-rich feeding grounds they could never reach on their own. The whale, meanwhile, doesn’t appear bothered. A few tiny shells on an animal that weighs tens of thousands of pounds impose no meaningful cost. The barnacles gain food access and transportation, while the whale’s daily life continues unchanged.
Cattle Egrets and Livestock
If you’ve ever driven past a pasture and seen white birds walking among cows, you’ve seen commensalism in action. Cattle egrets follow grazing livestock because the animals’ heavy footsteps flush insects out of the grass, making them easy pickings. Research on this relationship found that egrets are roughly 3.6 times more efficient at catching prey when foraging near cattle compared to foraging alone. The time between successful catches also drops significantly. The cattle, for their part, gain nothing from the birds’ presence and lose nothing either.
Orchids and Trees
In tropical and subtropical forests, orchids and bromeliads grow on the branches and trunks of large trees. These plants, called epiphytes, use the tree purely as a platform. They don’t tap into the tree’s nutrients or water supply the way a parasite would. Instead, they absorb moisture and nutrients from the air, rain, and debris that collects around their roots. The tree provides access to sunlight high in the canopy, which the epiphyte couldn’t reach from the forest floor.
The relationship isn’t random. Certain tree traits affect which epiphytes can establish themselves. Trees with rough, textured bark make it easier for seeds to lodge and germinate. Bark that retains moisture provides a more hospitable surface. Some trees even produce chemical compounds in their bark that inhibit epiphyte germination, which is why you’ll see some tree species loaded with orchids and others nearly bare.
Pseudoscorpions Riding Beetles
One of the more unusual examples involves tiny arachnids called pseudoscorpions that deliberately climb onto large beetles to travel between habitats. This behavior, known as phoresy, is not accidental. Pseudoscorpions detect beetles through smell and sound, then perform a specific sequence of actions to board. They pinch the beetle’s abdomen repeatedly, causing it to flex and partially open its wing covers, creating a small space the pseudoscorpion slips into for the ride.
The purpose is dispersal. Large numbers of adult pseudoscorpions board beetles on old, resource-depleted trees and disembark on freshly fallen ones where food and shelter are abundant. The beetle carries on unaffected. Researchers confirmed this by comparing pseudoscorpions found on trees to those riding beetles. The riders were actually in worse nutritional condition, ruling out the idea that they were climbing aboard to feed on the beetle’s mites. It’s purely a transportation arrangement.
Bacteria on Human Skin
Your body hosts trillions of microorganisms, and many of the bacteria living on your skin are considered commensals. The core skin community includes species from groups like Staphylococcus, Corynebacterium, and Streptococcus. These microbes colonize your skin, feed on oils and dead cells, and under normal conditions cause no disease.
One well-studied example is Staphylococcus epidermidis, a bacterium found at multiple body sites on virtually every healthy person. Unlike its more dangerous relative Staphylococcus aureus, S. epidermidis has low pathogenicity and is widely considered a beneficial or at least neutral resident. That said, the commensal label isn’t permanent. If the skin is injured or a medical device like a catheter is introduced, some of these normally harmless residents can behave as opportunistic pathogens. Context matters.
A similar story plays out inside the gut. Several species of amoeba, including Entamoeba coli, live in the large intestine without causing any symptoms. According to the CDC, these organisms colonize the gut lining noninvasively and are classified as nonpathogenic. They feed on bacteria and food particles in the intestinal environment while producing no disease in their human host.
Why True Commensalism Is Hard to Prove
Biologists define commensalism strictly: one organism benefits, and the other experiences zero effect, positive or negative. In practice, that “zero” is almost impossible to confirm. Even barnacles on a whale create some drag, however slight. Epiphytes on a tree add weight to branches. The effect may be trivially small, but proving it’s truly zero requires more data than most studies can provide.
This is why some ecologists argue that pure commensalism is rare or even theoretical. Many relationships classified as commensal may involve costs or benefits too small to measure with current methods. The examples above are the best-supported cases, but each one sits on a spectrum. Commensalism is less a clean category and more a useful label for relationships where the impact on one partner is negligible enough to be functionally zero.