Is a symbiotic relationship in which one organism benefits, while the other is harmed.

Symbiosis is a relationship between two organisms: it can be mutualistic (both benefit), commensal (one benefits), or parasitic.

Learning Objectives

• Compare Mutualism and Symbiosis

Key Points

• Mutualism, a relationship in which both species benefit, is common in nature. In microbiology, there are many examples of mutualistic bacteria in the gut that aid digestion in both humans and animals.
• Commensalism is a relationship between species in which one benefits and the other is unaffected. Humans are host to a variety of commensal bacteria in their bodies that do not harm them but rely on them for survival (e.g. bacteria that consume dead skin).
• Parasitic relationships, in which one species benefits and the other suffers, are very common in nature. Most of the microorganisms studied in medical microbiology are parasitic and feed on human tissue. For example, cholera, leshmaniasis, and Giardia are all parasitic microbes.
• Symbiotic relationships can also be classified by the physical relationship between the two species. Endosymbionts live inside the tissues of the host, while ectosymbionts live outside of their partner species.

Key Terms

• commensalism: A class of relationship between two organisms in which one organism benefits without affecting the other
• symbiosis: A close and often long-term interaction between two or more different biological species
• mutualism: A relationship between individuals of different species in which both individuals benefit

Symbiosis is any relationship between two or more biological species. Such relationships are usually long term and have a strong impact on the fitness of one or both organisms. Symbiotic relationships are categorized by the benefits and physical relationships experienced by each species.

Common types of symbiosis are categorized by the degree to which each species benefits from the interaction:

• Mutualism: In mutualistic interactions, both species benefit from the interaction. A classic example of mutualism is the relationship between insects that pollinate plants and the plants that provide those insects with nectar or pollen. Another classic example is the behavior of mutualistic bacteria in ecology and human health. Gut bacteria in particular are very important for digestion in humans and other species. In humans, gut bacteria assist in breaking down additional carbohydrates, out-competing harmful bacteria, and producing hormones to direct fat storage. Humans lacking healthy mutualistic gut flora can suffer a variety of diseases, such as irritable bowel syndrome. Some ruminant animals, like cows or deer, rely on special mutualistic bacteria to help them break down the tough cellulose in the plants they eat. In return, the bacteria get a steady supply of food.
• Commensalism: In commensalism, one organism benefits while the other organism neither benefits nor suffers from the interaction. For example, a spider may build a web on a plant and benefit substantially, while the plant remains unaffected. Similarly, a clown fish might live inside a sea anemone and receive protection from predators, while the anemone neither benefits nor suffers.
• Parasitism: Parasites are organisms that harm their symbiotic partners. Parasitism is incredibly common in nature: depending on the definition, more than half of all species may go through at least one parasitic stage in their life cycle. There are many well-documented examples of parasitic bacteria and microorganisms throughout this text.

Symbiosis can also be characterized by an organism’s physical relationship with its partner.

• Endosymbiosis: a relationship in which one of the symbiotic species lives inside the tissue the other. For example, Coral polyps have special algae called zooxanthelle that live inside their cells. Zooxanthelle provide sugars to the coral through photosynthesis. Similarly, nitrogen-fixing fungi often live inside the cells of plants, providing nitrogen in exchange for the sugars of photosynthesis.
• Ectosymbiosis: a relationship in which one species lives on the outside surface of the other. Barnacles that live on whales and bromeliads that live on tropical trees are examples of endosymbionts.

These categories can be paired with the above terms to better describe the species’ interactions. For example, you might say that a gut bacteria is an “endosymbiotic mutualist,” or that a flea is an “ectosymbiotic parasite. ”

Do interactions between species always result in harm?

A commensal shrimp sits on another sea organism, a sea slug. As a commensal shrimp, it neither brings a benefit nor has a negative effect on its host.

Symbiotic Relationships

Symbiosis is a close relationship between two species in which at least one species benefits. For the other species, the relationship may be positive, negative, or neutral. There are three basic types of symbiosis: mutualism, commensalism, and parasitism.

Mutualism

Mutualism is a symbiotic relationship in which both species benefit. An example of mutualism involves goby fish and shrimp (see Figure below). The nearly blind shrimp and the fish spend most of their time together. The shrimp maintains a burrow in the sand in which both the fish and shrimp live. When a predator comes near, the fish touches the shrimp with its tail as a warning. Then, both fish and shrimp retreat to the burrow until the predator is gone. From their relationship, the shrimp gets a warning of approaching danger. The fish gets a safe retreat and a place to lay its eggs.

The multicolored shrimp in the front and the green goby fish behind it have a mutualistic relationship.

Commensalism

Commensalism is a symbiotic relationship in which one species benefits while the other species is not affected. One species typically uses the other for a purpose other than food. For example, mites attach themselves to larger flying insects to get a “free ride.” Hermit crabs use the shells of dead snails for homes.

Parasitism

Parasitism is a symbiotic relationship in which one species (the parasite) benefits while the other species (the host) is harmed. Many species of animals are parasites, at least during some stage of their life. Most species are also hosts to one or more parasites.

Some parasites live on the surface of their host. Others live inside their host. They may enter the host through a break in the skin or in food or water. For example, roundworms are parasites of mammals, including humans, cats, and dogs (see Figure below). The worms produce huge numbers of eggs, which are passed in the host’s feces to the environment. Other individuals may be infected by swallowing the eggs in contaminated food or water.

Roundworms like this one might eventually fill a dog’s intestine unless it gets medical treatment.

Some parasites kill their host, but most do not. It’s easy to see why. If a parasite kills its host, the parasite is also likely to die. Instead, parasites usually cause relatively minor damage to their host.

Summary

• Symbiosis is a close relationship between two species in which at least one species benefits.
• Mutualism is a symbiotic relationship in which both species benefit.
• Commensalism is a symbiotic relationship in which one species benefits while the other species is not affected.
• Parasitism is a symbiotic relationship in which one species (the parasite) benefits while the other species (the host) is harmed.

Review

1. Define mutualism and commensalism.
2. Give examples of mutualism.
3. Explain why most parasites do not kill their host. Why is it in their own best interest to keep their host alive?

Roundworms like this one might eventually fill a dog’s intestine unless it gets medical treatment.

Some parasites kill their host, but most do not. It’s easy to see why. If a parasite kills its host, the parasite is also likely to die. Instead, parasites usually cause relatively minor damage to their host.