Symbiosis: Mutualism, Commensalism, and Parasitism

Symbiosis is a close, long-term relationship between two different species, and its main types are mutualism, commensalism and parasitism. In brief: in mutualism both species benefit, in commensalism one benefits and the other is unaffected, and in parasitism one benefits and the other is harmed. Bees and flowers are mutualists, barnacles on a whale are commensals, and a tapeworm in a gut is a parasite.

Almost every organism lives in some close relationship with others, from the bacteria in our guts to the fungi on tree roots. This guide defines the three main types, gives clear examples, shows how to decide which type a relationship is by looking at costs and benefits, and warns about difficult cases where categories blur.

What is symbiosis?

The word symbiosis comes from Greek words meaning “living together”. In its broad modern sense, it includes any close and lasting association between two species. Some textbooks use a narrower meaning and restrict the word to relationships that benefit both partners, so it is wise to check which definition a course uses. This article uses the broad meaning, which is common in school biology.

The partners are often very different in size. The larger partner is called the host, and the smaller one is the symbiont. Symbiotic relationships may be temporary or permanent, and a relationship can change with conditions.

What is mutualism?

In mutualism, both species benefit. The benefits may be food, shelter, protection or transport. Mutualism is common and is important in many ecosystems.

• Flowering plants and pollinators: The animal receives nectar or pollen, and the plant has its pollen transferred.
• Mycorrhizae: Fungi surround or enter plant roots. The fungus increases the root’s absorption of water and minerals, and the plant gives it sugars.
• Nitrogen-fixing bacteria in legume roots: Bacteria convert nitrogen gas into ammonium that the plant can use, and the plant gives them food and a protected home in nodules.
• Gut bacteria: Bacteria in the human intestine digest some fibers and make certain vitamins, while the body provides a warm environment with food.
• Cleaner fish: Small fish eat parasites from larger fish, so they are fed and the host is cleaned.
• Lichens: A fungus and an alga or cyanobacterium live together. The alga photosynthesizes, and the fungus provides structure and protection.

What is commensalism?

In commensalism, one species benefits and the other is neither helped nor harmed. The word comes from Latin for “sharing a table”.

• Epiphytes: Orchids and bromeliads grow on tree branches, where they receive more light, and they do not take nutrients from the tree.
• Remora fish: They attach to sharks and eat scraps of food, and the shark is largely unaffected.
• Barnacles on whales: They gain a place to live and access to food-rich water. The whale is usually not harmed, although heavy growth can add drag.
• Birds nesting in trees: They gain a safe site, and the tree is unaffected.

Pure commensalism is hard to prove, because a small effect on the host is often present but difficult to detect. Many ecologists regard it as one end of a range of relationships.

What is parasitism?

In parasitism, the parasite benefits and the host is harmed. The parasite lives on or in the host and takes nutrients from it. Unlike a predator, a parasite usually does not kill the host quickly, because it depends on the host for survival.

• Ectoparasites live on the surface of the host, such as fleas, ticks, lice and leeches.
• Endoparasites live inside the host, such as tapeworms, roundworms and the protozoan that causes malaria.
• Plant parasites include mistletoe, which taps into the water supply of its host tree, and dodder, which wraps around plants.
• Brood parasites such as cuckoos lay their eggs in other birds’ nests, so the host raises the cuckoo chick at the cost of its own young.

Hosts evolve defenses such as immune responses and grooming, and parasites evolve ways to avoid them. This pairing of changes is known as coevolution.

Types of symbiosis compared

Worked example: classifying relationships

To classify a relationship, ask what each partner gains or loses. Write a plus sign for a benefit, a minus sign for harm and a zero for no effect. Then compare the pair of signs.

1. Clownfish and sea anemone. The fish gets protection among the stinging tentacles, and it may clean the anemone and bring it food. Signs: (+, +). Mutualism.
2. Heron following cattle. The bird catches insects disturbed by the cattle, and the cattle are unaffected. Signs: (+, 0). Commensalism.
3. Tapeworm in a dog. The worm absorbs digested food, and the dog loses nutrients and may become ill. Signs: (+, −). Parasitism.
4. Mistletoe on an apple tree. The mistletoe takes water and minerals, and the tree is weakened. Signs: (+, −). Parasitism.

If a relationship is hard to classify, it may be because costs and benefits change. Gut bacteria that are helpful in the gut can cause infection if they enter the blood. The sign pairs are a tool, not a final answer.

How was symbiosis discovered?

The German botanist Heinrich Anton de Bary introduced the term symbiosis in 1879 to describe the living together of different organisms, from his studies of lichens. Simon Schwendener had shown a few years earlier that lichens are made of two organisms.

In the twentieth century, Lynn Margulis supported the endosymbiotic theory. According to this widely accepted theory, mitochondria and chloroplasts in cells originated from bacteria that once lived inside larger cells.

Why does symbiosis matter in practice?

Farmers plant legumes such as beans and clover to add nitrogen to soil, using the bacteria in their roots. Mycorrhizal fungi are added to some soils to improve plant growth.

Medicine pays attention to both sides. Parasitic diseases such as malaria affect hundreds of millions of people, and research on the gut microbiome studies how beneficial bacteria affect health. Coral reefs depend on a mutualism between corals and algae, and when this relationship breaks down in hot water, the corals bleach.

Which mistakes are common?

Mistake 1: Treating symbiosis as meaning only a helpful relationship

Under the broad definition used here, parasitism is also symbiosis. Always check how the term is defined in your course.

Mistake 2: Confusing parasitism with predation

A predator kills its prey, usually at once. A parasite lives with its host for a longer time and normally keeps it alive.

Mistake 3: Assuming a relationship is fixed

Conditions can change a mutualism into a parasitism. Shortage of nutrients may change what a partner provides.

Mistake 4: Calling any close contact commensalism

A zero effect must be shown, not assumed. Many apparent cases turn out to have a small cost or benefit.