Symbiont: Definition & Explanation

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  • 0:01 Symbionts
  • 0:45 Symbiotic Relationships
  • 1:11 Parasitism & Mutualism
  • 2:57 Commensalism & Amensalism
  • 4:28 Synnecrosis & Mutualism
  • 5:11 Lesson Summary
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Lesson Transcript
Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

In this lesson, you'll learn about different symbionts and symbiotic relationships, including parasitism, mutualism, commensalism, amensalism, synnecrosis, and neutralism.


A symbiont is an organism that is very closely associated with another, usually larger, organism. This larger organism is called a host. A symbiont can live on, in, or sometimes very near its host.

There are two general categories of symbionts: ectosymbionts and endosymbionts. Ectosymbionts live outside of their hosts' cells. For example, the bacteria living on your skin and in your digestive tract are ectosymbionts. These bacteria can be very helpful for fighting off harmful microorganisms.

Endosymbiont, on the other hand, live inside of their hosts' cells. Some plants, for example, have endosymbiotic bacteria living inside their root cells, helping the plants grow.

A symbiotic relationship is the interaction of a symbiont species and a host species. These interacting species can affect each other in positive or negative ways, or sometimes not at all. There are six possible types of symbiotic relationships:

  • Parasitism
  • Mutualism
  • Commensalism
  • Amensalism
  • Synnecrosis
  • Neutralism

Some of these are very common, while others are quite rare.

Parasitism and Mutualism

Parasitism is when the symbionts involved in a symbiotic relationship harm their hosts. These symbionts are called parasites. Just a few examples include lice, fleas, ticks, and tapeworms. These animals are ectosymbionts and benefit by feeding off of their hosts. The negative effects of these parasites aren't usually bad enough to cause diseases or death.

Some organisms, called vectors, can cause diseases indirectly by transferring parasites to other organisms. A tick carrying the bacterium that causes Lyme disease could transfer this parasite to its host while drinking blood. In this example, both the tick vector and the bacterium are considered parasites. A mosquito, which isn't a parasite, can be a vector if it carries the malaria-causing parasite. This parasite is an endosymbiont because it invades human red blood cells.

When both species in a symbiotic relationship benefit, it's called mutualism. Many species form mutualistic relationships. For example, some ants take care of aphids by protecting both adults and eggs from predators. The aphids provide the ants with sweet-tasting liquid to drink. As another example, many flowers rely on insects, birds, or bats to pollinate them. The pollinators ensure that the flowers can reproduce, while the flowers provide their pollinators with nectar to drink. The ant-protected aphids and flower pollinators are examples of ectosymbionts.

The bacteria that live in plant root cells are mutualistic endosymbionts. These bacteria take unusable nitrogen from the atmosphere and turn it into usable nitrogen. This enables the plant to grow and reproduce. Benefits for the bacteria include protection and nourishment.

Commensalism and Amensalism

In commensalism, the symbiont benefits from the host, while the host is unaffected. Some plants, such as orchids and moss, will grow out of large trees to gain easy access to sunlight. They neither help nor harm their host tree. Phoresy is a type of commensal relationship in which the symbiont hitches a ride on its host. For example, some barnacles will attach to the body of a whale. As the whale swims, it brings water and food particles to the barnacles, but is not affected by them.

Sometimes commensalism can be a tricky relationship to confirm. Often on closer examination of the species involved it's found that the host actually does benefit or is in fact harmed. For example, not all small plants growing on trees are commensal. Some, such as mistletoe, will take nutrients and water from their host tree. This makes the relationship parasitic.

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