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Symbiosis refers to two organisms of different species living together in a close relationship.

There are three types of sybiosis: mutualism, parasitism, and commensalism.

  • In the case of mutualism, both organisms benefit from the relationship. For example, corals (Class Anthozoa) often harbor symbiotic algae called zooxanthellae. The corals gain the products of algal photosynthesis (sugars) in an otherwise nutrient-poor environment. The algae gains a steady supply of organic material in the form of waste from the coral. In many cases, mutualism is necessary for the survival of both organisms.
  • In a parasitic relationship, one organism benefits while the other suffers. An example is a mosquito biting a mammal, this provides food for the mosquito but drains blood from the mammal.
  • In commensalism, one organism benefits while the other is unaffected. An example is how Lichens grow on the trunks and branches of trees, this does no harm to the tree but supports the lichens in an area where they can receive sunlight.

These exist on a spectrum, sometimes the relationship between organisms can vary depending on conditions. Vibro bacteria are common in salt water and make up a large part of the digestive tract fauna of many marine organisms, however in high concentrations Quorum sensing genes turn on and the bacteria become aggressive and invade the host tissue.

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