In the natural world all living organisms are connected, in one way or another. A complex network of many different types of relationships. These relationships can be parasitic, predator-prey, or a symbiotic relationship between two organisms. The term symbiosis stems from the Greek word meaning "living together” and this is precisely what it describes- two organisms whose lives are intertwined for the sake of joint benefit. Such cooperations can exist between organisms of every kingdom: plants, animals, bacteria and fungi. Both organisms benefit thanks to the symbiotic relationships, and under long periods of time they may even co-evolve in a way that makes them even more compatible with each other, sometimes to the level that they can no longer exist separately. The following video introduces the term symbiosis and presents a case study of symbiosis and co-evolution.

The lecture was delivered by David Gonzales as a part of the TedEd project.

Symbiotic relationships are commonly divided into two different types: Obligatory and Facultative. Obligatory relationships mean that the two collaborators require each other in order to survive and one cannot live without the other, while facultative relationships mean that although the symbiotic relationship is beneficial to both organisms, it is not vital for their existence. (The symbiotic relationship can be obligatory for one of the organisms but facultative for the other, meaning one cannot exist without the other, but the other could.)

An example of obligatory symbiosis are mitochondria, that can be found in every cell of every animal. The cell provides the mitochondria with the environment it requires for living and synthesis proteins for it, and the mitochondria provide energy for the cell. It is believed that the origins of mitochondria are from an ancient bacterium that entered the cells during evolution, and so the relationship began and over time became obligatory. This form of symbiosis, where one of the organisms is “engulfed” by the other, is called endosymbiosis (the meaning of the word “endo” is within).

Famous symbiosis examples are the many luminous fish residing at the depths of the ocean, which are equipped with special organs with the purpose of creating niches for luminous bacteria. The bacteria gain a cozy habitat and protection, while the fish gain a luminescent organ with which they can locate prey, attract females and navigate through the dark deep water.

However, symbiotic relationships do not have to strictly occur between a microscopic organism and a larger animal, they can also occur between two different animals such as the sea anemone and the clownfish. The sea anemone provides protection to the clownfish from predators as it hides within the anemone’s tendrils, and in return the clownfish protects the sea anemone from its own predators.

Some regard symbiosis as only a mutual relationship where both sides gain, also called Mutualism. However, some consider symbiosis to be an umbrella term that also includes other types of reciprocal relations: parasitism and commensalism. Commensalism is a situation where one side benefits from the relationship and the other side is merely not harmed. In a parasitic relationship, one side benefits at the expense of the other side. Most pet owners recognize at least one type of parasitic relationships – fleas that reside on the skin of other animals and feed off them. Most parasites are much smaller than the host carrying them and require only relatively small amounts of energy that they can leech off the host without effort. Each one of the three relationship types mentioned above has a myriad of examples among animals and plants. These structures form the basis of the interactions between organisms in the complex network of living things.