Symbiosis (pl. symbioses) means living together. It describes close and long-term relationships between different species. The term was used by Anton de Bary in 1869, as "the living together of unlike organisms".^[1][2][3]

A symbiont is an organism living in a relationship with another species in which one or both get benefits.^[4] When one species lives inside another species, or a microscopic symbiont lives inside the cells of a host, it is called an endosymbiont.

The relevance of symbiosis is its frequency and its evolutionary significance. There appear to be no higher plants or animals without symbionts. Those symbionts are of great importance to the larger organisms, who in most cases would be unable to live as they do without their symbionts. Mycorrhiza in higher plants, and gut flora in insects and vertebrates are examples. Humans are no exception.^[5][6]

Furthermore, most of these associations are between organisms not just from different species, but from different kingdoms. And lastly, the cells of all eukaryotes contain organelles which are descendants of symbiotic relationships which began at least a billion years ago. Mitochondria and plastids are examples. The conclusion must be that symbiosis has been highly significant in the evolution of life.

The definition of symbiosis has been controversial. Some believe symbiosis should only refer to persistent mutualisms, while others believe it should apply to all kinds of long-term biological interactions.^[7]

After 130 years of debate,^[8] current biology and ecology textbooks now use the latter "de Bary" definition or an even broader definition (where symbiosis means all species interactions). The restrictive definition (where symbiosis means mutualism only) is no longer used.^[9]

The widest definition includes parasitism (in which one organism is helped and the other organism is hurt), mutualism (in which both organisms are helped), commensalism (in which one organism is helped and the other is not affected), and competition (in which both organisms are hurt).

The various forms of symbiosis include:

Symbiosis does not always benefit both partners. This is what may happen:

• parasitism, in which the association has disadvantages for one of the two. One may even destroy or kill the other. (+ –)
• mutualism, in which the association has advantages for both (+ +)
• commensalism, in which one member of the association benefits while the other is not affected (+ 0)
• competition, in which both members of the association are fighting over food or other needs. (– –)

This kind of symbiosis is called endosymbiosis. Examples are:

• Rhizobia: nitrogen-fixing bacteria which live in root nodules on plants of the pea family.
• Singly-celled forams which include a single-celled alga inside the cell. This is 'facultative', which means they may or may not do it. A more far-reaching version is idioplastic endosymbiosis. Here, the foram consumes the alga, but keeps its chloroplasts in working order.
• Green algae inside marine polychaete worms.
• Single-celled algae inside reef-building corals.
• Bacteria inside the guts of those insects and vertebrates which digest plant cellulose. These are 'obligate' symbionts, which means the host must have them.^[11]
• Lichen: fungus+alga or bacterium.
• Mycorrhiza: fungal hyphae and roots of trees.

Almost for certain, this happened to form the eukaryote cell. That's the type of cell all animals and plants are made of. The organelles inside the cell, such as mitochondria and chloroplasts, contain some DNA. This DNA is the remnant of a once separate bacterium. The theory is that the eukaryote cell evolved by the fusion of several bacteria or archaea organisms.^{[12][13][14][15]}

• An example of mutual symbiosis is the relationship between clownfish that live among the tentacles of tropical sea anemones. The clownfish protects the anemone from other fish. Clownfish waste excreted provides vital nutrients, and they can boost their hosts’ oxygen supplies at night too. The stinging tentacles of the anemone protect the anemone fish from its predators. A special mucus on the clownfish protects it from the stinging tentacles.^[16]

• Another example is the goby fish, which sometimes lives together with a shrimp. The shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby live. The shrimp is almost blind, and is vulnerable to predators when above ground. When a predator approaches, the goby touches the shrimp with its tail as a signal. When that happens both the shrimp and goby quickly retreat into the burrow.

• A lichen is an intimate combination of a fungus with an alga. The alga lives inside the fungus, which must have the alga to survive. The alga, on the other hand, can survive on its own. The result of the union is a flat, coloured lichen which grows on rocks and other surfaces in the open air.

• Herbivores are host to gut bacteria which help them digest plant material. Plant cell walls are made of cellulose, and almost no animal has developed an enzyme to digest this material. Therefore, at least for herbivores which eat leaves, the bacteria are essential.

• Some species of ants 'farm' aphids, protecting them from predators, and moving them from one feeding site to another. The ants consume the sweet sticky fluid which aphids secrete after sucking plant sap.