A symbiotic relationship is a close and long-term interaction between two different biological species. The term was first used in 1879 by German botanist Heinrich Anton de Bary to describe the cohabitation of different organisms, according to Britannica. These relationships are ubiquitous in nature and are critical for the function of many ecosystems. The organisms involved in a symbiotic relationship are called symbionts.

Symbiotic relationships can be classified based on the physical relationship between the symbionts. In ectosymbiosis, one organism lives on the surface of another, such as lice on a human. In endosymbiosis, one organism lives within the tissues of another, such as the bacteria that live in the human gut, as described by the National Human Genome Research Institute. The relationships can also be obligate, meaning one or both symbionts depend on the relationship for survival, or facultative, where the organisms can live independently.

Types of Symbiosis

The most common classification of symbiosis is based on the outcome of the interaction for each partner. There are three main types:

Mutualism

Mutualism is a type of symbiotic relationship where both species benefit from the interaction. A well-known example is the relationship between bees and flowers. Bees get nectar (food) from the flowers, and in the process, they carry pollen from one flower to another, facilitating pollination and reproduction for the plant, as detailed by the National Park Service. Another example is the lichen, which is a composite organism arising from algae or cyanobacteria living among filaments of multiple fungi in a mutualistic relationship. The fungus provides a protected environment, and the algae provide carbohydrates via photosynthesis.

Commensalism

Commensalism is a relationship in which one organism benefits, and the other is neither significantly harmed nor helped. This type of interaction is often difficult to prove, as it can be challenging to demonstrate a complete lack of effect on one partner. A classic example is the relationship between remoras and sharks, as explained by National Geographic. The remora, a type of suckerfish, attaches itself to the shark and feeds on the scraps left over from the shark's meals. The shark is generally unaffected by the remora's presence.

Parasitism

In parasitism, one organism, the parasite, benefits at the expense of the other, the host. The parasite lives on or in the host's body and derives nutrients from it, often causing harm, disease, or even death. Examples of parasites include tapeworms living in the intestines of vertebrates, fleas feeding on the blood of mammals, and cuckoos, which engage in brood parasitism by laying their eggs in the nests of other bird species, as noted by the University of California Museum of Paleontology. The host bird then raises the cuckoo chick, often to the detriment of its own offspring.

Evolutionary Significance

Symbiosis is a major driving force in evolution. Over long periods, these close interactions can lead to coevolution, where two or more species reciprocally affect each other's evolution. A key example is the Endosymbiosis theory, proposed by biologist Lynn Margulis. This theory posits that organelles such as mitochondria and chloroplasts in eukaryotic cells were once free-living prokaryotic organisms that were engulfed by a host cell, establishing a permanent endosymbiotic relationship, a concept detailed by Nature Education. This event was a pivotal step in the evolution of complex life.