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Mutualism is an interaction between two species, in which the organizations involved benefit from this relationship. This is called an interaction with reciprocal benefits. However, mutualism in the strict sense is not a binding relationship between the individuals involved. In this case, we speak of mutual symbiosis, which is a particular form of symbiosis in which the species concerned, the symbionts (or symbionts) and their respective hosts live in direct contact with each other and benefit each other. Unlike commensalism, there are adaptations in the two associated species, since the modification of one can affect the survival and reproduction of the other.

At the evolutionary level, many mutual relationships probably derive from predator-prey or host-parasite interactions. Every living organism on earth is involved in at least one mutualist interaction during its life.

Parasitism is a biological relationship between two living beings where one of the protagonists - the parasite - takes advantage of a host organism to feed itself, shelter or recurrence1. Organisms that are not parasitic are referred to as "free". Parasites are found throughout the living world. Some groups are composed almost exclusively of parasites (examples: monogenous platyhelminths), although most include both parasite and free species (eg nematodes). Vertebrates have very few parasitic species, only fish: lamprey gnaw the skin of pelagic fish, vampire fish (or candirús) suck the blood of large amazonian silurids, some pearl fish (or aurins) parasitize sea ​​cucumbers. Finally there is an intra-specific parasitism in the abyssal monkfish (ceratiidae): the males, tiny, attach themselves to the female, at the expense of which they spend all their life. Many parasites can modify the behavior of their host, to the advantage of the parasite2, a phenomenon now classified as a lasting interaction. Although rarely used, the definition of parasitism corresponds perfectly to the interactions between a virus or a pathogenic bacterium and its host. Parasites are sometimes themselves victims of other parasites, which are then called hyperparasites3. The parasitoids are organisms which, during their development, systematically kill their host, which makes them outside the framework of parasitism in the strict sense.

Commensalism is a concept theorized by Pierre-Joseph van Beneden (1809-1894) during the second half of the nineteenth century. The Belgian zoologist thus records in his book Les commensaux et parasites in the animal kingdom published in 1875, 264 examples of associations which he classifies within the commensalism. Some species (such as certain bacteria in the intestinal flora) have become "commensal" species of humans that have unintentionally introduced them into many ecosystems and regions where they did not exist. Moreover, the concept of commensalism is present in the present study of the microbiome. If the host supplies part of its own food to the commensal, it does not obtain any obvious counterpart of the latter (the relation is to non-reciprocal benefit). Unlike parasitism, commensalism is a non-destructive association for the host; the latter can quite continue to live and evolve in the presence of the commensal and, more often than not, "ignore" everything about the relationship. The survival of both organizations is independent.

Bibliography: https://fr.wikipedia.org/wiki/Commensalisme

https://fr.wikipedia.org/wiki/Parasitisme

https://fr.wikipedia.org/wiki/Mutualisme_(biologie)