Planet earth is home to millions of species of very diverse, living organisms that include plants, animals, fungus, algae, and unicellular organisms. Likewise, there are millions of complex relationships taking place between and among these differing species of organisms. Many organisms live together in diverse, intricate relationships. The term Symbiosis (sym meaning “together” and biosis meaning “living,” thus living together”) descried any association, either temporary or permanent, between at least two living organisms of different species. Each member of this association is called a symbiont.
The most of animals live independently in their natural habitats, obtaining their own food material and utilizing free water and oxygen for their metabolic processes. These animals when live in close association, develop a variety of patterns of association. These are mainly of five types:
1. Parasitism
2. Commensalism
3. Phoresis
4. Mutualism
5. Symbiosis.
Of these associations, commensalisms is a loose association made on a basis of shelter, defence of food-obtaining mechanisms where as parasitism, mutualism and symbiosis are intimate associations in which the metabolism of an individual of one species is dependent to some degree on permanent association with an individual of another species.

Parasitism
It is an intimate association between two specifically distinct organisms in which one partner usually smaller lives on or within the other usually larger in order to obtain sustenance. The one which is benefited is known as the parasite while the other which is injured, the loser to a smaller or a larger extent, but certainly does not gain anything by this association is known as the host or parasitism is a way of living in which an organism, the parasite, uses an organism of a different species, the host – both as a habitat and as food.
Commensalism
The literal meaning of the commensalisms is eating at the same table. It is a loose association in which two animals or organisms of different species live together without either being metabolically dependent on the other although one animal may receive some benefit called commensal but the other neither get benefit nor harm from the other. However, it is not obligatory for their existence, e, g.
1. Many intestinal protozoa such as Entamoeba coli and Trichomonas sp. live in intestine as commensal without causing any harm to host.
2. Pilot fish and shark: The pilot fish (Naucrates doctor) accompanies the shark in a free-swimming manner eating the fragments of food that become available as the shark tears apart its prey.
3. Remora and shark : The remora (Echeneis remora) actually attaches itself to the shark by its dorsal fin and feeds on “left over” and does not harm the shark.
4. Crab and sea-urchin: The crab (Eumedon conuictor) lives I a large cavity found on the back of the sea-urchin (Echinothrix turca). The crab does not exert any energy in swimming around the host, on the other hand, it shares the food of the sea-urchin and enjoys its protection.
5. Similar relationship is also seen between Pocillopora and Hapalocarcinus marsupialis (a crab).

Mutualism
It is derived from the Latin word mutuus means exchange. It is an intimate association or relationship of two animals or individuals in which both the partners are benefited, However, this relationship is not permanent and obligatory. Either of the partners can break it without any loss to either of them, e.g.
1. Hermit crab and sea anemone lives on the back of a crab. The anemone is benefited by being moved to new hunting grounds and obtaining morsels of food torn off by the crab, while the crab is protected by the bulk and stinging tentacles of the bulk and stinging tentacles of the anemone.
2. In the herbivores ruminal flora help in the digestion of cellulose in ruminants and in return get the protection and food.

Symbiosis
It comes from the greeek word symbioun means to live together. This term was first used by de Bary in 1879. It is an intimate or close association or relationship of two individuals in which both the partners are benefited like mutualism but in this he participating species are dependent on each other for existence and are obligatory, e.g.

1. Termites and their intestinal protozoa The termites provide the habitat and the food in the form of wood (cellulose) which they cannot digest. The protozoa in the intestinal habitat, however, are capable of hydrolyzing the wood for their own and the termites use.
2. Zoochlorella and Hydra Zoochlorella lives in the cells of hydra, hence gest the protection. Secondly the CO2 produced by hydra, is taken and utilized in food making and release the O2 which is utilized by the hydra. In this way both are benefited.

Phoresis
The term means “Traveling together”. It is a temporary association between two individuals or living beings in which where is no metabolic dependence. Example: Bacteria and amoebae on the legs of fly or fungal spores on the legs of a beetle.