Aquatic ecology and Biodiversity

Unit 8- Animal associations and interactions

8.5. Mutualism

A relationship between two dissimilar organisms that are mutually beneficial is called mutualism. The term mutualism literally meaning ‘living together’ was first coined by De Bary (1877). The association may be obligatory or continuous, facultative or transitory. While in continuous or obligatory mutualism, the individuals or symbionts cannot live apart, in facultative mutualism the individuals can successfully live apart from each other. The organisms involved in mutual relationship cover a wide range, namely plant-plant, animal-animal or plant-animal associations.
Obligate Mutualism
This is a permanent type of relationship in which two symbionts are in close contact and interdependent on each other. It is also referred as mutualism with continuous contact
Examples:
1. Hermit crab and sea Anemone- Hermit crab (Eupagurus sp) lives in the empty shell of a snail. The outer surface of the shell is inhabited by the sea anemone (Adamsia sp). The hermit crab is protected from the enemies by the nematocysts of the sea anemones. The sea anemone derives food from hermit crabs and gets transported from one place to another.
2. Giant clam and zooxanthellae- Giant clam, Tridacna gigas lives in the well lighted shallow coastal water and its mantle cavity harbours huge population of the algal symbiotic dinoflagellate species, which is commonly called Zooxanthellae (Symbiodinium microadriaticum). Since the clam shells are very thick, the shells cannot be closed and this helps the symbiotic algae to receive good sun light and carry out photosynthesis. Some of the older cells of these algae are in turn consumed by this giant clam.
II. Facultative mutualism
In this type of mutualism, partners or symbionts are rarely attached to each other and if so it is only for a short duration. It is also called as mutualism without continuous contact. Though both the partners are benefited from each other but the nutritional need is primary at least for one of the species. A commonly cited example of this type of mutualism is found in crocodile (Crocodylus spp.) and crocodile-bird (Pluvianus aegyptius), which removes leaches from around the teeth of the crocodile which allows the bird to enter its mouth for the search.