Ans. It is a relationship between members of different species, in which they live together for longer or shorter periods of time.
Types of symbiosis: Symbiosis is of three types: parasitism, mutualism, and commensalism
a) Parasitism
It is a type of symbiosis (between members of different species), in which smaller partner (parasite) derives food and shelter from the body of larger partner (host) and, in turn, harms it.Temporary parasitism: In temporary parasitism, the parasite spends most of its life cycle as independent free-living organism. Only a part of its life cycle is spent as a parasite. Leech, bed bug, mosquito are common temporary parasites of human.
Permanent parasitism: In permanent parasitism, the parasites spend their whole life cycle as parasites. Many disease causing bacteria and all viruses are permanent parasites.
Parasites may also be classified as ectoparasites and endoparasites.
Ectoparasites live outside i.e. on the surface of host's body and get food from there. Mosquitoes leeches, lice etc. are the examples of ectoparasites.
Endoparasites live inside the body of host and get food and shelter. Bacteria, viruses, tapeworm, Ascaris, Entamoeba, Plasmodium etc. are the examples of endoparasites.
Some plants (e.g. Cuscuta, also called dodder) are parasites on other plants. Parasitic plants grow special types of roots (haustoria) into host body and suck the required nutrients from the vascular tissues of host.
b) Mutualism
In this type of symbiotic interaction, both partners (of different species) get benefit and neither is harmed.
Examples
1) Termites eat wood but are not able to digest it. A protozoan lives in their intestine. It secretes 'cellulase' enzyme to digest the cellulose of wood. In return, the termite provides food and shelter to the protozoan.
2) The nitrogen fixer bacteria Rhizobium live in the root nodules of leguminous plants like pea, gram etc. The bacteria obtain food and shelter from plants while in return they fix gaseous nitrogen into nitrates for the plant which is required for their growth. Figure 16.9,
c) Commensalism
It is a type of symbiosis in which one partner is benefited while the other is neither benefited nor harmed.
• Epiphytes are small plants found growing on nodules
other larger plants for space only. They absorb water and minerals from atmosphere and prepare their own food. The larger plants are neither benefited nor harmed in any way.
• Sucker fish attaches to the surface of sharks by its sucker. In this way, the shark provides easy transport to the sucker fish to new feeding grounds.