Applied Zoology (ZOO516DB)
Unit-1 Host-parasite Relationship and epidemiology
1.1 Types of hosts, types of relationships (Parasitism, Symbiosis, Commensalism)
Symbiosis: Symbiotic relationships, or symbioses (plural), are close interactions between individuals
of different species over an extended period of time which impact the abundance and distribution of
the associating populations. Any association between two species populations that live together is
symbiotic, whether the species benefit, harm, or have no effect on one another. Both positive
(beneficial) and negative (unfavourable to harmful) associations are therefore included, and the
members are called symbionts. Symbiosis, is any of several living arrangements between members of
two different species, including mutualism, commensalism, and parasitism.
Mutualism: In past it was termed symbiosis. Mutualism, is association between organisms of two
different species in which each benefits. Mutualistic arrangements are most likely to develop between
organisms with widely different living requirements. In this case both of the species derive benefits
and there exists a close and often permanent and obligatory relationship which is more or less
essential for survival of each..
For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut. The
termite benefits from the ability of bacterial symbionts within the protozoa to digest cellulose. The
termite itself cannot do this, and without the protozoa, it would not be able to obtain energy from its
food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbionts benefit
by having a protective environment and a constant supply of food from the wood chewing actions of
the termite.
Lichens have a mutualistic relationship between fungus and photosynthetic algae or bacteria. As these
symbionts grow together, the glucose produced by the algae provides nourishment for both
organisms, whereas the physical structure of the lichen protects the algae from the elements and
makes certain nutrients in the atmosphere more available to the algae.
Commensalism: Commensalism, in biology, is a relationship between individuals of two species in
which one species obtains food or other benefits from the other without either harming or benefiting
the latter. The commensal—the species that benefits from the association—may obtain nutrients,
shelter, support, or locomotion from the host species, which is unaffected. The commensal relation is
often between a larger host and a smaller commensal. The host organism is essentially unchanged by
the interaction, whereas the commensal species may show great morphological adaptation. This
relationship can be contrasted with mutualism, in which both species benefit.
One of the best-known examples of a commensal is the remora (fish) that rides attached to sharks
and other fishes. Remoras have evolved on the top of their heads a flat oval sucking disk structure that
adheres to the bodies of their hosts. Both remoras and pilot fishes feed on the leftovers of their hosts’
meals.
Other examples of commensals include bird species, such as the great egret (Ardea alba), that feed
on insects turned up by grazing mammals or on soil organisms stirred up by plowing. Various biting
lice, fleas, and louse flies are commensals in that they feed harmlessly on the feathers of birds and on
sloughed-off flakes of skin from mammals.
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