parasitism
Parasitism
can be defined as “a
relationship between two different
species in, which the smaller (parasite) is physiologically dependent upon the larger (host),
prevalence of parasites and the intensity of infection in the host population
are over-dispersed or non-random, and the parasite species has a higher
reproductive potential than the host species”.
Parasitism is an interaction
between two organisms, in which one organism (the parasite) attains all the
benefits of the close relationship. Parasitism can be considered a special case
of predation since their effects on the host are similarly, though not equivalently,
detrimental. Parasites that live inside the body of the host are called endoparasites (e.g., hookworms and
tapeworms that live in the host gut) and those that live on the outside are
called ectoparasites (e.g., ticks, lice,
fleas). A parasite that kills its host is called a parasitoid whilst klepto-parasitism
involves the parasite stealing food that the host has caught or otherwise
prepared.
•The majority of animals are free-living;Live
in natural habitat, find own food, water and oxygen for metabolic functions, Some animals, have developed a variety of
patterns of association (simple & complex)Homogenousassociation is formed between individuals of same genotype e.g. antelopes,
ants, bees, flamingoes etc. (simple)Heterogenous
association is formed between individuals of different genotypes e.g. rhino
& ox-pecker, shark & remora fish etc. (complex)
Commensalism
Commensalism can be defined as “a type
of loose association in which two animals of different species live together, without either being metabolically dependent on the other, although one or both may receive some
benefit from the association. It is a variant of parasitism in which there is
no discernible damage to the host”. In commensalism one member of the
association benefits while the other is not affected.
The
term commensalism is derived from the Latin word “com mensa”, meaning sharing a table. Originally it was
used to describe the use of waste food by second animals, like the carcass
eaters who follow hunting animals, but wait until they have finished their
meal. In ecology, commensalism is an interaction between two living
organisms, where one creature benefits and the other is neither harmed nor
helped.
An example of commensals is the relationship
between fish belonging to the
genus Fierasfier, which live within the
respiratory trees of holothurians, or occasionally in starfish. These fish are
relatively helpless and are readily attacked and devoured by other species.
The holothurians appear to be undisturbed by the presence of the fish.
Phoresis
Phoresis exists when two symbionts are
merely “traveling together”, and there is no physiological or biochemical
dependence on the part of either participant. Usually, the phoront is smaller than the other and is
mechanically carried about by its larger companion.
•This
type of phenomenon could clearly represent a stage similar to that in the early evolution of parasitism, since
chance contact followed by the use of one species as shelter
by the other is likely to have been the first step in an association leading to
the parasitic,way
of life. An example of phoresis is afforded by barnacles and some crabs e.g.
Poecilasma kaempferi and Neolithodes grimaldi, respectively
There is so much controversy regarding proper
definition of parasitism. However, it can be defined as “a relationship in
which one of the partners, the parasite, either harms its host or in some sense
lives at the expense of the host. The parasite may cause mechanical
injury to the host, stimulate a damaging inflammatory or immune response or rob
the host of nutrition. Sometimes it may be a combination of the above;Parasites, which live on the outer surface of
the host body are called ectoparasites, whilst internal ones are known as endoparasites
An obligate parasite
cannot complete its lifecycle without spending at least part of the time in a
parasitic relationship.A facultative parasite is
normally not parasitic but can become so when it is
accidentally eaten or enters a wound or other body orifice. Infection of such
parasites may be fatal to the host e.g. Micronema sp. (nematode)
An accidental or incidental parasite is one, which parasitses a host other than its normal one. For
example, a dog flea may occasionally bite man. Usually, such a relationship
culminates in a dead end for one or both partners. A permanent parasite lives its entire life on or in the host,
whilst an intermittent parasite feeds then leaves or drops off the host e.g.
filarial worms and bedbugs, respectively
•Intermittent parasites are
sometimes referred to as micro-predators because they prey on different preys/hosts.
However, it should be borne in mind that usually, a “good parasite” does not
kill its host.
Parasitoids are insects, typically
wasps or flies, whose immature stages feed on their host’s body, usually
another insect, but finally kill the host. In most cases they are host-specific
Hosts
A host is categorised depending on the role it plays in the
lifecycle of the parasite A definitive host is
one in which the parasite attains sexual maturityAn intermediate
host
is one in which the parasite does not reach sexual maturity
A paratenic or transport host is one in which the parasite
does not undergo any development, but in which it remains alive and infective
to another host.Host-specificity refers to the degree of flexibility in
infectivity of different hosts by the same parasite
A reservoir host is
one that harbours an
infection that can be transmitted to humansHyper-parasitism refers to parasitization of a parasite by another An association in which both associates
benefit has long been referred to as mutualism (mutuus = latin word for “exchanged”) by some authors and symbiosis (symbioun = latin word
for “to live together”) by others. The term symbiosis however, is commonly used
to refer to a relationship in which the participating partners are dependent on
each other for existenceFrom
the metabolic point of view, both mutualism and symbiosis are recognised as
special cases of parasitism in which some metabolic products of the parasite
are of value to the host (e.g. wood-eating termites and hyper-flagellates in
their intestines)- this is symbiotic
parasitism i.e. termites are entirely dependent on the
flagellates for the supply of Nitrogen and carbohydrates resulting from
breakdown of wood
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