Parasitic Adaptations

What is meant by Adaptation?

Adaptation definition: ‘Any feature of an organism or its part which enables it to exist under conditions of its habitat is called adaptation.’ The adaptations are mainly to withstand the adverse conditions of the environment and to use the maximum benefit of the environment.

What are Parasites?

A parasite is an organism which lives in or on another organism (called host) and benefits by deriving shelter and nutrients from them. The parasitism is a type of negative ecological / biological interaction in nature where one organism gets benefited (the parasite) and the other is harmed (the host).

The parasites may be microbes such as bacteria, virus and Mycoplasma, or animals such as liver fluke, worms, nematodes, some insects and plants such as Loranthus, Cuscuta etc. All types of parasites show peculiar adaptations to survive in or on the host system and to get maximum benefit from them.

Learn more: Positive and Negative Interactions in an Ecosystem

Parasites show three level adaptations, they are

(1).     Structural Adaptations (Morphological and Anatomical Adaptations)

(2).     Physiological Adaptations

(3).     Reproductive Adaptations

(1).  Structural Adaptations (Morphological and Anatomical) Adaptations of Parasites:

Ø  Feeding organs are usually absent in endoparasites.

Ø  Fluid feeding insects such as aphids have highly specialized mouth parts for the easy absorption of cell sap from the host.

Ø  The presence of well-developed piercing device in some parasites enables them to invade into the host tissue. Example: the stylet in nematodes.

Ø  Since endoparasites show restricted movements, the locomotory organs are generally absent or highly reduced in them. Example: gut parasites such as Fasciola and Taenia.

Ø  The presence of attachment organs such as rostellum, hooks or suckers for the securely connecting to the organs of the host as in Fasciola and Taenia.

Ø  The outer covering of the endoparasite is resistant to the enzymatic digestion of the host. Example: Fasciola

Ø  Sensory organs are highly reduced in internal parasites since the environmental conditions of endoparasites are relatively constant.

Ø  The nervous system is highly reduced in most of the parasites.

Ø  Parasites usually have the reduced body size to occupy the niche in the internal or external surface of the host.

Ø  Most of the internal parasites are dorsi-ventrally flattened, which help the easy attachment to the host surface without any friction.

Ø  Some endoparasites such as Ascaris have highly muscular pharynx for the easy absorption of food materials from the host.

Ø  Development of haustoria in some parasitic plants for the absorption of nutrients from the host. Example: Cuscuta.

Ø  The haustoria in partial plant parasites (such as Loranthus) are attached to the xylem of the host to absorb minerals and water from them.

Ø  The haustoria in complete plant parasites (such as Cuscuta) are attached to the phloem of the host to absorb the prepared food material from them.

(2). Physiological Adaptations of Parasites

Ø  Parasites produce hydrolyzing enzymes to digest the host the tissue. Example: Fungi produce a variety of hydrolyzing enzymes such as cutinase, cellulose, hemicellulose and pectinase to digest the cell wall of the host plant.

Learn more: Enzymes in Pathogenesis

Ø These hydrolyzing enzymes are exoenymes, produced in the external surroundings of the parasite.

Ø  Blood feeding parasites produce anticoagulant to prevent the clotting of blood in the gut of parasites (Example: Hirudinaria)

Ø  Internal parasites have high chemo-sensitivity to find the best location in the host.

Ø  Internal parasites produce digestive enzymes for the penetration of host tissues or organs.

Learn more: Enzymes Introduction

Ø  Internal parasites can respire anaerobically in the absence of oxygen or in anaerobic condition.

Ø  There is a progressive tendency towards the reduction of the content of gut in some endoparasites since; many internal parasites have the capacity to absorb nutrients through the body surface.

Ø  Those internal parasites which don’t absorb the nutrients through body surface will develop thick cuticle around the body protect from host enzymes.

Ø  All parasites, especially endoparasites have high resistance to toxins or metabolites of the host origin.

Learn more: Toxins in Pathogenesis

Ø  Internal parasites have the ability to escape from the immunological reactions of the host.

Ø  Parasites generally attach host proteins to its external surface to escape from the host immunological responses.

Ø  Some parasites such as HIV and Plasmodium often change their antigenic components to escape from the immunological reactions of the host.

Ø  Strategies by which parasites escape from the immunological reactions of the host are: absorption of host antigen, antigenic variation, occupation of immunologically privileged sites, disruption of the host’s immune response, molecular mimicry and loss or masking of surface antigens.

Ø  Endoparasites produce some anti-enzyme compounds which will neutralize or inactivate the digestive enzymes of the host.

Ø  Endoparasites keep their osmotic concentration equal to or slightly lower than the host cell for the easy absorption of water and nutrients from the host.

Ø  The digestive tracts of most of the endoparasites are simple and there is no provision for storage due to the availability of continuous supply of food.

(3). Reproductive Adaptations of Parasites

Ø  Some parasites are hermaphrodites (bisexual). This allows them to self-fertilize in the absence of the opposite sex in the internal condition.

Ø  The fecundity rate of parasites is very high since there is only a remote chance to find the correct host by the progenies.

Ø  Most of the parasites produce a large number of reproductive bodies such as egg or sperm or cysts than their free-living relatives.

Ø  Example: e.g., Taenia soium lays about 35,000 eggs/day., Fasciola hepatica lays about 10,000 eggs/day, Ascaris lumbricoides lays about  2,00,000 of eggs/day.

Ø  Reproductive bodies are tough and resistant when they are external to the host.

Ø  The reproductive particles that secrete out of the host body are protected with very thick coat to prevent desiccation as in cysts.

Ø  Some parasites have highly complicated and specialized reproductive cycle.

Ø  Some parasites use of secondary hosts as vectors to increase the chance of their survival.

Ø  Most of the endoparasites show one or more larval stages with more multiplication capacity (due to polyembryony) to increase the number of progenies. Example: Nematodes

Ø  The retroactive particles of a parasite can survive for a long time in the absence of the host.

Ø  Rapid maturation capacity and extended life span in most of the endoparasite is another reproductive adaptation.

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