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1800-102-2727We all live in a community and live in harmony with other organisms. But do you think an organism can live in complete isolation? The answer to this would be no. Even if we are considering green plants which prepare their own food by photosynthesis, they still require the microbes present in the soil to break down the organic matter in the soil to release the nutrients for absorption. We are going to look at one such interesting interaction called parasitism. Parasitism is an inter-specific (between species) interaction among two organisms. Examples include ticks living on a dog.
Fig: Ticks on dog
A parasite is a plant or an animal that lives on, with, or inside a larger species and extract nutrients. The parasite causes harm to the host in most cases. They can divert resources for their growth, reproduction, and survival without any rewards for the hosts. We cannot say that parasitism is restricted to a few taxonomic groups since parasitic lifestyle has been seen in groups as diverse as viruses, bacteria, protozoans, invertebrates, and vertebrates. This diversity is seen due to different ranges of life cycles, host exploitation strategies, transmission methods, and virulence. In this article we are going to discuss in depth about parasitism.
Parasitism is a type of population interaction between two living species in which one organism is benefited at the expense of the other. The organism that is benefitted is called the parasite, while the one that is harmed is called the host. Examples include liver flukes (Fasciola hepatica) that live inside human beings (host).
Fig: Infected human liver with liver fluke
Parasitism is observed in a diverse range of organisms including animals, plants, fungi, protozoans, bacteria, and viruses. Parasitism is a widespread phenomenon in the Animal Kingdom. Based on the location on the host the parasites can be classified as ectoparasites and endoparasites.
Ectoparasites |
Endoparasites |
It is a parasite that lives on the surface of the host |
It is a parasite that lives inside the host |
They may be temporary, intermittent or permanent on the host |
They are generally permanent inside the host |
They have digestive tracts |
They usually lack digestive tracts |
They are relatively less harmful to the host |
They are harmful to the host and in some cases may even cause its death |
They can be hemiparasites or holoparasites |
They are usually holoparasites |
They exhibit aerobic respiration |
They show anaerobic respiration |
Examples include mosquitoes, mites, fleas, and ticks |
Examples include roundworms, tapeworms, and liver flukes |
Fig: Ticks on dog |
Fig: Infected human liver with liver fluke |
Parasites also have varying levels of dependency on the host for their survival. Some of the parasites are more independent and take only a few substances from the host while others are dependent upon the host to complete their life cycle and will not exist without the host.
Facultative parasite |
Obligate parasite |
They are parasites that are not dependent upon the host for completing their lifecycle |
They are parasites that depend upon the host for completing their lifecycle |
They become parasites once comes in contact with the host |
They lead only a parasitic life |
They have their own metabolic pathway and are capable of reproducing by themselves |
They do not have their own metabolic pathway and are incapable of reproducing by themselves and require the host machinery |
They have a simple life cycle as they are independent |
They have a complex life cycle as they are dependent on the host |
Examples include Hirudinaria granulosa (Leech) |
Examples include Ascaris (Roundworm) and Taenia (Tapeworm) |
Fig: Leech |
Fig: Roundworm |
Parasites are classified into two types based on the disease causing ability as follows:
Pathogenic parasites |
Non pathogenic parasites |
These parasites derives only nourishment from the host and cause no diseases to the host |
These parasites not only derives nourishment from the host but also cause diseases to the host |
Examples include Plasmodium (malarial parasite) and Entamoeba histolytica (causes amoebic dysentery) |
Examples include non pathogenic intestinal protozoans like Entamoeba coli, and Entamoeba polecki |
Fig: Entamoeba histolytica |
Fig: Entamoeba coli |
Parasites can be classified into the following types based on their dependency on the host:
Holoparasite |
Hemiparasite |
These parasites are completely dependent on the host for all their requirements |
These parasites receives only a part of their nourishment from the host |
Examples include Cuscuta |
Examples include Loranthus |
Fig: Cuscuta |
Fig: Loranthus |
Parasites are classified into three types based on the duration they spend on the host as follows:
Temporary parasite |
Permanent parasite |
Intermittent parasite |
These parasites live in contact with the host only for a pat of their life cycle |
These parasites live in contact with the host throughout their life |
These parasites visits the host only at the time of feeding |
Examples include Glochidium larva of freshwater mussel |
Examples include Ascaris (Roundworm) |
Examples include leech (Hirudinaria granulosa) |
Fig: Ascaris |
Fig: Leech |
These parasites live on another parasite. Examples include bacteriophages feeding on bacteria.
Fig: Bacteriophage
Brood parasitism is a peculiar type of parasitism exhibited by a few organisms where they rely on others to raise their young ones. This strategy is seen among birds, insects, and fishes. It is a type of parasitism in which the parasitic bird lays its eggs in the nest of its host and lets the host incubate them. The eggs of the parasitic bird have evolved to resemble the host’s egg in size as well as colour to reduce the chances of the host bird detecting the foreign eggs and ejecting them from the nest. Examples include koel laying eggs in the crow’s nest. It can also be considered as an evolutionary strategy, where parasitic organisms are no longer required to expend energy on rearing their young ones.
Fig: Crow feeding koel
Hosts have evolved a variety of defensive measures against their parasites. Some of the adaptations of the host against pathogens are as follows:
Parasites have also evolved a variety of measures to overcome the defences of the hosts.It includes the structural adaptations, physiological adaptations, and reproductive adaptations. Some of the adaptations of the parasites are as follows:
Fig: Haustoria of Loranthus
Fig: Life cycle of liver fluke
The parasites employ general or specialised pathology to reduce the host's fitness. Parasites in turn increase their own chances of survival by exploiting the resources of the host. They utilise several methods in order to infect or invade the host. The association between the hosts and parasites causes them to sometimes evolve together and may sometimes lead to a mutualistic relationship. It is a population interaction which is required to maintain the population of the host.
Q1. Choose the incorrect statement related to the characteristics of endoparasites.
(A) Loss of unnecessary sense organs
(B)Presence of adhesive organs or suckers
(C ) Loss of digestive system
(D) Development of cryptically coloured bodies
Solution: Parasitism is a type of population interaction between two living species in which one organism is benefited at the expense of the other (+/- interaction). The organism that is benefitted is called the parasite, while the other one that is harmed is called the host. Many parasites have evolved to parasitise only a single species of host and is said to be host-specific. Parasites have evolved several special adaptations to successfully and efficiently parasitise the host. The following are the common adaptations shown by the parasites:
Fig: Head of hookworm without major sense organs
Fig: Head of tapeworm with suckers
Camouflage or cryptic colouration, is a defence mechanism that organisms use to disguise their appearance (mimicry), usually to blend in with their surroundings. Camouflaged bodies are present in some insects, butterflies and frogs. It is an adaptation evolved to avoid being detected by its predators. For example, an owl butterfly resembling the eye of an owl. Hence the correct option is (D).
Fig: Owl butterfly resembling the the eye of an owl
Q2. The plant that lacks chlorophyll and obtains its nutrition from other plants is ____________.
(A) Calotropis
(B) Cuscuta
(C) Balanus
(D) Chthamalus
Solution: Parasitism is a type of population interaction between two living species in which one organism is benefited at the expense of the other (+/- interaction). The organism that is benefitted is called the parasite, while the one that is harmed is called the host. Cuscuta is a parasitic plant that parasitises hedge plants (planted along the border like Bougainvillaea). It lacks chlorophyll and leaves. It produces specialised roots called haustoria that penetrate the host vascular tissues (xylem and phloem) and derives its nutrition from the host plant.
Fig: Cuscuta
Q3. Match column I with column II and select the correct option.
Column I |
Column II |
|
|
|
2. Helminth |
C. Ascaris lumbricoides |
3. Crustacea |
D. Copepods |
4. Protozoa |
(A) A - 4, B - 1, C - 2 and D - 3
(B) A - 2, B - 1, C - 4 and D - 3
(C ) A - 3, B - 2, C - 1 and D - 4
(D) A - 2, B - 3, C - 2 and D - 4
Solution: Populations of different species interact with each other in various ways in their habitats. Parasitism is a type of population interaction, where one organism (parasite) lives and derives nutrition from another specific organism (host). In the above table names of a few common parasites along with the group they belong to are given.
Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery in humans. Protozoans are the single-celled microscopic animals, which include amoebas, flagellates etc. Common symptoms of the disease include diarrhoea with belly cramps, fever, nausea, vomiting and blood or mucus in the stool.
Fig: Entamoeba histolytica
Ascaris lumbricoides (roundworm) is a helminth. Helminths are worm-like parasites that survive by feeding on a living host (humans). Ascariasis is an infection of the small intestine caused by roundworm. Common symptoms of the disease include abdominal pain, nausea, vomiting, diarrhoea and bloody stools.
Fig: Ascaris lumbricoides
Candida albicans is a fungal parasite that causes candidiasis in humans. It is a type of yeast, normally lives on the skin or inside the body, in places such as the mouth, throat, gut, and vagina, without causing any problems. In immune compromised people (immunity is low, in cases like AIDS), it appears like white spots that have the appearance of cottage cheese and may bleed when touched.
Fig: Candida albicans
Copepods are a group of organisms belonging to crustacea, under phylum Arthropoda. They act as parasites on freshwater and marine fishes. They are frequently found on the body, around the mouth, and on the gills of the fishes. Some copepods crawl freely over the surface and move from fish to fish. Hence the correct option is (A).
Fig: Copepod on fish
Q4. Which of the following is a parasite of small marine fishes?
(A) Cyanobacteria
(A) Barnacle
(C) Snails
(D) Copepods
Solution: Parasitism is a type of population interaction between two living species in which one organism is benefited at the expense of the other (+/- interaction). The organism that is benefitted is called the parasite, while the one that is harmed is called the host.
Copepods are small crustaceans that are ectoparasites on many fishes. Crustaceans are shelled organisms belonging to the phylum Arthropoda. A variety of different parasitic copepods can cause external infestations (colonisation) of freshwater and marine fishes. So they are commonly referred to as fish lice. They are normally found on the body, around the mouth, and on the gills of the fishes. Hence option (D) is correct.
Fig: Copepods on the mouth of fish
Q1. What is the interaction between two species called?
Answer: The interaction between two species is called interspecific interaction.
Q2. Define ectoparasite and endoparasite, and give suitable examples for them?
Answer: Ectoparasites are parasites that live on the external surface of the host organism. Examples include mosquitos, mites, fleas and ticks. Endoparasites are those that live inside the body of the host. Examples include roundworms, tapeworms, and liver flukes.
Q3. Can we consider a mosquito as a parasite?
Answer: Parasitism is a type of population interaction between two living species in which one organism is benefited at the expense of the other. The organism that is benefitted is called the parasite, while the one that is harmed is called the host. Female mosquitoes feed on their host's blood, but they do not live on their hosts like head lice. Hence they are not considered as parasites.
Q4. Which is the largest parasite?
Answer: Dioctophyme renale or the giant kidney worm, is considered as the largest known parasitic nematode infecting humans.