agra,ahmedabad,ajmer,akola,aligarh,ambala,amravati,amritsar,aurangabad,ayodhya,bangalore,bareilly,bathinda,bhagalpur,bhilai,bhiwani,bhopal,bhubaneswar,bikaner,bilaspur,bokaro,chandigarh,chennai,coimbatore,cuttack,dehradun,delhi ncr,dhanbad,dibrugarh,durgapur,faridabad,ferozpur,gandhinagar,gaya,ghaziabad,goa,gorakhpur,greater noida,gurugram,guwahati,gwalior,haldwani,haridwar,hisar,hyderabad,indore,jabalpur,jaipur,jalandhar,jammu,jamshedpur,jhansi,jodhpur,jorhat,kaithal,kanpur,karimnagar,karnal,kashipur,khammam,kharagpur,kochi,kolhapur,kolkata,kota,kottayam,kozhikode,kurnool,kurukshetra,latur,lucknow,ludhiana,madurai,mangaluru,mathura,meerut,moradabad,mumbai,muzaffarpur,mysore,nagpur,nanded,narnaul,nashik,nellore,noida,palwal,panchkula,panipat,pathankot,patiala,patna,prayagraj,puducherry,pune,raipur,rajahmundry,ranchi,rewa,rewari,rohtak,rudrapur,saharanpur,salem,secunderabad,silchar,siliguri,sirsa,solapur,sri-ganganagar,srinagar,surat,thrissur,tinsukia,tiruchirapalli,tirupati,trivandrum,udaipur,udhampur,ujjain,vadodara,vapi,varanasi,vellore,vijayawada,visakhapatnam,warangal,yamuna-nagar

Parasitism: Definition, classifications, adaptations, Practice Problems and FAQs

We 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.

Please enter alt text

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.

Table of contents

  • Parasitism
  • Classification of parasites based on the location on the host
  • Classification of parasites based on lifestyle
  • Classification of parasites based on disease causing ability
  • Classification of parasites based on dependency on the host
  • Classification of parasites based on duration on the host body
  • Brood parasitism
  • Adaptations of the host against parasites
  • Counter adaptations of parasites against host defence mechanisms
  • Significance of parasitism
  • Practice Problems
  • FAQs

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

Classification of parasites based on location on the host

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

Classification of parasites based on lifestyle

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

Classification of parasites based on disease causing ability

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

Classification of parasites based on dependency on the host

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

Classification of parasites based on duration

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

Hyperparasite

These parasites live on another parasite. Examples include bacteriophages feeding on bacteria.

Fig: Bacteriophage

Brood Parasitism

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

Adaptations of the host against parasites

Hosts have evolved a variety of defensive measures against their parasites. Some of the adaptations of the host against pathogens are as follows:

  • Some host organisms have skin as an external barrier to parasites.
  • Intestinal parasites in most cases have to overcome the acidity of the host stomach to establish in the intestine.
  • There are intercellular and intracellular receptors that trigger the T cells and B cells which are able to recognise parasites.
  • Plants are also capable of resisting parasites by producing defensive chemicals such as secondary metabolites which inhibit parasite growth.
  • The immune system helps to resist the parasites.

Counter adaptations of parasites against host defence mechanisms

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:

Structural adaptations

  • In endoparasites the feeding organs are absent.
  • Insects that feed on fluids such as aphids have mouth parts that are highly adapted to absorb cell sap from the host.
  • Parasites that invade the hosts have well developed piercing devices such as stylet seen in the head of plant-parasitic nematodes.
  • The endoparasites have highly reduced locomotory organs since they have restricted movement within the body of the host.
  • They exhibit the presence of attachment organs such as hooks and suckers.
  • Loss of unnecessary sense organs in endoparasites.
  • Endoparasites have an outer covering that is resistant to the enzymatic digestion of the host.
  • Parasitic plants develop structures such as haustoria to absorb nutrients from the host.

Fig: Haustoria of Loranthus

Physiological adaptations

  • Ectoparasites that feed on blood such as leech produce anticoagulants to prevent the clotting of host blood.
  • Parasites have a high resistivity to the toxins produced by the host.
  • In order to neutralise or inactivate the digestive enzymes of the host, the parasites produce some anti-enzyme compounds.
  • In order to absorb water and nutrients from the host, endoparasites maintain their osmotic concentration equal to or lower than the host cell.

Reproductive adaptations

  • Some parasites such as leeches have developed the ability to self fertilise.
  • Parasites produce a larger number of reproductive structures such as eggs or sperm than free living organisms to increase the chance of successful reproduction.
  • The ectoparasites have tough and resistant reproductive bodies.
  • The parasites sometimes exhibit a highly specialised reproductive cycle.
  • The life cycle of parasites is often complex, involving one or two intermediate hosts or vectors. For example, the human liver fluke completes its life cycle in two intermediate hosts such as snail and fish.

Fig: Life cycle of liver fluke

Significance of parasitism

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.

Practice Problems

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:

  • Loss of unnecessary sense organs - Sense organs (eyes, ears) are necessary for quick and efficient response to the environmental stimuli in free-living organisms. Since endoparasites live inside the host’s body where the environment is more or less uniform, the sense organs are highly reduced and sometimes entirely absent. Examples include hookworms (Ancylostoma duodenale).

Fig: Head of hookworm without major sense organs

  • Presence of adhesive organs or suckers to cling on to the host - For a firm grip on or inside the host’s body, some special organs of adhesion are necessary. For example tapeworms possess suckers with which they can hang on to the gut lining of their hosts.

Fig: Head of tapeworm with suckers

  • Loss of digestive system - Since parasites consume digested or semi-digested food of the host, therefore alimentary canal and digestive glands are partially or wholly lost. For example, in all tapeworms the digestive system is absent.

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

  1. Entamoeba histolytica
  1. Fungi
  1. Candida albicans

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

FAQs

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.

 

Talk to our expert
By submitting up, I agree to receive all the Whatsapp communication on my registered number and Aakash terms and conditions and privacy policy