9 min readConcept sheet1-mark MCQ or assertion-reason, a 2-mark question on parasite adaptations or brood parasitism, and a 3-mark answer; Cuscuta is also a spotting specimen

Population interactions: parasitism

In parasitism (+, −) the parasite lives on or in a host and harms it. Many parasites are host-specific and co-evolve with their hosts.

Parasitism(+, −): the parasite gets free lodging and food; the host is harmed

Host-specific parasiteCan parasitise only one species of host; host and parasite co-evolve

Human liver flukeA trematode needing two intermediate hosts: a snail and a fish

Brood parasitismThe koel (a cuckoo) lays its eggs in a crow's nest; its eggs resemble the host's eggs

What is parasitism?

A parasite gets free lodging and meals from another organism, its host. This way of life pays so well that it has evolved in many groups, from plants to higher vertebrates. Many parasites are host-specific: they can parasitise only one species of host. Host and parasite then co-evolve. If the host evolves a way to reject or resist the parasite, the parasite must evolve a way to counter it. Most parasites reduce the survival, growth and reproduction of the host and lower its population density. They may also make the host physically weak, so predators catch it more easily.

Adaptations of parasites

Loss of unnecessary sense organs.
Adhesive organs or suckers to cling to the host.
Loss of the digestive system in many, since they take in food already digested by the host.
High reproductive capacity, so that some offspring reach a new host.
Complex life cycles with one or two intermediate hosts or vectors. The human liver fluke needs a snail and a fish; the malarial parasite needs a mosquito as a vector to reach new hosts.

Ectoparasites vs endoparasites

EctoparasitesFeed on the outer surface of the host. Examples: lice on humans, ticks on dogs, copepods on many marine fish, and the plant Cuscuta on hedge plants. Cuscuta has lost its chlorophyll and leaves in evolution and draws its food from the host plant.

EndoparasitesLive inside the host, in the liver, kidney, lungs, red blood cells and other sites. Examples: the liver fluke and the malarial parasite. Their life cycles are more complex because they are highly specialised. Their body structure is greatly simplified, while their reproductive potential is very high.

The female mosquito is not called a parasite, though it needs our blood to make its eggs. It visits a host only briefly and does not live on or in it.

Brood parasitism in birds

In brood parasitism, a bird lays its eggs in the nest of a host bird and lets the host incubate them. Over evolution, the parasite's eggs have come to resemble the host's eggs in size and colour. This lowers the chance that the host spots the foreign eggs and throws them out. In India, the koel (a cuckoo) lays its eggs in the crow's nest. You can watch this in a park during the breeding season, from spring to summer.

In real life

Lice, ticks on pet dogs and roundworms in children are common parasites in India; the deworming tablets given to schoolchildren target gut endoparasites. Each spring you can hear the koel that leaves its eggs in a crow's nest.

Exam angle

2 marks: “Why is the female mosquito not considered a parasite, though it feeds on human blood?” It does not live on or in the human body. It visits only briefly for a blood meal, which it needs to produce eggs.

2 marks: “What is brood parasitism? Give an example.” A bird lays its eggs in another bird's nest and lets the host incubate them, e.g. the koel in a crow's nest. The parasite's eggs resemble the host's eggs in size and colour.

3 marks: “List the adaptations of parasites to their way of life.” Loss of unnecessary sense organs; adhesive organs or suckers; loss of digestive system; high reproductive capacity; complex life cycles with intermediate hosts or vectors (any three, with an example such as the liver fluke).

Remember it

A parasite's kit is SALT: Suckers, Absent gut, Lost sense organs, Tons of eggs.