→ Many parasites are host-specific (they can parasitize only a single host species). They tend to co-evolve. i.e., if the host evolves special mechanisms against the parasite, the parasite also evolves mechanisms to counteract them to remain with the same host species.
→ Adaptations of parasites : Loss of sense organs, presence of adhesive organs or suckers to cling on to the host, loss of digestive system, high reproductive capacity etc.
→ Life cycles of parasites are often complex. E.g.
→ Human liver fluke depends on 2 intermediate hosts (a snail & a fish) to complete its life cycle.
→ Malarial parasite needs mosquito to spread to other hosts.
→ Parasites harm the host. They may reduce the survival, population density, growth and reproduction of the host. They may make the host physically weak and more vulnerable to predation.
Types of parasites :
1. Ectoparasites
→ Parasites that feed on the external surface of host. E.g.
→ Lice on humans, Ticks on dogs, Copepods on many marine fishes, Cuscuta plant on hedge plants.
→ Cuscuta has no chlorophyll and leaves. It derives its nutrition from the host plant.
→ Female mosquito is not considered a parasite, because it needs our blood only for reproduction, not as food.
2. Endoparasites
→ Parasites that live inside the host body at different sites (liver, kidney, lungs, RBC etc).
→ The life cycles of endoparasites are more complex.
→ They have simple morphological & anatomical features and high reproductive potential.
Brood parasitism in birds :
→ Here, the parasitic birds lay eggs in the nest of its host and lets the host incubate them.
→ During evolution, eggs of the parasitic bird have evolved to resemble the host's egg in size and colour. So the host bird cannot detect and eject the foreign eggs easily.
E.g. Brood parasitism between cuckoo and crow.