We commonly use the term parasite as quite an apt description of self-interested human hangers-on who benefit from the work of others without contributing anything themselves. The term used more scientifically, generally refers to an individual organism (parasite) which has a close association with an individual organism of another species (host) in which the parasite takes from the host and does it harm.
Parasitism is very common, many species have parasitic cycles in their life and most free living animals are hosts of parasites.
Types of parasites common in free living animals are:
Endoparasites living within the cells of the host eg Protozoans
Endoparasites living within the body spaces of the host eg Helminths (worms)
Ectoparasites living on the surface of the host eg ticks, fleas, lice and mites
Parasites and their hosts are in a never-ending battle. The host’s behaviour and immune system develop to avoid, repel, and destroy parasites while the parasites adapt to changes in the host defences.
In some cases a stable relationship between parasite and host may lead to reduced harm to the host to the extent that parasite absence may cause harm eg it is now thought that parasitic worms may reduce the effects of autoimmune disorders in animals and humans.
There is evidence that some parasites do not just weaken their hosts, they actually change their host’s behaviour and appearance to increase the chances of being consumed by a predator, who becomes the next host. Some fish and insects infested by worms start to behave strangely so they are more vulnerable to predators. Flukes change the appearance of coral polyps to make them more obvious to predatory fish.
Because they must move from host to host many parasites may have complex life cycles. For instance lancet fluke eggs are shed in the manure of a grazing animal. These are picked up by a snail, the eggs hatch in the snail and the fluke’s larvae are then shed in the snail’s slime. An ant eats the slime, the larvae then control the ant’s brain, so at the end of the day the ant does not return to its nest, instead it climbs a blade of grass and hangs on. This continues every day until the ant becomes part of a mouthful of grass consumed by a grazing animal, so the fluke returns to its ultimate host.
There are very few vertebrate parasites. Some species of birds such as cuckoos, cowbirds, honeyguides are brood parasites. They remove an egg from the host bird’s nest then lay one mimic egg. The young cuckoo hatches first, grows rapidly and has a huge mouth. It pushes out the host nestlings and demands the greatest share of food from the host parents. Observations have indicated that cuckoos visit the nests they have parasitised and may attack the nest and nestlings if they see that their egg or nestling has been rejected by the host.