Parasitism

Parasitism has evolved independently across a very wide range of taxonomic groups, from plants right up to the higher vertebrates. Many parasites are host-specific, meaning they can parasitise only a single host species, so that the host and its parasite tend to co-evolve against one another over time. Parasites that feed on the outer surface of their host are called ectoparasites -- familiar examples include lice on humans, ticks on dogs, and various ectoparasitic copepods that infest marine fish. Cuscuta, a parasitic plant commonly seen growing over hedge plants, has lost its own chlorophyll and leaves in the course of its evolution, and derives its nutrition entirely from the host plant it parasitises. Parasites that live inside the host's body, or endoparasites, show a different suite of adaptations in keeping with their lifestyle -- loss of sense organs they no longer need, adhesive organs or suckers to cling on to host tissue, loss of a functioning digestive system, and unusually high reproductive capacity. The life cycles of such parasites are often complex, involving one or more intermediate hosts or vectors to help them reach a new host -- the malarial parasite Plasmodium vivax, for instance, needs a mosquito as a vector to spread from one host to another. …