Parasitism
Phylum Class/Order No . of species Ecto- paras Vert def . host No . of hosts Marine Fresh- water Terres- trial
Cnidaria Myxozoa 1350 Yes 2 or more Yes Yes
Flatworms Trematodes 15,000 Yes 2 or more Yes Yes Yes
Flatworms Monogeneans 20,000 Yes Yes 1 Yes Yes
Flatworms Cestodes 5,000 Yes 2 or more Yes Yes Yes
Horsehair worms 350 1 or more Yes Yes
Nematodes 10,500 Yes 1 or more Yes Yes Yes
Acanthocephala 1,200 Yes 2 or more Yes Yes Yes
Annelids Leeches 400 Yes Yes 1 Yes Yes
Molluscs Bivalves 600 Yes Yes 1 Yes
Molluscs Gastropods 5,000 Yes Yes 1 Yes
Arthropods Ticks 800 Yes Yes 1 or more Yes
Arthropods Mites 30,000 Yes Yes 1 Yes Yes Yes
Arthropods Copepods 4,000 Yes Yes 1 Yes Yes
Arthropods Lice 4,000 Yes Yes 1 Yes
Arthropods Fleas 2,500 Yes Yes 1 Yes
Arthropods True flies 2,300 Yes Yes 1 Yes
Arthropods Twisted-wing flies 600 1 Yes
Arthropods Parasitoid wasps 650,000 Yes 1 Yes
Taxonomic range -- Animals
Further information : List of parasitic organisms
In evolutionary biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has characterised parasites as predators that eat prey in units of less than one. Parasites include protozoans such as the agents of malaria, sleeping sickness, and amoebic dysentery; animals such as hookworms, lice, mosquitoes, and vampire bats; fungi such as honey fungus and the agents of ringworm; and plants such as mistletoe, dodder, and the broomrapes. There are six major parasitic strategies of exploitation of animal hosts, namely parasitic castration, directly transmitted parasitism (by contact), trophically transmitted parasitism (by being eaten), vector-transmitted parasitism, parasitoidism, and micropredation. Like predation, parasitism is a type of consumer-resource interaction, but unlike predators, parasites, with the exception of parasitoids, are typically much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. Parasites of animals are highly specialised, and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the malaria-causing Plasmodium species, and fleas. Parasites reduce host fitness by general or specialised pathology, from parasitic castration to modification of host behaviour. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate (secondary) hosts to assist in their transmission from one definitive (primary) host to another. Although parasitism is often unambiguous, it is part of a spectrum of interactions between species, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic. People have known about parasites such as roundworms and tapeworms since ancient Egypt, Greece, and Rome. In Early Modern times, Antonie van Leeuwenhoek observed Giardia lamblia in his microscope in 1681, while Francesco Redi described internal and external parasites including sheep liver fluke and ticks.