 Types of parasitism
Types of parasitism
Parasites that live inside the body of the host are called endoparasites (e.g., hookworms that live in the host's gut) and those that live on the outside are called ectoparasites (e.g., some mites). An epiparasite is a parasite that feeds on another parasite. This relationship is also sometimes referred to as "hyperparasitoidism", especially in the insect world. For example a wasp or fly larva may be an endoparasite of an Ichneumon wasp larva, which is in turn an endoparasite of a wood-boring beetle larva. Therefore the ovipositing adult female hyperparasitoid must find the host of her host, namely the beetle larva, and oviposit into that beetle larva, after which her egg hatches within the beetle larva and seeks out the Ichneumon larva, ultimately burrowing into it and becoming an endoparasite. It is most likely that in this case, the ovipositing female adult hyperparasite locates the beetle larva by chemical cues it emits as a result of being parasitized itself by the Ichneumon wasp larva.
Many endoparasites acquire hosts by passive mechanisms, such as the nematode Ascaris lumbricoides, an endoparasite of the human intestine. A. lumbricoides produces large numbers of eggs which are passed from the host's digestive tract into the external environment, relying on other humans to inadvertently ingest them in places without good sanitation. Ectoparasites, on the other hand, often have elaborate mechanisms and strategies for finding hosts. Some aquatic leeches, for example, locate hosts by sensing movement and then confirm their identity through skin temperature and chemical cues before attaching.
Necrotrophs are parasites that use another organism's tissue for their own nutritional benefit until the host dies from loss of needed tissues or nutrients. Necrotrophs are also known as parasitoids. Biotrophic parasites cannot survive in a dead host and therefore keep their hosts alive. Many viruses, for example, are biotrophic because they use the host's genetic and cellular processes to multiply.
Some parasites are social parasites, taking advantage of interactions between members of a social host species such as ants or termites to their detriment. Kleptoparasitism involves the parasite stealing food that the host has caught or otherwise prepared. A specialized type of kleptoparasitism is brood parasitism, such as that engaged in by many species of cuckoo. Many cuckoos use other birds as lifetime "babysitters"; cuckoo young are raised and fed by adults of the host species, while adult cuckoos fend for themselves.
Cheating or exploitation types of parasitism are often found in situations where there are generalized non-specific mutualisms between broad classes of organisms, such as mycorrhizal relationships between plants and many types of fungi. Some myco-heterotrophic plants behave as "mycorrhizal cheaters", establishing mycorrhiza-like interactions with a fungal symbiont, but taking carbon from the fungus (which the fungus, in turn, gets from other plants) rather than donating carbon. | 
January 11th, 2007, 01:13 PM
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