What Is a Parasite

Parasite comes from the Greek parasitos. Originally, parasitos referred to a dinner guest. However, around 400 BC, Greek comedies began featuring a stereotyped character: a hard to get rid of guest whose obnoxious nature becomes tiresome. Hereafter, parasitos took on the meaning of a freeloader.

Tapeworms, digenetic trematodes, and acanthocepha-lans are familiar parasitic taxa. Among the animals, there are nine entirely parasitic phyla and 22 predominantly (>99%) predatory phyla. For the remaining 11 phyla that have a mix of parasitic and free-living species (e.g., Nematoda, Platyhelminthes, and Arthropoda), clades within a phylum are often either entirely parasitic or predatory. Nonetheless, parasitism lacks a single evolutionary origin and is spread among phyla.

The field of parasitology traditionally limits its focus to animal groups that one can see clearly with a microscope. These include protozoans (amebas, flagellates, ciliates, apicomplexans, myxozoans, and mesozoans) and metazoans. Of the metazoans, several parasitic groups are familiar: platyhelminthes (monogenes, trematodes, ces-todes), nematodes, acanthocephalans, pentastomids, and arthropods (crustaceans and insects). Some of these groups (particularly the dipterans) are best defined as micropredators. Small and nonanimal parasitic taxa (viruses, bacteria, fungi, and plants) are not included in the field of parasitology. These groups are often termed pathogens. For these reasons, an ecological/evolutionary concept of parasitism does not necessarily conform to the taxa treated by the field of parasitology.

Natural enemies of all taxa take nourishment from a victim (host) or victims (prey) using a variety of trophic strategies. The particular strategy an individual natural enemy uses can vary from one victim to the next. How do parasites differ from predators? At first glance, it appears that relative body size is the key factor. Parasites are generally much smaller than their hosts and predators are larger than their prey. But a wolfis not a parasite ofthe larger moose and body size is not a particularly useful means of distinguishing among trophic strategies, despite the fact that it appears to be a key correlate and evolutionary driver of trophic strategies.

The durability of the enemy-victim interaction has been argued as a useful distinction between predators and parasites (i.e., parasitic interactions are durable). While parasites are generally intimately associated with their hosts, intimacy is difficult to apply as a dichotomous operational definition. There is no obvious durability criterion that would clearly allow us to distinguish predator from parasite. For instance, a lion may stay for several days feeding on a zebra carcass while the nymph of a tick takes one blood meal from a deer and drops off within a few hours.

If intimacy is difficult to apply as a categorical definition of parasitism, what other criteria are available? Population models provide a possible perspective of trophic strategies. In infectious disease models, a parasite exploits only one host during a particular phase of its ontogeny. In contrast, most population models in ecology incorporate a functional response that considers how predators eat many prey. In other words, population modelers have established a well-accepted dichotomy between parasites and predators based on the number of hosts or prey attacked during a particular life-history stage. Attacking one versus more than one victim is easier to apply as a means to distinguish parasites from predators than durability of an attack, though the two views are not in conflict (exploiting only one victim during a life-history stage is necessarily a durable attack).

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