Parasite: a special consumer

All species of horsehair worms are generally acknowledged to be parasites

(Hanelt et al., 2005), included the species found emerging from their hosts and nearly

half of the species which are "believed to be" parasites since their hosts have not yet been known (Poinar and Chandler, 2004; Schmidt-Rhaesa, 2012). The term "parasite"

is widely used and it is commonly accepted that a parasite adopts one of the most unique strategy to be a consumer. Nevertheless, parasitism, despite it has been suggested for more than one century from 1879

(Zelmer, 1998), is not a constant

concept in each assay. As the opposite concept of the free-living organism, parasites are generally considered to be the organisms live in their hosts. To date, using the host as both source of nourishment and habitat is the most common definition of parasitism

(Zelmer, 1998; Poulin, 2007). However, this rule groups various parasitic

organisms which is much more than people image. As almost all organisms have other organisms living in it (including parasites infected by other parasites, termed hyperparasitism), the free-living strategy may be more unique than parasitism

(Matthews, 1998). Thus, parasite discussed is always further defined by the limited

taxa or cases (Matthews, 1998), despite parasitism is more likely to be an ecological concept instead a systematic group (Eggleton and Belshaw, 1992; Zelmer, 1998).

The predator is always considered to be most closely related to the parasite. The most significant difference between parasite and predators is that if the prey are alive during being consuming. This property makes parasitoids, which consume the live hosts but finally kill them, become the special parasitic organisms. There are two properties mainly applied in characterizing a parasitoid: 1) the free-living stage in the life cycle (Eggleton and Gaston, 1990; Zelmer, 1998) and killing the host in the end of

the parasitic stage (Eggleton and Gaston, 1990; Zelmer, 1998; Kuris and Lafferty,

2000). The former property emphasizes the habitat role of the host to a parasite and

makes the parasitoid more likely to be a predator which, comparing with the normal predators, spend longer time on exploiting the food resource from its prey (Zelmer,

1998). This concept also rules out the "insidious and consumptive predators" usually

has been considered ecto-parasites like mosquitos, which do not really live on their hosts. Since the emphasis on the habitat role of the host, whether the host is killed is not the essential property to characterize a parasitoid. This point of view is not much of the problem in the parasitoid wasps, but is debatable in the parasitoid flies. In the early concept, the parasitoid is suggested to specially describe the insect parasite especially the parasitoid wasps (Eggleton and Gaston, 1990). In many literatures to date, the parasitoid is still usually equal to the "parasitic" Hymenoptera (Eggleton and

Belshaw, 1992). With the highly host specific, the parasitoid wasps generally

parasitized with insect hosts and kill it in the end of the parasitic stage (Eggleton and

Belshaw, 1992). However, the flies, as also common "parasitic" insects, highlights the

non-essential property of killing host by its diverse host taxa. As the different way in evolving the parasitic life style, host range of the "parasitic flies" is much wider than the parasitoid wasp which can be invertebrates or vertebrates (Eggleton and Belshaw,

1992). In the fly species parasitizing insect hosts, the "parasitic flies" usually kill their

hosts before pupation whose behavior is similar to the parasitoid wasps. Whereas the obligatory myiasis (e.g., screw worms, bot flies and warbles) which lay eggs on mammal hosts is usually less harmful to cause the lethal damage

(Eggleton and

Belshaw, 1992). Whether the host die or not among the parasitism of flies and wasps

might be mainly due to the host relative size (Zelmer, 1998). Thus, death of the hosts is more likely to be an extended property but not a defining character. Since

considering the host as a prey instead of habitat, the parasitoids frequently kill their hosts by the extreme virulence or damage, which tends to be reduced in the parasites

(Alizon et al., 2009).

The high virulence in the parasitoids, than the parasites, indicates their diverged way in exploiting their hosts. In contrast to the definition emphasizing on the habitat role of hosts, the host death as the character makes the parasitism more close to be a foraging strategy. The foraging model causes one special dichotomy for dividing the parasite from other consumers: the number of host/prey attacked during the particular life-history phase (Kuris and Lafferty, 2000). The functional response, which is the important component of the most widely known Lotka-Volterra predator prey model, describe how predators eat "many" prey (Kuris and Lafferty, 2000). This model is obviously not applicable in describing the parasite-host system as a parasite generally attacks one host in a particular life-history phase. Under this rule, typical parasites, the parasite contains typical parasites, parasitoids, and micro-predators (e.g., ecto-parasites, insect herbivores) whose size are smaller than their victims. However, comparing with the disease model mainly based on epidemiology, parasitoid models are derived more from predator-prey theory (Kuris and Lafferty, 2000). One of the example is the Nicholson-Bailey model. This model describes how the competition of female parasitoids search and lay eggs in the hosts, which is also known as the competition model

(Royama, 1971). This model emphasizes the predator property of

parasitoids in the free-living stage and implies the parasitic larval stage as the mean of handling the prey. Thus, the key difference between parasitoids and parasites, killing the host, could be considered as the predator property of the mother parasitoids. The separation of the free-living adult phase and the parasitic larval phase makes it more clear to characterize the foraging strategies in different developmental stages.

Although it is less mentioned in the parasitoid model, during the parasitic larval phase, parasitoids still show the high similarity with typical parasite.

During evolutionary processes, the parasitic strategy independently evolved in wide phylogenetic taxa. It can be fixed in the population under different environmental conditions. Thus, it might be difficult and impractical to group all the parasitism evolved with different adaptations by a signal or few properties. Zelmer

(1998) summarized different definitions of parasitism into a property that all parasitic

organisms have the ability to evade the immune response of the host. However, this definition is rarely applied and actually does not highlight the significance of parasitism from other foraging strategies or ecological relationship. In the contrast, many characters of the parasitic organisms including the smaller size, feeding alive organisms, evading the host immune response, are all related to the habitat role of hosts. Thus, it is not surprisingly that "the habitat and nourishment role" become two critical parasite's properties most commonly applied, despite there are still a gray area between the parasitic organism and others.

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