tooth‐like bones that develop at the tips of his jaws (his actual teeth degenerate during metamorphosis). After attaching, the male grows (although he remains much smaller than his consort), and his testes mature. His skin and blood vessels fuse with hers at the site of attachment, and he remains attached for the rest of his life and draws all his nourishment from her. Some authors suggest that the male must find a virgin female. However, although most females carry only a single male, there are records of females with three or more males attached to them. This is presumably an adaptation to life in the deep‐sea regions in which the opportunity to locate mates is limited. Nevertheless, this raises questions about how sexual selection occurs because it is unusual in nature for a female to mate with just one male for life, especially if that male is the first one to turn up. This type of relationship is not found in all ceratioid anglerfish; in some species, the males are facultative parasites rather than obligate ones as described in the above scenario, whilst in other species the males are free‐living, capable of capturing their own food, and form only temporary attachments to females. Molecular evidence suggests that the development of the parasitic males is a relatively plastic phenomenon among anglerfish and has evolved and subsequently become lost on several occasions (Pietsch 2005).
1.2.6 Parasitoids
The term parasitoid is restricted to certain parasitic insects whose hosts are almost exclusively other insects – although a few species attack certain crustacea, spiders, millipedes, centipedes, and earthworms. Some parasites cause mortality and may even depend on the death of their host to effect transmission to the next stage of their life cycle, but host death is not inevitable. By contrast, parasitoids slowly consume their host’s tissues so that the host remains alive until the parasitoid has completed its development. At this point, the host dies either through the loss of vital tissues or through the parasitoid physically eating its way out of its host. Parasitoids are all parasitic during their larval stage, and the adult insect is free living and feeds on nectar, pollen, dead organic matter, or is predatory, depending upon the species. Parasitoids can develop as endoparasites within their host or as ectoparasites attached to the outside but with their mouthparts buried deep within the host’s body. The larva has only the one host in or on which it develops and those that are endoparasites tend to exhibit the most host specificity. This lifestyle is therefore distinct from those insects such as warble flies (e.g., Hypoderma bovis) and bot flies (e.g., Gasterophilus intestinalis), which exhibit a more ‘traditional’ parasitic way of life that does not inevitably result in the death of the host. Many species of the order Hymenoptera (bees, ants, wasps) are parasitoids, and it is also a common lifestyle among the Diptera (true flies), but it is absent or very rare among the other orders. By contrast, most of the insect orders are hosts to parasitoids. Hyperparasitism is also common in which a parasitoid parasitizes another species of parasitoid. Parasitoids are effective for the control of agricultural pests, particularly within closed environments such as greenhouses. However, they have had limited success as control agents for parasites, their vectors, or intermediate hosts.
The parasitoid lifecycle typically begins with the adult female locating its host and either injecting one or more eggs or attaching them to the host’s outer surface. Sometimes she also injects a toxin that temporarily or permanently disables her victim. The host is chosen based on its stage of development, which may be anywhere from the egg to the adult stage.
Parasitoid: Virus Interactions
Some endoparasitic wasps belonging to the families Ichneumonidae and Braconidae have a fascinating relationship with polydnaviruses. The polydnaviruses from these two wasp families are morphologically distinct, and they probably arose from the ‘domestication’ of two different viruses. However, through convergent evolution they exhibit many biological similarities (Drezen et al. 2017; Strand and Burke 2019).
The viruses replicate within the calyx cells of the wasps’ ovaries and are then secreted into the oviducts. Therefore, when a wasp injects her eggs into a suitable host, usually a lepidopteran caterpillar, she also injects the virus. The viruses cannot replicate within the caterpillar, but they do invade several of its cell types. Within these cells, the viruses integrate into the caterpillar’s genome and cause the expression of substances that facilitate the establishment of the parasitoid. For example, one of the main immune responses that insects express in response to an invader is encapsulation. Encapsulation depends upon recognition of the invader and then a co‐ordinated physiological response: amoeboid‐like cells present in the haemolymph surround the invader and then either kill it through the production of toxic chemicals and/or lack of oxygen or physically isolate it and thereby prevent it damaging the host.
If one implants wasp eggs without the virus into a host, then these are rapidly encapsulated and killed. The protective effect of the virus probably results from it causing the caterpillar to express protein tyrosine phosphatase enzymes and thereby interfering with the encapsulation process. Protein tyrosine phosphatases dephosphorylate the tyrosine residues of several regulatory proteins and are therefore closely involved in the regulation of signal transduction. Altering the levels of regulatory proteins makes it impossible for the host to express an effective immune response and therefore the parasitoid egg develops unmolested. The viruses also have other effects on the caterpillar including preventing its further development once it reaches the stage at which the parasitoid is to emerge. Consequently, the polydnaviruses have a mutualist‐like relationship with the parasitoid within which they replicate. They are transmitted vertically as an endogenous provirus that integrates into the wasp genome but have a pathogenic relationship with the parasitoid’s host, within which it cannot replicate.
It is probable that there are many other examples of symbiotic virus‐parasitoid/parasite relationships awaiting discovery. However, not all wasp parasitoids have relationships with viruses and these inject toxins that cause similar disruptions to the host immune response and host development.
1.2.7 The Concept of Harm
The term ‘harm’ is often employed when describing interactions between organisms but is particularly pertinent to any discussion of host: parasite relationships. Unfortunately, harm is a difficult term to define and is not always easy to measure. For example, parasites are usually much smaller than their host and a single parasite may have such a minor impact that its effect on the physiology and well‐being of the host is too trivial to measure. By contrast, a large number of the same parasite could cause serious illness or even death. Similarly, a low parasite burden may have little impact upon a healthy, well‐nourished adult host, but the same number of parasites infecting an unhealthy, starving young host may prove fatal. Consequently, harbouring a pathogen (being infected) and expressing the signs and symptoms of being infected (suffering from a disease) are not necessarily synonymous. A common analogy is that a single glass of water will not harm you and may even do you good, but the rapid consumption of a thousand glasses of water would kill you. Does that mean that water is beneficial or poisonous? Clearly, it can be both and, likewise, harm is dependent upon the context in which it is being considered. For human parasites, one should also consider the context and psychological consequences. Among some poor communities, being infected by lice and parasitic worms may be considered an unremarkable fact of everyday life. By contrast, in affluent communities, the very thought of harbouring worms inside the body or being bitten by fleas may cause mental torment far above any physical harm caused. It is therefore not a good idea to make the ability to record measurable harm as a pre‐requisite for the classification of the relationship between two organisms. Indeed, in certain instances, low levels of parasitic infection may benefit the well‐being of the host (Maizels 2020). Nevertheless, many parasite species have the capacity to cause morbidity, that is, a diseased state, and some may cause mortality (death). We discuss the possible beneficial consequences of low parasite burdens in more detail in Chapter 12.
The morbidity that parasite infections induce is often reflected in a reduction in the host’s fitness as measured in terms of its
