provides camouflage that is beneficial is uncertain. When the Hydra reproduces by budding, its algal partner is passed on to the offspring; the algae are not essential to the budding process, but H. viridissima seldom undergoes sexual reproduction if the algae are absent. Experiments in which the algae are removed from the Hydra by exposure to high light intensities (Habetha et al. 2003) indicate that the nature of the relationship depends upon the environmental conditions. Like other Hydra species, H. viridissima obtains its food by capturing prey on tentacles that are armed with nematocysts, whilst the alga carries out photosynthesis and releases the sugars maltose and glucose‐6‐phosphate that can potentially be used by H. viridissima. If there is suitable illumination and plenty of prey for the Hydra, the growth of H. viridissima with and without algae is similar. This indicates that, under these conditions, the sugars released by the algae have little importance for the Hydra. If, however, there is illumination but no food for the Hydra, then those lacking algae die after a few weeks, whilst those containing algae shrink but can survive for at least 3 months and commence feeding again if presented with food. Therefore, the symbiotic algae play an important role in the survival of H. viridissima whose normal food supply is low/absent. By contrast, if H. viridissima are kept in the dark but with plenty of prey available, those lacking algae grow much better than those containing them. Furthermore, the algal population declines by about 60% although they are not lost entirely and the H. viridissima remain pale green. This indicates that under these conditions, the algae receive nutrients from the Hydra to such an extent that the relationship changes from mutualism to one akin to parasitism.
1.2.5 Parasitism
Parasitism is a surprisingly difficult term to describe, and there are numerous definitions in the literature. We have adopted the definition that: ‘parasitism is a close relationship in which one organism, the parasite, is dependent on another organism, the host, feeding at its expense during the whole or part of its life (− +)’. Parasitism is frequently a highly specific relationship that always involves a degree of metabolic dependence of the parasite upon its host and often, though not always, results in measurable harm to the host. The association is usually prolonged, and although it may ultimately result in the death of the host, this is not usually the case. It is therefore distinct from predation in which the predator usually quickly kills and consumes its prey. However, owing to the complexities of animal relationships, there are always ‘grey areas’ in which any definition starts to become unstuck. This is particularly apparent in the case of animals that feed on blood. Mosquitoes and tsetse flies are not considered parasites because they only feed for a few seconds or minutes before departing. By contrast, hookworms and crab lice are parasites, because they live in permanent associated with their host. Blood‐feeding leeches and lampreys, however, are free‐living organisms that attach to their victim for several hours whilst taking a blood meal; some authors consider them parasites, whilst others define their feeding as a type of predation.
From Welcome Guest to Villain: The Derivation of the Term ‘Parasite’
The word ‘parasite’ derives from the Greek παρά (‘para’) meaning ‘beside’ and σῖτος (‘sitos’) that means ‘food’. In Ancient Greece, the term ‘parasite’ had religious connotations and nothing to do with infectious organisms. According to a stone tablet discovered in the temple of Heracles (Hercules) in Cynosarges, the priest was required to make monthly sacrifices in the presence of parasites who were to be drawn from men of mixed descent. Declining a request to act as a parasite was a punishable offence. (Cynosarges was an area near to the city walls of Athens. In addition to the temple there was also a gymnasium, and it was here that the Cynic philosophers taught.) Subsequently, the word came to mean someone who shared one’s food in return for providing amusement and flattery. The ‘parasitus ridiculosissimus’ was a popular character in Greek and early Roman comedies and they even had joke books to help them should they run out of witticisms. The greed of the parasite was a constant source of fun for dramatists, and he was often given crude nicknames such as ‘little brush – because he swept the table clean’. Double entendres were as popular over 2000 years ago as they are today and the Latin for little brush ‘peniculus’ is also a diminutive for a penis (Maltby 1999).
An obligate parasite is one that has no alternative but to develop as a parasite of its host. On the other hand, a facultative parasite can develop as a parasite or a free‐living organism depending upon the circumstances. For example, the larvae of the warble fly Hypoderma bovis must develop as parasites of cattle and are therefore obligate parasites. By contrast, the larvae of the blowfly Lucilia sericata are facultative parasites. This is because if the female fly lays her eggs upon a live sheep, the larvae will feed on living tissue and therefore be parasites. Conversely, if she lays her eggs on a dead sheep, the larvae will feed as free‐living detritivores. Similarly, the amoeba Naegleria fowleri can live as a free‐living organism in ponds and lakes but if it enters the nostrils of someone swimming in the water, then it can become an opportunistic parasite and infect their brain.
As mentioned above, some organisms, such as the human body louse Pediculus humanus, are parasitic at all stages of their life cycle, whilst others are only parasitic at one or more stages. For example, the blood fluke Schistosoma haematobium parasitises us during its adult stage and snails during two of its larval stages but it also has two non‐feeding free‐living stages. The act of being a parasite is therefore stage specific. Some estimates suggest that as many as 50% of all known species are parasites at some point in their life cycle. However, this estimate is subject to the caveat that there is no consensus about what constitutes a species, especially among the prokaryotes. The number of known species is also a reflection of the interests of biologists in different groups of animals. For example, the fact that insects account for 72% of all known species is, at least partly, a consequence of them being studied intensively for over 200 years. In one insect order alone, the Hymenoptera (ants, bees, wasps), there are approximately 100,000 parasitoid species. By contrast, fewer people have studied mites and nematodes and the diversity of their parasitic species is probably vastly underestimated. Nevertheless, parasitism is a remarkably common lifestyle and parasites (and their hosts) exist in all the major groups of living organisms including the archaea, bacteria, fungi, plantae, protozoa, invertebrates, and vertebrates.
There is an endless debate as to whether viruses are parasitic organisms. At one level, this would appear to be self‐evident since viruses are incapable of maintaining themselves or reproducing except when within their host cell. However, being composed of complex organic molecules and having the capacity to evolve is not necessarily synonymous with being a living entity, especially when those attributes are dependent upon existing within a host cell. We will discuss this topic further in Section 2.2.
1.2.5.1 Intra‐specific Parasites
Although most parasitic relationships involve two different species, intra‐specific parasitism also occurs. Brood parasitism is a common example of intra‐specific parasitism among many birds (Tomás et al. 2017) and some social wasps (Oliveira et al. 2016). It involves a female laying her eggs in the nest of a conspecific (member of the same species) – this means that the costs of rearing, the young will be borne by another individual. Intra‐specific parasitism sometimes occurs during sexual reproduction when the male attaches to the female and becomes dependent upon her for the provision of nutrients. For example, in certain deep‐sea angler fish belonging to the suborder Ceratioidea, the larval fish develop in the upper 30 m of sea water and then gradually descend to deeper regions as they metamorphose into adults. The adolescent males have a very different morphology to the females: they are much smaller; they have larger eyes, and, in some species, they develop a large nasal organ that presumably helps in their search for females. Furthermore, the males cease feeding and rely upon reserves laid down in their liver during the larval period to fuel their swimming. Upon finding a
