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Mutualistic (Latin, mutuus meaning ‘reciprocal’) relationships are those in which both species benefit from the association in terms of their growth and survival (++). Some authors further restrict the definition to one in which neither partner can live on its own, whilst others are less prescriptive. The association between Wolbachia bacteria and O. volvulus is clearly mutualistic. The bacteria live within the cells of the reproductive tissues and hypodermis in the adult female worms and provide them with essential metabolites. In the absence of the bacteria, the worms cannot establish themselves in their host and grow and adult females become infertile. The bacteria are therefore a potential target for the chemotherapy of filarial nematode infections (Jacobs et al. 2019; Taylor et al. 2019).

Whether the relationship between the Cnidarian Hydra viridissima and its algal partner Chlorella is mutualistic depends upon the strictness of one’s definition. Hydra viridissima can grow and reproduce in the absence of their algal partner, but it is uncertain whether the strains/species of Chlorella associated with H. viridissima can survive independently. The algae live within vacuoles in the endodermal cells of the Hydra and thereby impart it with its characteristic green coloration. Whether this 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.