gut (Tasiemski et al. 2015). This reduces the diversity of the gut microbial flora and emphasises the close relationship between the two organisms. Aeromonas veronii is not always beneficial: in humans, it causes wound infections, septicaemia, and gastroenteritis. Blood‐feeding leeches are useful in modern medicine, particularly to aid wound drainage following reconstructive surgery, but there is a risk of them facilitating an Aeromonas infection in the patient. The infections are often trivial, but they can become serious and lead to abscesses or cellulitis. This is a difficult problem to solve because the symbiotic bacteria are essential for the long‐term survival of the leech. One cannot develop a strain or culture of Aeromonas‐free leeches. However, treating the leeches 1–4 weeks before use with an antibiotic such as ciprofloxacin removes the bacteria without compromising the willingness of the leech to feed (Mumcuoglu et al. 2010). A leech is only used once in reconstructive surgery because of their potential to transmit diseases between patients. Consequently, the long‐term survival of antibiotic‐treated leeches is not a concern.
The nymphs and adults of the human body louse Pediculus humanus humanus (sub‐order Anoplura) (Figure 7.11) have a symbiotic relationship with the gamma (γ) proteobacterium Riesia pediculicola – also referred to as Candidatus Riesia pediculicola. The term ‘Candidatus’ is used in prokaryote taxonomy for an organism that may be well characterised from molecular and other studies but cannot be cultured in the laboratory. Riesia pediculicola are primary intracellular endosymbionts that provide their hosts with pantothenic acid (vitamin B5) and are essential to the survival of the lice. In nymphs and adult male lice, the symbionts live within a mycetome that some texts refer to as the ‘stomach disc’. This is an unfortunate term because lice, like other insects, do not have a stomach in the mammalian sense of the word. Anyway, the mycetome is located on the ventral side of the mid‐gut but unlike the leeches mentioned previously, there is no connection between the mycetome and the gut lumen (Perotti et al. 2008). In adult female lice, the bacteria re‐locate to the oviducts and the developing eggs. Interestingly, molecular phylogenetic analysis cannot distinguish between the symbiotic bacteria of human head lice (Pediculus humanus capitis) and human body lice (Pediculus humanus humanus). This adds support to phylogenetic analysis of the lice themselves that indicates that although head lice and body lice occupy different ecological niches and body lice tend to lay their eggs on clothing whilst head lice attach their eggs to hair shafts, they are two morphotypes of the same species rather than two separate species. One suggestion is that body lice evolved from head lice relatively recently in human evolution once we started wearing clothing. The association between Riesia and Pediculus is between 12.95 and 25 million years old – which makes it one of the youngest host: primary endosymbiont relationships so far recorded (Allen et al. 2009). In common with other primary endosymbionts, Riesia has undergone a reduction in genome complexity and lost genes (Moran and Bennett 2014): this is because it has come to rely on its host for the provision of many nutrients and protection from the environment etc. In addition, because its transmission is via the eggs of its host, each louse symbiont population is in reproductive isolation and unable to undergo recombination with other strains of Riesia in other lice. This has led to the suggestion that Riesia will lack the capacity to develop rapid resistance mechanisms to antibiotics – and because the Riesia is essential for the lice, killing the symbiont would result in host mortality.
1.2.2 Commensalism
The term ‘commensalism’ derives from the Latin commensalis and means ‘at the same table together’. Most definitions state that one species benefits from the association and the other is unharmed (0+). Including the concept of ‘harm’ within any definition is seldom a good idea because harm is difficult to measure and varies with the circumstances. Similarly, a ‘benefit’ may not be immediately apparent, and some associations commonly cited as commensal might involve a degree of benefit to both parties (++) albeit they may not benefit to the same extent. A commensal association may be ‘facultative’, in which both species can live independently of one another or ‘obligatory’, in which one of the associates must live in association with its partner. For example, in many warmer parts of the world, the cattle egret (Bubulcus ibis) perches on the back of cattle and big game from which it swoops down periodically to capture lizards and insects that are disturbed as its ride moves through the undergrowth. The egret is perfectly capable of living apart from cattle, but it benefits from its mobile vantage point‐cum‐beater. The egrets probably do not remove many ectoparasites from the cattle and they get their Arabic name Abu Qerdan ‘father of ticks’ from the abundance of ticks associated with their nesting colonies. The cattle, therefore, appear to gain little from the relationship although the egret acts as an early warning system of the approach of predators. African Cape Buffalo (Syncerus caffer) have a good sense of smell but a notoriously poor eyesight: they are therefore vulnerable to predators approaching from downwind. The red‐billed oxpecker (Buphagus erythrorhynchus) is sometimes said to have a similar commensal relationship with cattle, but this is almost certainly false. Unlike cattle egrets, the red‐billed oxpecker has an obligatory relationship with cattle and big game and far from removing ticks it feeds primarily on scabs and wound tissue pecked from their host. Their feeding delays wound healing and thereby makes the affected animal vulnerable to infections and infestations with blowfly larvae.
The amoeba, Entamoeba coli (not to be confused with the gastro‐intestinal bacterium Escherichia coli, which is also abbreviated to E. coli) is a common commensal that lives in our large intestine. Unlike its pathogenic cousin, Entamoeba histolytica, E. coli feeds on bacteria and gut contents and does not invade the gut mucosa or consume red blood cells. Therefore, E. coli is of little interest per se, although a study in Mexico suggested an association between moderate‐heavy infestations and childhood obesity (Zavala et al. 2016). The most important feature of E. coli is that its morphological similarity to E. histolytica means that one must be careful to distinguish between the two species in microscope surveys of faecal samples.
1.2.3 Phoresis
The term ‘phoresis’ derives from the Greek verb ϕέρω (‘phero’) meaning to bear/carry. This association involves one species providing shelter, support, or transport for another organism of a different species and may be temporary or permanent. For example, apart from during their first instar, the larvae, and pupae of the blackfly Simulium neavei attach themselves to the outer surface of freshwater crabs. The larvae feed by filtering out phytoplankton and detritus from the water and the crabs act as a firm yet mobile substrate on which to attach. An appreciation of this association is important because adult S. neavei are vectors of the filarial nematode Onchocerca volvulus that causes ‘River Blindness’.
1.2.4 Mutualism
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
