ABSTRACT
Cleaning in gobies is defi nitely a more muted aff air than it is in Indo-Pacifi c cleaner wrasses. Th e service is less conspicuous, by virtue of the smaller size of gobies and the lack of advertisement dancing, the transaction more straightforward, owing to the absence of tactics to counter cheating, and the outcome in terms of parasite removal only marginally benefi cial, at least from the client’s point of view. Th ere has even been a suggestion that adopting a cleaning lifestyle is making the best of a bad situation, based on the observation that the growth rates of juvenile and survival rates of adult E. evelynae that live on sponges (where they do not clean) are higher than those of E. evelynae that live on coral (where they do clean) (White et al., 2007). Th is is clearly not the case everywhere or for every cleaning goby species with dichotomous habits (e.g., Whiteman and Côté, 2004a), but such observations highlight the fact that cleaning behaviour, particularly in gobies, is far from well understood, either evolutionarily or ecologically. Key questions remain to be answered. One concerns the outcome of cleaning. How substantial is the benefi t of being cleaned by cleaning gobies? Th e studies that have focused on ectoparasite numbers suggest a very small eff ect, but other metrics might be important. Th e issue of ectoparasites as vectors for diseases was raised earlier. In addition, there is evidence that cortisol levels-a marker of physiological stress-are lower in clients interacting with cleanerfi sh (Bshary et al., 2007). Th us, as a result of being cleaned, the general health of clients may therefore be improved in ways that are poorly refl ected by simple parasite counts. Another important question relates to the ultimate cause of the discrepancies observed between cleaner wrasses and cleaning gobies. We outlined earlier how a preference by cleaning gobies for ectoparasites over mucus or other client-gleaned items could potentially explain these large interspecifi c diff erences, but the evidence to date is only circumstantial. Experiments of the type carried out with cleaner wrasses, in which cleaners are simultaneously off ered diff erent food items (e.g., Grutter and Bshary, 2003, 2004) or allowed to forage on an anaesthetised client that cannot retaliate to cheating (Bshary and Grutter, 2002a), need to be performed with cleaning gobies to measure their foraging preferences. Alternative causes should also be considered. For example, if cleaning gobies infl ict very small costs on their clients when cheating, the cleaner control tactics displayed by clients of cleaner wrasses would be largely unnecessary, as might be the advertisement of (honest) cleaning service. In addition, punishing a cleaner
that misbehaves is useful to a client only if the cleaner changes its behaviour in subsequent interactions with the punisher. Th e cognitive ability required is considerable, as the cleaner needs to recall the act that led to punishment as well as the identity of the individual that retaliated. Cleaner wrasses, which have been described as having a ‘Machiavellian intelligence’ (Bshary and Würth, 2001), are able to do this (Tebbich et al., 2002; Bshary and Grutter, 2002a), but we do not know whether cleaning gobies can. Finally, the apparently limited role played by cleaning gobies in determining client abundance or community structure needs to be confi rmed (or refuted). Interested researchers should note in this context that the earliest cleaner wrasse removal experiments invariably concluded that cleaner wrasses had little or no eff ect on client populations (Youngbluth, 1968; Gorlick et al., 1987; Grutter, 1996, 1997). Yet, later eff orts with longerrunning and better-replicated fi eld experiments concluded the opposite (Bshary, 2003; Grutter et al., 2003). Given that so much of our recent understanding of cleaning behaviour has been influenced by research on cleaner wrasses, should we study cleaning gobies at all? Our answer is an emphatic yes! Work on cleaning gobies has revealed that cleaner wrasses are not a universal model to which all cleanerfi sh conform. Th is comparison has given, and will continue to provide, deep insights into the evolution of cleaning interactions. We have only begun to unravel the mysteries of cleaning gobies.
