The performance of cleaner wrasse, Labroides dimidiatus, in a reversal learning task varies across experimental paradigms.

Testing performance in controlled laboratory experiments is a powerful tool for understanding the extent and evolution of cognitive abilities in non-human animals. However, cognitive testing is prone to a number of potential biases, which, if unnoticed or unaccounted for, may affect the conclusions drawn. We examined whether slight modifications to the experimental procedure and apparatus used in a spatial task and reversal learning task affected performance outcomes in the bluestreak cleaner wrasse, Labroides dimidiatus (hereafter "cleaners"). Using two-alternative forced-choice tests, fish had to learn to associate a food reward with a side (left or right) in their holding aquarium. Individuals were tested in one of four experimental treatments that differed slightly in procedure and/or physical set-up. Cleaners from all four treatment groups were equally able to solve the initial spatial task. However, groups differed in their ability to solve the reversal learning task: no individuals solved the reversal task when tested in small tanks with a transparent partition separating the two options, whereas over 50% of individuals solved the task when performed in a larger tank, or with an opaque partition. These results clearly show that seemingly insignificant details to the experimental set-up matter when testing performance in a spatial task and might significantly influence the outcome of experiments. These results echo previous calls for researchers to exercise caution when designing methodologies for cognition tasks to avoid misinterpretations.

Mutualistic cleaner fish maintains high escape performance despite privileged relationship with predators.

Predatory reef fishes regularly visit mutualistic cleaner fish (Labroides dimidiatus) to get their ectoparasites removed but show no interest in eating them. The concept of compensated trait loss posits that characters can be lost if a mutualistic relationship reduces the need for a given trait. Thus, selective pressures on escape performance might have relaxed in L. dimidiatus due to its privileged relationship with predators. However, the cost of failing to escape a predatory strike is extreme even if predation events on cleaners are exceptionally rare. Additionally, cleaners must escape from non-predatory clients that regularly punish them for eating mucus instead of parasites. Therefore, strong escape capabilities might instead be maintained in cleaner fish because they must be able to flee when in close proximity to predators or dissatisfied clients. We compared the fast-start escape performance of L. dimidiatus with that of five closely related wrasse species and found that the mutualistic relationship that cleaners entertain with predators has not led to reduced escape performance. Instead, conflicts in cleaning interactions appear to have maintained selective pressures on this trait, suggesting that compensated trait loss might only evolve in cases of high interdependence between mutualistic partners that are not tempted to cheat.

Social cognition in fishes.

Brain evolution has often been correlated with the cognitive demands of social life. Further progress depends on our ability to link cognitive processes to corresponding brain part sizes and structures, and, ultimately, to demonstrate causality. Recent research suggests that fishes are suitable to test general hypotheses about vertebrate social cognition and its evolution: brain structure and physiology are rather conserved among vertebrates, and fish are able to perform complex decisions in social context. Here, we outline the opportunities for experimentation and comparative studies using fish as model systems, as well as some current shortcomings in fish social cognition research.

Power and temptation cause shifts between exploitation and cooperation in a cleaner wrasse mutualism.

In many instances of cooperation, only one individual has both the potential and the incentive to 'cheat' and exploit its partner. Under these asymmetric conditions, a simple model predicts that variation in the temptation to cheat and in the potential victim's capacity for partner control leads to shifts between exploitation and cooperation. Here, we show that the threat of early termination of an interaction was sufficient to induce cleaner wrasse Labroides dimidiatus to feed selectively against their preference (which corresponds to cooperatively eating client fish ectoparasites), provided that their preference for alternative food was weak. Under opposite conditions, cleaners fed selectively according to their own preference (which corresponds to cheating by eating client mucus). By contrast, a non-cleaning fish species, Halichoeres melanurus, failed to adjust its foraging behaviour under these same conditions. Thus, cleaners appear to have evolved the power to strategically adjust their levels of cooperation according to the circumstances.