Abstract
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.
Highlights
Evolutionary theory predicts that anti-predator traits should decay and resources be reallocated elsewhere if predation pressure relaxes on a given species [1,2,3]
We ask whether reduced predation pressure on fish that provide cleaning services to predators can lead to decreased escape performance via compensated trait loss
Pseudocheilinus hexataenia and L. dimidiatus were the two species that had the shortest response latency: their performance was significantly better than that of the four other species
Summary
Evolutionary theory predicts that anti-predator traits should decay and resources be reallocated elsewhere if predation pressure relaxes on a given species [1,2,3]. We tested for interspecific differences in five measures of escape performance using linear mixed-effects models (LMM): escape latency (ms), Umax (cm s21), Amax (m s22), Desc (cm) and turning rate (8 ms21) In addition to these five standard kinematic variables, we computed the cumulative distance travelled in 34 ms (Desc_stim), which corresponded to the time between the onset of the stimulus to the end of stage 2 (approximately). We did not control for body length for three reasons: (i) size and fish species were collinear (see the electronic supplementary material, figure S1), (ii) Desc and Umax are thought to be sizeindependent when measured in a fixed time interval [51] and (iii) we were interested in absolute (i.e. in terms of centimetres) rather than relative performance (i.e. body lengths). All analyses were done in R v. 3.2.2 [54]
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