Description
Background: Marine cleaning interactions in which cleaner fish or shrimps remove parasites from
visiting 'client' reef fish are a textbook example of mutualism. However, there is yet no conclusive
evidence that cleaning organisms significantly improve the health of their clients. We tested the
stress response of wild caught individuals of two client species, Chromis dimidiata and Pseudanthias
squamipinnis, that had either access to a cleaner wrasse Labroides dimidiatus, or to cleaner shrimps
Stenopus hispidus and Periclimenes longicarpus, or no access to cleaning organisms.
Results: For both client species, we found an association between the presence of cleaner
organisms and a reduction in the short term stress response of client fish to capture, transport and
one hour confinement in small aquaria, as measured with cortisol levels.
Conclusion: It is conceivable that individuals who are more easily stressed than others pay a
fitness cost in the long run. Thus, our data suggest that marine cleaning mutualisms are indeed
mutualistic. More generally, measures of stress responses or basal levels may provide a useful tool
to assess the impact of interspecific interactions on the partner species.
; Oliveira, Rui Filipe 
