Are Caribbean cleaning symbioses mutualistic? Costs and benefits of visiting cleaning stations to longfin damselfish

Cleaning symbioses in the marine environment have long been held to be mutualistic interactions in which cleaners glean food from the surface of their fish clients while client ectoparasite load is reduced. However, there is limited evidence to show that clients benefit from being cleaned. We investigated the nature of a cleaner-client relationship by quantifying the costs to territorial longfin damselfish, Stegastes diencaeus, of being cleaned by cleaning gobies (Elacatinus evelynae and E. prochilos) in terms of travelling time, aggression and territorial intrusions incurred while seeking cleaners, and the benefits in terms of reduction in ectoparasite load. Travelling time to seek cleaners increased with distance from a damselfish's territory, as did the number of aggressive attacks by other territorial fish sustained by the travelling damselfish. The number of intrusions by fish on to the unguarded territory also increased, although not significantly, with time spent away from the territory. As a result, damselfish visited cleaning stations less as the distance between its territory and cleaning station increased. This variation in visit rate was related to a slight but significant reduction in the number of parasitic gnathiid isopod larvae per damselfish, suggesting that cleaning gobies significantly reduce client ectoparasite load. Longfin damselfish appeared willing to pay only limited costs to be cleaned. They travelled much further and staved away longer from their territories to perform reproductive and social activities than they did to seek cleaners. Distance-dependent variability in the costs of seeking cleaners allows damselfish to scale these costs in relation to the benefits gained and ensure that their relationship with cleaners remains mutualistic. (C) 2001 The Association for the Study of Animal Behaviour