Seeking service: How client behavior determines cleaning station clustering

Cleaner shrimp engage in a mutualistic relationship with reef fish, providing cleaning services in exchange for nutritional benefits. These shrimp inhabit stationary sea anemones, forming "cleaning stations" that rely on mobile fish clients to locate and revisit them. To investigate how fish movement behaviors influence the spatial distribution of cleaning stations, we developed an individual-based model that explicitly incorporates both stochastic fish movement and two forms of directed fish movement strategies: taxis (i.e. gradient-following) and memory-based movement. Our results reveal that directed movement, whether through taxis or memory, promotes the formation of spatially clustered cleaning stations, but only when the range of directed movement outweighs the homogenizing effects of random dispersal. Specifically, memory-based clustering requires the memory range to exceed dispersal distance, while taxis-based clustering emerges even with taxis-based movement ranges exceeded by dispersal distance. By parameterizing the model with empirical data on client fish visitation frequencies, we further show that reefs dominated by "Choosy" fish (those willing to travel farther for preferred stations) exhibit stronger station clustering compared to reefs with "Resident" fish (territorial species with limited movement). These findings highlight how client behavior shapes the spatial ecology of cleaning mutualisms, with implications for understanding partner encounter dynamics in other species interactions.