Water flow controls distribution and feeding behavior of two co-occurring coral reef fishes: II. Laboratory experiments

The chaenopsid blenny Acanthemblemaria spinosa occupies topographically high locations on coral reefs where flow speeds and turbulence are frequently greater than those experienced by its congener, A. aspera, which occupies locations close to the reef surface. To investigate the adaptive mechanisms resulting in this microhabitat differentiation, the foraging effort and success of these fishes were determined in laboratory flumes that produced flow conditions approximating those experienced in the field. Individual fish were subjected to unidirectional (smooth and turbulent) and oscillatory flows while they fed on calanoid copepods, Acartia tonsa, whose vulnerability to predation varies with water flow. In unidirectional flow both blenny species had their greatest foraging success at intermediate flow speeds (ca. 10 cm s⁻¹) and under turbulent flow. Under all conditions, Acanthemblemaria spinosa exhibited greater foraging effort and attacked at greater distances, greater mean water speeds, and in oscillatory flow, over a greater proportion of the wave cycle than did A. aspera. A. spinosa also exhibited greater foraging success under turbulent flow conditions. These differences in feeding patterns allow A. spinosa, with its higher metabolic rate, to occupy the more energetic higher locations in corals where planktonic food is more abundant. A. aspera occupies the poorer quality habitat in terms of planktonic food availability but its lower metabolic rate allows it to thrive there. Consequently, these species divide the resource in short supply, i.e., shelter holes, based on their differing abilities to capture prey in energetic water conditions in conjunction with their differing food energy requirements. Communicated by Biology Editor Dr. Mark McCormick