Flow Mediates Predator-Prey Interactions Between Triclad Flatworms And Larval Black Flies

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The velocity preferences of suspension-feeding larval black flies (Simulium vittatum) and predatory triclads (Dugesia dorotocephala), both alone and together, were determined in a laboratory flow chamber in which a gradient of current velocities was maintained and manipulated. Larval densities were highest in regions of the flow chamber with high velocities. In a comparable velocity gradient, triclad densities were lowest where the velocities were greatest. When triclads were added to a flow chamber containing larvae, they readily attacked and consumed the black fly larvae. As would be predicted from their respective flow preferences, the majority of encounters between triclads and larvae occurred in the slower regions, with few encounters in areas of high velocity. Predation efficiency (as measured by percentage of attacks resulting in successful capture) was likewise nonrandomly distributed within the flow chamber, with the greatest efficiency occurring in the slowest flow regimes. We attribute this phenomenon to hydromechanical constraints on prey handling ability. Therefore larvae in high velocities have a reduced predation risk due to two factors: (1) reduced encounters with predators; and (2) reduced probability of capture following an encounter. These results clearly demonstrate the potential importance of flow in mediating predator-prey interactions in benthic stream communities.

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