Collaborative Research: Parasite or partner? Causes and consequences of conditional outcomes in a cleaning symbiosis

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For decades, ecologists have focused on antagonistic interactions such as competition and predation in studies of ecosystems and food webs. But in recent years it has become apparent that cooperative interactions among species, i.e., mutualisms, can be equally important to ecosystem structure and function. One widespread mutualism is that between pollinators and plants, which is of major economic importance to agricultural systems. However, other types of mutualisms are prevalent in nature, but poorly understood. This project will study a model system involving a cleaning mutualism, a species interaction in which one organism feeds by cleaning another. The mutualism involves crayfish and small annelid worms, which live on the gills of crayfish. The worms clean the crayfish gills by feeding on organisms that attach to the gills; this benefits both the worms and the crayfish. However, if the worms become too abundant relative to their food, they can switch to act as parasites, feeding directly on the crayfish's gills. This shift from mutualism to parasitism is not unique to the crayfish-worm system. It has been hypothesized that such shifts may be widespread in nature in response to certain environmental changes. This project will test that general hypothesis using both field and laboratory experiments to study the crayfish-worm model system. The project will examine the importance of species behaviors and other traits in maintaining the interaction at mutually beneficial levels, and explore how the mutualism is shaped by environmental context. The project will also explore the broader consequences of a shift from mutualism to parasitism to the rest of the food web and stream ecosystem. This project has substantial broader impacts; it is a collaboration among faculty at three universities, one of which is a predominantly undergraduate institution. Many undergraduate students will be training in aquatic ecology, and will participate in team-structured, multi-year research projects. A number of graduate students will also be supported by this project, and will lead outreach efforts through partnerships with local watershed associations and university extension programs.