Impacts of Zebra Mussels on Texas Unionid Mussels
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Zebra mussels (Dreissena polymorpha) are an invasive species known to detrimentally affect native unionid mussels, a highly imperiled group of organisms. Zebra mussels infest (directly attach to) the shells of unionid mussels and compete with them for food as both unionid and zebra mussels are filter feeders. Previous studies, mostly from the northeastern US, have shown that both competition for food (indirect interaction) and zebra mussel infestation (direct interaction) can affect the body condition of unionid mussels. However, no study has examined this in Central Texas, the southern edge of zebra mussel distribution nor compared the impact of zebra mussel presence (in the same system as unionid mussels) and infestation (direct attachment to unionid mussel shells) on glycogen storage under controlled conditions. Hence, the objectives of this study were to 1) examine the impact of infestation versus presence of zebra mussels with experiments in the laboratory and 2) collect data on glycogen concentrations of unionid mussels at different field sites with and without zebra mussels. In the experiment, tissue samples were collected after 30 days from treatment tanks where 1) Threeridge (Amblema plicata) were artificially infested with zebra mussels, 2) zebra mussels were present with their biomass being comparable with the first treatment (44 ± 14 g wet biomass), but shells of A. plicata were not infested, and 3) control tanks where zebra mussels were absent. Tissue samples from A. plicata were collected from 20 individuals at each of the 10 field sites. Results from the experiments showed zebra mussel presence and infestation reduced glycogen 38% and 66% respectively. Results from the field supported these findings. Variation in glycogen concentrations of mussels collected in the field was best explained by chlorophyll-a concentrations (coarse measure of food resource) and total number zebra mussels. A larger percentage of infested mussels were burrowed in the lab experiment compared to control mussels, potentially to avoid unfavorable conditions created by both the presence and infestation of zebra mussels. Reduction of glycogen stores by zebra mussels can reduce short- and long-term fitness of unionid mussels which needs to be considered when creating and implementing management practices.