Positive interactions between suspension-feeding bivalves and seagrass - afacultative mutualism

In shallow coastal waters, suspension-feeding bivalves often dominate the b enthos in numbers as well as biomass. In the Gulf of Mexico and the Caribbe an Sea, these filter feeders are usually associated with seagrass. Two simu ltaneously conducted field experiments (a mussel density manipulation and a mussel predation experiment) were conducted to examine the potential posit ive interactions between the suspension-feeding mussel Modiolus americanus (Leach) and the seagrass Thalassia testudinum Banks ex Konig. The mussel de nsity manipulations resulted in a doubling of the total nitrogen and total phosphorus levels of sediments, and a significant reduction in leaf tissue C:N, N:P and C:P ratios, demonstrating that the mussels increased the sedim ent nutrient content and that these increased nutrients were biologically a vailable to the plant. T. testudinnm responded to the presence of mussels b y significantly increasing leaf widths and lengths. In addition, productivi ty significantly increased in the mussel-addition treatments. Another respo nse to the presence of mussels included a significantly reduced epiphytic l oad on the seagrass leaves. The mussel predation experiment evaluated the e ffects of seagrass on the survivorship of the associated mussel, M. america nus. Mean survival was significantly greater in vegetated habitats than in unvegetated sediments. Consequently, when mussels are present in seagrass m eadows, they elevate seagrass productivity through either increased nutrien t resource pools or reduced epiphytic loads on the leaves, while the seagra ss increases mussel survivorship. Thus, this study demonstrates the recipro cal positive interactions of these organisms when associated and suggests t hat seagrass meadows may exist as a mosaic of nutrient and productivity 'ho t spots' when suspension-feeding organisms are present. These positive inte ractions may have important consequences in the development, structure and organization of seagrass communities.