Positive interactions between suspension-feeding bivalves and seagrass-a facultative mutualism

Abstract

In shallow coastal waters, suspension-feeding bivalves often dominate the benthos in numbers as well as biomass. In the Gulf of Mexico and the Caribbean Sea, these filter feeders are usually associated with seagrass. Two simultaneously conducted field experiments (a mussel density manipulation and a mussel predation experiment) were conducted to examine the potential positive interactions between the suspension-feeding mussel Modiolus americanus (Leach) and the seagrass Thalassia testudinum Banks ex Konig. The mussel density 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 sediment nutrient content and that these increased nutrients were biologically available to the plant. T. testudinum responded to the presence of mussels by significantly increasing leaf widths and lengths. In addition, productivity sig- nificantly increased in the mussel-addition treatments. Another response to the presence of mussels included a significantly reduced epiphytic load on the seagrass leaves. The mussel predation exper- iment evaluated the effects of seagrass on the survivorship of the associated mussel, M. americanus. Mean survival was significantly greater in vegetated habitats than in unvegetated sediments. Con- sequently, when mussels are present in seagrass meadows, they elevate seagrass productivity through either increased nutrient resource pools or reduced epiphytic loads on the leaves, while the seagrass increases mussel survivorship. Thus, this study demonstrates the reciprocal positive inter- actions of these organisms when associated and suggests that seagrass meadows may exist as a mosaic of nutrient and productivity 'hot spots' when suspension-feeding organisms are present. These positive interactions may have important consequences in the development, structure and organization of seagrass communities.