The reciprocal relationship between hydrodynamics and bivalves
Delavan, Sarah Kelly
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The focus of this study was to determine the effect of clam presence and behavior on the crossflow of the ambient horizontal flow and the effect of ambient horizontal flow characteristics influence the clam feeding behavior. Hence, there is a reciprocal relationship between organisms and the physical environment, and this study ultimately addressed the role of hydrodynamics in the predator-prey relationship between bivalve clams, Mercenaria mercenaria, and their predators, blue crabs and whelks. The study concludes that clams alter the chemical odorant source characteristics and control the transmission of the chemical signal through altering the crossflow. The first part of the study is a field experiment designed to quantify the effect of the presence and behavior of clams on the crossflow of the horizontal crossflow. The second part of this study is a two-part laboratory experiment designed to isolate the influence of environmental factors on clam behavior. One experiment quantifies the unsteadiness of the clam excurrent jet velocity time record according to environmental cues such as the horizontal crossflow velocity, the density of the clam patch, and the size of the clam. The second laboratory experiment quantifies the unsteadiness of the jet velocity values according to the presence of predator cues in the upstream flow. Clams are found, using an ADV system in the field, to alter the vertical distribution of velocity according to the sediment in which they are buried. Also, turbulence characteristics, such as Turbulent Kinetic Energy and Reynolds shear stress, are altered in the presence of clams according to the ambient horizontal crossflow velocity and treatment site. The laboratory flume PIV system captured vector plots for two-dimensional planes that bisect the clam excurrent siphons and clam jet velocity time records were extracted. A fractal analysis and a lacunarity analysis of the jet velocity time records found that clams alter their jet excurrent velocity unsteadiness according to the horizontal crossflow velocity. This behavioral change may contribute to the differences in the turbulence characteristics in the field experiment. Another result from the laboratory experiments is that the effect of clam patch density on the feeding activity was dependent on the size of the organism. This size/density dependent relationship suggests that predation by blue crabs dominates the system since larger clams are no longer susceptible to blue crab predation, whereas clams of all sizes are susceptible to whelk predation. Finally, clams increase the randomness of their excurrent jet velocity values when predator cues are located in the upstream flume flow. This suggests that the presence of predators elicits clam behavior that promotes the mixing and dilution of their chemical metabolites.