| Hard clams are slow even by clam standards (Doering 1982), and when found by predators, they are often injured and/or consumed (Irlandi and Peterson, Nakaoka 2000). Thus, their best survival strategy is to avoid predators since they lack the ability to escape or defend themselves against consumers. Results from these experiments indicate that clams detect blue crabs using chemical signals and react by reducing their feeding time. Clams were unresponsive to crabs that were starved, but displayed similar reactions to crabs whether they had eaten fish vs. clams. Since blue crabs are generalist predators, their diet is unrelated to the risk level they pose to clams, and it is perhaps unsurprising that clams responded similarly to crabs regardless of their diet. Starved predators may actually pose a greater threat to clams than those recently fed, as hungry predators tend to search longer and more frequently for food and decrease the threshold for detecting potential food before initiating a search (Zimmer-Faust and Case 1982). Thus, an inability to detect starved predators may actually increase clam vulnerability in hungry consumers. Clams did react to injured conspecifics, which may be a mechanism that allows them to respond to risk when predators are undetectable.; In addition to blue crabs and injured conspecifics, clams also reacted to knobbed whelks by reducing their feeding behavior. When clams were placed in the field with knobbed whelks or blue crabs caged nearby, clam survival was much higher than in controls with empty cages. This suggests that predator-induced feeding reductions are beneficial to clams, and by minimizing the amount of attractive chemicals they release into the environment, clams may avoid detection by predators.; Using a laboratory flume, we examined clam reactions to blue crabs and knobbed whelks in slow and fast flows to determine the impact of hydrodynamics on clam ability to respond to predators. Clam reactions to knobbed whelks were consistent regardless of flow speed, but clam responses to blue crab predators decreased in the high velocity flow. Interestingly, whelks are more likely to locate clams in fast and/or turbulent flows than are crabs (Ferner and Weissburg 2005), and clams were more responsive to whelks in these conditions.; Since hydrodynamics reduced clam reactions to blue crabs in laboratory assays, we conducted a follow-up field study to verify that turbulent flows indeed diminished clam ability to detect blue crabs. We established clam plots in the field, and compared clam survival when caged blue crabs were placed either 0.5 m or 2.0 m away in low and high levels of turbulence to survival in plots near empty cage controls. As in the earlier field study, clam survival was much greater than controls when blue crabs were caged near clam beds, even when crabs were caged 2.0 m away. However, when blue crabs were 2.0 m away in turbulent flows, clam survival was almost identical to that measured in controls, indicating that the reactive distance of clams to blue crabs had been diminished by turbulence, and validating the earlier behavioral results obtained in the flume.; Since turbulence diminished clam responses to crabs, and had previously been shown to reduce blue crab ability to find clams (e.g., Weissburg and Zimmer-Faust 1993), the role of turbulence in modulating the outcomes of clam-crab predatory interactions was unclear. We predicted that the animal (clam or crab) whose perceptive abilities were least affected by increasing turbulence levels would have a sensory advantage and prevail in turbulent flows. We established pairs of clam plots in four field sites that differed in their mean flow velocities and turbulence levels. One member of each pair was surrounded with a ring of sun-bleached oyster shells to increase turbulence, and we compared crab predation levels both within and between sites. By increasing turbulence levels within sites, we were able to expose clams to similar c... |
