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To determine whether hard clams Mercenaria mercenaria alter excurrent jet velocity as a function of environmental factors, this laboratory study quantified the excurrent jet velocity as a function of bulk mean crossflow velocity, density of clam patch, and clam size. The excurrent velocity was measured via the non-intrusive particle image velocimetry technique. The flow and unsteady patterns in the time records of vertical velocity near the tip of the excurrent siphon were analyzed by calculating the mean, variance, power spectral density, fractal dimension, lacunarity, and mean jet-to-crossflow ratio. The results revealed that clams vary their excurrent flow characteristics according to bulk mean crossflow velocity; in particular, the texture of the time record of velocity varied. Further, the jet-to-crossflow velocity ratio was larger for smaller clams, and the response of clams to the density of the clam patch was dependent on the size of the animal. These observed behaviors may impact the predation success of blue crabs Callinectes sapidus and knobbed whelks Busycon carica under various environmental conditions. In this context, the results indicated that blue crabs dominate the predator-prey system because of the dependence on clam size and bulk mean crossflow velocity. Alternatively, the varying clam excurrent siphon velocity behavior may provide hydrodynamic signaling of the clam patch recruitment status to settling larvae.KEY WORDS: Predator-prey · Olfactory predation · Jets-in-crossflow · Bivalve · Non-consumptive predator effects · Lacunarity Resale or republication not permitted without written consent of the publisherAquat Biol 13: [175][176][177][178][179][180][181][182][183][184][185][186][187][188][189][190][191] 2011 acteristics of the excurrent flow as a function of bulk mean crossflow velocity and clam size are of interest because the predators of this system, blue crabs and knobbed whelks, both consume clams and use olfactory navigation as a means of locating prey, but the turbulence regimes in which they are successful are highly disparate. Blue crabs are less successful predators during highly turbulent hydrodynamic conditions (Jackson et al. 2007). In contrast, knobbed whelks are successful in highly turbulent regimes (Ferner & Weissburg 2005). Also, blue crabs prefer clams of a small size range despite the fact that they can consume prey over a wide size range (Micheli 1995). Therefore, as clams increase in size, they become less susceptible to predation by blue crabs while maintaining their susceptibility to whelk predation. Thus, there may be alternate strategies for predator avoidance depending on the size of the clams and the turbulence regime of the environment in which they live.The influence of the density of the clam patch was also of interest because there may be predator avoidance strategies depending on the proximity of conspecifics. High-density patches may appear as one large, hard to eat prey to blue crabs, and small clams may have higher survival rates in these ...

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