Unsteadiness of bivalve clam jet flow according to environmental conditions

Abstract

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 ve- locity was measured via the non-intrusive particle image velocimetry technique. The flow and un- steady patterns in the time records of vertical velocity near the tip of the excurrent siphon were an- alyzed 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 charac- teristics 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 re- sponse of clams to the density of the clam patch was dependent on the size of the animal. These ob- served 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 be - havior may provide hydrodynamic signaling of the clam patch recruitment status to settling larvae.