Effects of flow speed, turbulence, and orientation on growth of juvenile bay scallops Argopecten irradiansconcentricus (Say)

Abstract

Experiments were conducted using controlled pipe flows to test the extent to which growth of juvenile bay scallops Argopecten irradians concentricus (Say) is affected by flow speed, turbulence, and orientation of individuals relative to flow direction. When flow speed was held within a narrow range (1.7–3.9 cm · s −1), scallop growth was unaffected by the presence and intensity of turbulence in flows. Scallops with exhalent pseudosiphonal openings (posteriors) oriented upstream grew significantly less than those with anteriors oriented upstream at identical flow speeds. Shell growth declined gradually with increasing velocity from 1.4–17.2 cm · s −1, but the extent of flow effects on growth varied significantly with orientation to the flow. Negative effects of higher flows on growth were apparent for scallops with posteriors oriented upstream, whereas in the reverse position effects of flow speed on growth were less obvious. Flume tests suggested that the unfavorable position, with posterior pointed upstream, was not often maintained at flow speeds > 9 cm · s −1, whereas the more favorable reverse orientation was maintained at all flow speeds examined. Velocity profiles measured in natural scallop beds, and in situ observations of the elevation of juvenile bay scallops above the bottom, revealed that juveniles routinely experienced current speeds ⩾ 17 cm · s −1. Thus, changes in orientation may provide a significant means of maintaining high growth rates under field conditions. The effect of orientation suggests that feeding by juvenile Argopecten is inhibited by back pressure established at the exhalent opening by external flow. Hydrodynamic scaling arguments indicate that adult scallops should be less sensitive than juveniles to back pressure, yet, paradoxically, previous evidence, coupled with our results, indicate that growth of adult Argopecten is more sensitive to current speed than is the growth of juveniles.