HYDRODYNAMIC EFFECTS OF DRILLHOLES ON POST-MORTEM TRANSPORT OF BIVALVE SHELLS

Predatory drill holes in marine invertebrates represent an important source of information on the nature of biotic interactions and have often been used to explore the ecological and evolutionary roles of such interactions. Measures of drilling frequencies, and of valve and site stereotypy represent the raw data for inferring the intensity and selectivity of drilling predation. Any biases that may affect these measures are therefore of special interest as they may impact the ecological and evolutionary interpretations of the data. One potential source of bias explored in this study relates to the hydrodynamic properties of shells: presence of drill holes or drill hole position may influence how shells behave when subjected to moving fluids.

In a preliminary flow tank study with bivalves, we found that the threshold current velocity for the entrainment of undrilled convex-up shells is significantly lower than for centrally drilled shells. One possible explanation for the observed differences is the effect of the drill hole on lift. According to Bernoulli's principle, a pressure gradient develops as current moves over the convex-up shell. In the presence of a drill hole, the pressure gradient may drive water through the hole from the inside to the outside lessening the pressure on the shell: the use of dye injected into the convex-up shell confirmed the existence of such flow. Direct measurements of lift on shells subjected to a range of current velocities corroborate this hypothesis: centrally drilled shells experience lower lift force than undrilled shells. This difference in lift may be the cause for the lower entrainment velocity of undrilled shells.

The position of the drill hole on a shell also affects its hydrodynamic properties. Previously, Lever et al. (1961) reported lower entrainment velocity for upstream facing umbonally drilled shells compared to undrilled shells. They suggested that this may be due to fluidization of sediments beneath the shell leading to the shell's destabilization. We found a similar pattern: a lower entrainment velocity of upstream facing umbonally drilled shells. However, we did not use sediments in our experiments and thus “fluidization” alone cannot be the cause; instead, fluid flowing into the shell through the drill hole creating turbulence within may reduce entrainment velocity.