DO PREDATORY DRILL HOLES INFLUENCE THE TRANSPORT AND DEPOSITION OF GASTROPOD SHELLS?

Drill holes made by predatory gastropods are an appealing source of information about predation rates through time because they are frequently well preserved, and common in Cenozoic molluscan communities. A problem with using drill holes to interpret predation intensity is the assumption that drilled and undrilled mollusc shells are hydrodynamically similar. If this is not the case, shell accumulations are more likely to be biased by preferential transport and deposition of those shells that are more easily entrained in moving water. Although attempts have been made to address this problem in the case of bivalves, no workers have compared the hydrodynamic properties of drilled and undrilled gastropod shells.

To test the hypothesis that drilled and undrilled gastropod shells may entrain at different flow velocities, we ran multiple entrainment trials using similarly sized specimens of two morphologically distinct taxa, Olivella biplicata and Euspira lewisii. Experiments were conducted using a recirculating flume tank with a smooth plexiglass bottom, which eliminated variation in entrainment velocity due to changes in the character of the substrate. Individual specimens of each taxon (n = 92 Olivella, 42 undrilled, 50 drilled; n = 46 Euspira, 25 undrilled, 21 drilled) were observed five times from rest (in stable orientation) until entrainment. During each trial, flow velocity was slowly increased so as to avoid pulses in acceleration. Drilled and undrilled samples of both taxa show moderate correlations between shell height and average entrainment velocity (Pearson product-moment correlation; Olivella: undrilled r = 0.34, p = 0.03, drilled r = 0.38, p = 0.006; Euspira: undrilled r = 0.49, p = 0.014, drilled r = 0.71, p = 0.00028).

Comparing specimens from a similar size range, drilled and undrilled shells of both Olivella and Euspira show non-significant differences in entrainment velocity (Fisher's transformation; Olivella: z = - 0.21, p = 0.4168; Euspira: z = -1.1, p = 0.1357). These results suggest that, in general, predatory drill holes do not affect the hydrodynamic behavior of gastropod shells. As a result, gastropod shell accumulations are unlikely to be biased towards an overabundance of drilled or undrilled shells as a result of depositional processes alone.