EFFECTS OF ENVIRONMENTAL VARIATION AND SAMPLING METHODS ON THE RECORD OF DRILLING PREDATION ON THE GASTROPOD CREPIDULA FORNICATA FROM LONG ISLAND, NEW YORK

Paleontologists have used death assemblages on beaches to understand the paleoecology of fossil assemblages better, and especially the record of shell drilling by predatory gastropods. We examined the gastropod Crepidula fornicata (primarily preyed upon by muricid gastropods) from modern death assemblages on the North Fork of Long Island, NY. To study the effect of environmental variation on the record of predation, shells were obtained from three beaches on Peconic Bay (a sheltered environment) and three beaches on Long Island Sound (a harsher environment). We investigated bias related to sampling method and collector experience. Three collectors (one trained, two inexperienced) used targeted and bulk sampling methods to collect Crepidula. The trained collector had taken a paleontology class, whereas the two inexperienced collectors were only taught to recognize Crepidula fornicata. This approach allowed us to test the effect of sampling method and collector experience on the drilling predation record.

Between early June and early July, 2018, 4582 Crepidula fornicata were collected, with 2201 from the Peconic Bay side and 2381 from the Sound side. For each Crepidula specimen, we measured shell length, recorded the quality of preservation, and tallied all complete muricid drill holes. Of the shells collected, 113 had complete drill holes, with 61 from the Bay and 52 on the Sound, yielding an overall drilling frequency (DF) of 0.025. We hypothesized that samples from the harsher Sound environment would have poorer shell preservation and a lower DF. Shell quality differed as expected [Mann-Whitney U (MWU) test, p <<0.001], but DF did not differ between Bay and Sound localities (p = 0.20). Across all samples, shell size and quality were correlated (Spearman’s rho = 0.524, p = 0.02). We tested whether collector experience affected size or quality of shells collected (t- and MWU tests). At all Bay locations, no significant differences occurred, but at all beaches on the Sound, the inexperienced collectors targeted larger, better preserved shells. The only significant difference we found in comparing collecting methods is that the bulk-sampled shells were, on average, smaller than the target-sampled shells. Drilling frequencies and specimen quality did not differ significantly (MWU test) for target and bulk sampling.