LATITUDINAL PATTERNS IN DRILLING PREDATION AMONG MODERN CREPIDULA SPECIES ALONG THE EAST COAST OF THE UNITED STATES: EFFECTS OF ENVIRONMENTAL CONDITIONS AND TAPHONOMIC PROCESSES
We sampled 32 beaches from different latitudes along the East Coast of the U.S. At each locality we collected Crepidula species (~50-100 specimens/site), bulk samples of mollusk shells when possible, and data on surface water temperature, salinity and pH. In addition to the field data, we obtained average yearly water temperature data from 24 NOAA buoys in proximity to the beaches sampled.
Every Crepidula specimen collected was identified to species and assigned a taphonomic grade. Length, width, and thickness were measured and evidence of drilling predation was recorded. We calculated drilling frequency (DF) and prey effectiveness (PE) for each beach and province. We used the Chi-squared test, Fisher exact test and Spearman’s Rank Correlation Coefficient to determine if significant differences in drilling occurred with latitude and what variables correlated with one another.
With the exception of Peconic Bay, NY, no significant differences in drilling occurred between targeted sampling and bulk sampling at beaches where both sampling methods were used. Drilling frequency was greatest in the Carolinian province (0.108), but not significantly more than in the Gulf province, and DF decreased northward to 0 in the Nova Scotian province. PE was greatest in the Virginian province (0.341), decreasing to the south. All three environmental parameters tested were significantly correlated with latitude, but taphonomic grade was not. DF and latitude had a very high statistically significant inverse correlation (Rs = -0.523; p = 0.004), which is consistent with a significant positive correlation between DF and temperature (both for field and NOAA data). Increased drilling in warmer climates may in part be responsible for temporal patterns in drilling in the fossil record. Crepidula shells were thinner to the north and smaller in the south. Multiple species found at a single beach did not differ in drilling frequency despite differences in morphology.