BED-BY-BED ANALYSIS OF PLIOCENE PREDATOR-PREY INTERACTIONS FROM THE TJÖRNES PENINSULA, ICELAND, ACROSS THE TRANS-ARCTIC INVASION

The trans-Arctic invasion (TAI) is characterized by asymmetrical faunal interchange between the Pacific and Atlantic Ocean (~3.5Ma), resulting from the closure of the Central American Seaway coupled with the opening of the Bering Strait, which may have reorganized North Atlantic oceanic currents. We investigated whether naticid gastropod drilling predation on bivalves in the Tjörnes beds of northeast Iceland was altered due to migration of fauna from the Pacific into the Atlantic Ocean.

The Tjörnes strata represent three zones subdivided into 25 horizons: the Tapes (horizons1-5) and Mactra (6-12) zones are pre-invasion, whereas the Serripes zone (13-25) is post-invasion. The 25 horizons allow for bed-by-bed analyses with temporal resolution sufficient to track transitions in drilling predation across the invasion. Specimens from the Tjörnes peninsula, Iceland, were analyzed in collections at the Icelandic Institute of Natural History. Height and length of bivalve specimens were measured. The occurrence of complete and incomplete (unsuccessful) drillholes and drillhole diameter were recorded for all ~whole bivalves. Drilling frequency (DF) and prey effectiveness (PE = proportion of attempted drillholes that were incomplete) were calculated. These data were combined with the Serripes data of McCoy (2007).

Pre-invasion samples included 28 bivalve species (857 specimens), whereas 37 bivalve species (2209 specimens) were recorded in post-invasion samples. DF significantly increased between the pre-invasion zones and the Serripes zone (0.03 vs 0.18). Drilling metrics changed as invasive species became more integrated. Bed-by-bed analyses show a statistically significant peak in DF (0.27) in horizon 14; Spearman’s rank correlation indicated a significant increase in DF from beds 1 to 14 and a significant decrease from beds 14 to 25. Incomplete drillholes were too rare to yield meaningful patterns. Lentidium (the only bivalve consistently present throughout the section) showed significant differences in valve length and height across the TAI based on two-tailed t-tests; valve size decreased at the start of the invasion but increased in the upper Serripes zone. Naticids chose larger Lentidium prey earlier in the invasion than later, but differences were not statistically significant.