DECREASED NATICID GASTROPOD PREDATION FOLLOWING THE TRANS-ARCTIC INVASION IN ICELAND

Though biotic invasions, both anthropogenic and natural, are of increasing ecological concern, the long-term implications are unknowable without investigating the fossil record. We looked at changes in naticid gastropod predation on bivalves in Iceland since the trans-Arctic Invasion (TAI). The TAI began after the Bering Strait opened at 5.4-5.5 Ma, reconnecting the Pacific and Arctic Ocean basins for the first time in100 m.y. The arrival of the first Pacific molluscs, including new species of naticid gastropods, in the Atlantic Ocean occurred at ~ 3.6 Ma and is recorded in the Tjörnes beds of NE Iceland. Six samples were collected from the Serripes zone (B14, U14, 15, B17, U17, B18; B=base, U=upper; bed number uses Bardarson's classification). These samples postdate the invasion but record the evolutionary integration of the new species into the community. For each sample, bivalve specimens were identified to the genus level, and drilling frequency (DF) and prey effectiveness (PE = incidence of failed drilling) were calculated for the assemblage and for selected taxa. In all, 1132 bivalve shells were identified.

Some specimens were preserved as molds but nevertheless record drill holes. DF for moldic and shelled specimens were not significantly different, indicating lack of bias due to mode of preservation. Overall, DF decreased upsection: B14 (DF = 0.38) was significantly different from bed 15 (0.21), B17 (0.18), U17 (0.16), and B18 (0.16). In addition, the combined DF of the lower two levels (0.37) is significantly different from that for the upper 3 levels combined (0.17), suggesting that the major changes in DF occurred between beds 14 and 17. Within Macoma, DF also decreased upsection, with significant differences between B14 and B18 and between bed 14 and B18. Only B14 and B17 contained Thracia specimens, and DF is significantly greater for B14 than B17. No significant difference is seen in DF of Lentidium specimens (B17 vs B18). Overall, PE is very low; we found 9 incomplete drillholes in the common thick-shelled Arctica of B14 and one in the thin-shelled Macoma of U14.

Changes in DF suggest that prey defenses increased following the invasion of naticid predators. Future work will examine changes in predator behavior based on drillhole size and location and selectivity of valve, prey size and species.