LATITUDINAL VARIATION IN DRILLING PREDATION ON MUSSELS FROM EASTERN SOUTH AMERICA

Temporal patterns of drilling predation can be used to examine the history of predator-prey interactions; however, variation in geographic coverage of fossil deposits can lead to challenges in interpretation of trends through time. Patterns of drilling may be impacted by latitudinal effects that may vary geographically such as temperature, seasonality, and precipitation of calcium carbonate. Time-averaged molluscan assemblages on modern beaches offer an opportunity to study geographic variation in predation with implications for the fossil record. Previous work has focused primarily on molluscan communities from North America and/or Europe; Visaggi & Kelley (2015) reported on patterns of drilling in infaunal Recent bivalves along the coast of Brazil. Their research indicated a decrease in drilling poleward, and similar patterns were revealed in preliminary analysis of infaunal bivalves from Argentina. This work continues to explore latitudinal patterns in drilling predation along eastern South America but focuses instead on epifauna by analyzing mussels.

Modern samples collected from 18 beaches between 36S-52S were analyzed for evidence of drilling predation on mussels. Specimens were separated for study of drilling by analyzing only “whole” (>85% of the shell intact) vs. fragmented shells. Drilling predation recorded on nearly 6000 whole shells belonging to several species of mussels included both complete drillholes (=230) and incomplete drillholes (=13). Faunal provinces represented by these latitudes include the southern end of the Argentinean Province (36S-40S) and the Magellanic Province (42S-52S). Ecoregions are represented by the Uruguay-Buenos Aires Shelf (36S-40S), Northern Patagonian Gulfs (42S-46S), and the Patagonian Shelf (48S-52S). Preliminary results indicate that drilling frequency (DF) on mussels decreases poleward at the scale of provinces (AR = 17%, MA = 8%) and ecoregions (UBA = 17%, NPG = 11%, PS = 7%). Beaches sampled in the Magellanic Province included more rocky shorelines yielding increased proportions of mussels (and muricid predators), yet many more of these specimens were undrilled leading to lower DFs. Future work will include an examination of drilling on mussels farther north in Brazil to see how patterns of predation compare across a broader range of latitudes.