LATITUDINAL TRENDS IN DRILLING PREDATION ON RECENT INFAUNAL BIVALVES IN ARGENTINA: PRELIMINARY RESULTS FROM THE URUGUAY-BUENOS AIRES SHELF

Trends in predator-prey interactions across latitude can be studied by examining infaunal bivalves for evidence of drilling predation by naticid gastropods. Modern spatial patterns have implications for the fossil record, but most available data are from North America and Europe. Previous work by Visaggi & Kelley (2015) in South America (6°S-34°S) revealed that drilling frequency increased equatorward across four ecoregions in Brazil. This project represents an extension of that research into Argentina with preliminary results reported here for the southernmost ecoregion of the Argentinean Province (Uruguay-Buenos Aires Shelf).

Beach-collected shells were obtained from localities along 36°S-52°S in Argentina in 2010. Work in progress has incorporated ~4100 infaunal bivalves from 36°S, 38°S, and 40°S, of which nearly 70% of shells were suitable for study of drilling (>85% of valve intact and umbo preserved). Assemblages were identified to genus level and were often characterized by several key recurring genera. Evidence of drilling through complete and incomplete drillholes was noted for Mactra, Glycymeris, and Amiantis. Drilling frequency (=DF) was calculated as the number of drilled valves divided by half the number of “whole” valves.

DFs fluctuated at the scale of localities and latitudes with an overall ecoregion DF of ~12%. For the Argentinean Province in Brazil, ecoregion DFs of ~10% (Southeastern Brazil, 23°S-28°S) and <1% (Rio Grande, 30°S-34°S) were previously reported with a province DF of <5%. Although drilling is higher in the Uruguay-Buenos Aires Shelf contrary to the equatorward increase observed across ecoregions in Brazil (as intensity is more similar to the DF for Southeastern Brazil relative to the Rio Grande), adding data from this study yields a revised overall DF for the Argentinean Province at ~6.5%. This DF still reflects greater drilling at lower latitudes for the Western Atlantic in South America given that the DF for the Brazilian Province is 12.5% as earlier documented. Interestingly, more incomplete drillholes were noted here and from fewer shells compared to each of the ecoregions studied in Brazil. Ongoing research will examine potential factors that might influence these patterns and eventually integrate data from the polar-influenced Magellanic Province as well.