PRELIMINARY RESULTS: LATITUDINAL VARIATION IN DRILLING PREDATION ON BIVALVES ACROSS FOUR ECOREGIONS IN BRAZIL

The fossil record of drilling predation provides much of the evidence for the controversial hypothesis of escalation. However, understanding the impact of spatial variation in drilling on apparent patterns of predator-prey evolution has been difficult to assess due to limited fossil deposits. Modern molluscan assemblages offer an opportunity to explore geographic variation in drilling frequency (=DF) as contemporary spatial patterns have implications for paleontological interpretations. Previous investigations of latitudinal variation in DF using modern mollusks have supported either a peak in drilling at mid-latitudes or an increase or decrease in drilling with latitude. Available data are contradictory and only incorporate assemblages derived from North America. To our knowledge, this study is the first to assess patterns in drilling predation across a broad latitudinal gradient in the Southern Hemisphere.

Bulk samples of shells were collected from ~30 beaches along the coast of Brazil every few degrees latitude from 6°S - 34°S in April 2009. Fieldwork covered four biogeographic ecoregions (Northeastern Brazil, Eastern Brazil, Southeastern Brazil, Rio Grande) representing both tropical and temperate marine environments. Preliminary results are presented here for four localities (one per ecoregion), each separated by 8° of latitude (10°S, 18°S, 26°S, 34°S).

All four ecoregions are represented by a single bulk sample in this preliminary analysis. A total of >4,000 bivalve shells were identified (a minimum of >250 shells represents each ecoregion). DF was highest at 10°S (20.8%) and 26°S (22.4%). The difference in DF between these ecoregions (Northeastern Brazil vs. Southeastern Brazil) was not statistically significant (p=0.243). Drilling was low at 18°S (0.9%) and absent at 34°S. No significant difference existed between these ecoregions (Eastern Brazil vs. Rio Grande; p=0.320). Significant differences in DF were detected for all other comparisons (p=0.000). This unexpected pattern in drilling with latitude needs further clarification; ongoing work shall incorporate additional specimens collected in Brazil. Future sampling into Argentina including the polar-influenced Magellanic Province will allow for extended latitudinal comparisons of DF along the Western Atlantic of South America.