Predation by shell-breaking crabs on a marine gastropod along a latitudinal gradient in the southwestern Atlantic: influence of extrinsic and intrinsic factors.

Abstract

Biotic interactions-and predation in particular-are thought to follow a latitudinal gradient, increasing towards the tropics; yet empirical evidence remains contradictory and largely based on studies from the Northern Hemisphere. Moreover, the role of environmental variables shaping latitudinal gradients of predation intensity has seldom been tested. Here, we quantify predation by shell-breaking crabs on modern shells of the marine gastropod Trophon geversianus along a latitudinal gradient (40°-54° S) on the southwestern Atlantic coast. We further evaluate how intrinsic factors (four shell morphometric traits) and extrinsic factors (seven environmental variables and the biogeographic region) jointly influence predation patterns. Fragmentation from crushing predation affected 37% of the shells (544 out of 1480), with the most frequent damage types being major body whorl damage (28%), deep aperture chips (11%) and extensive aperture chips (6%). When analysed by biogeographic province, fragmentation increased significantly towards the south in the Magellan province. Notably, random forest modelling revealed that intrinsic factors-particularly shell size and thickness-were stronger predictors than extrinsic factors in driving latitudinal variability of shell-breaking crab predation. By highlighting the dominant influence of intrinsic factors over extrinsic ones, this study emphasizes the crucial role of species-specific traits in shaping predator-prey interactions across biogeographic regions.