Olive-headed sea snakes Disteria major shift seagrass microhabitats to avoid shark predation

Predators elicit prey spatial shifts that may influence prey resources. The nature of these indirect effects is difficult to predict, however, in part because the means by which prey differentiate safe from dangerous space are poorly understood. Prey often avoid their predators, but they may instead favor predator-rich areas that facilitate escape, or discourage attack, when a predator is encountered. We investigated how olive-headed sea snakes Disteria major index their risk of tiger shark Galeocerdo cuvier predation over seagrass bank microhabitats (edges, interiors) in Shark Bay, Australia. D. major is equally likely to escape sharks in both microhabitats, so we expected to observe avoidance of predator-rich space, Supporting our prediction, snakes used microhabitats roughly in proportion to food supply when sharks were scarce and avoided edges, which are preferred by sharks, when sharks were abundant. Thus, D. major appears to measure danger across seagrass banks using variability in predator density and to seek low-encounter microhabitats when antipredator investment is needed. Our results suggest that the influence of predators on sea snakes is underappreciated and, in the context of previous work, that sympatric prey species sharing predators may show opposite spatial shifts when threatened, potentially leading to different predator indirect effects.