Seaweed traits linked to wave exposure determine predator avoidance

Greater attention to habitat-specific species interactions should improve our ability to predict the context-dependency of species interactions. Predator avoidance depends upon resource traits that can vary predictably between habitats, but few studies have linked predator avoidance to habitat-specific resource traits. For example, predator avoidance in herbivores may depend upon wave exposure, because wave-exposed and wave-protected seaweeds display strong intraspecific differences in morphology and physiology that likely influence their value to herbivores. In 2 sets of laboratory mesocosm experiments we compared predator avoidance in the herbivorous snail Littorina obtusata offered the bladder wrack Fucus vesiculosus from either wave-exposed or protected shores. The effect of predation risk by the green crab Carcinus mae- nas on avoidance by L. obtusata depended on wave exposure, with a stronger effect of predation risk for snails on wave-exposed F. vesiculosus. To explore the traits responsible for this pattern, we conducted multi-choice feeding experiments with F. vesiculosus from 3 wave-exposed and 3 pro- tected sites. F. vesiculosus from protected sites (where avoidance behaviors were weak) was more palatable than F. vesiculosus from wave-exposed sites. Although several morphological (e.g. blade width and vesicle number) and physiological traits (e.g. C:N ratio) of F. vesiculosus depended upon wave exposure, these were unrelated to snail feeding and predator avoidance. Therefore, L. obtusata displayed stronger predator avoidance on less palatable, wave-exposed seaweed. We suggest that the different effects of predators on rocky shores may partly reflect the influence of exposure-specific seaweed traits that determine predator avoidance.