Habitat choice and predator avoidance by Antarctic amphipods: the roles of algal chemistry and morphology

Habitat choice is an important aspect of community structure. Perhaps the most important factor influencing choice by prey species is predation risk. Predators may directly influence prey habitat choice via consumption or, secondarily, through 'intimidation'. We investigated these forces in the near-shore ecosystem of the western Antarctic Peninsula. This system is dominated by macroalgae and exceedingly abundant amphipods, with the omnivorous fish Notothenia coriiceps feeding on both. Amphipod densities are very high on the dominant brown alga Desmarestia menziesii, which is both structurally complex and chemically defended from herbivory. Amphipod densities are low on the simply structured alga Palmaria decipiens, which is a preferred food of fish and some amphipods. We used these 2 species of algae and plastic and gel algal analogues to experimentally separate the roles of algal structure and chemistry in influencing amphipod habitat preference. In the absence of predators, the amphipod Gondogeneia antarctica preferred Palmaria decipiens and had no preference between structural analogues. In the presence of predator cues, its preferences changed to D. menziesii and the structurally complex analogue. In the absence of predator cues, the amphipod Prostebbingia gracilis preferred the chemically defended, structurally complex D. menziesii to all other choices, and preferred the structurally complex analogue to the simple one. Both D. menziesii and the structurally complex analogue decreased predation risk for Prostebbingia gracilis. Based on these results, natural abundances of amphipods are likely driven both by actual predation and the threat of predation (i.e. non-consumptive effects).