Changes in amphipod densities among macroalgal habitats in day versus night collections along the Western Antarctic Peninsula

Amphipods along the western Antarctic Peninsula appear to gain refuge from predators by associating with chemically defended macroalgae rather than palatable macroalgae. However, nothing is known about amphipod activity at night. If foraging on non-chemically defended macroalgae regularly occurs, then nocturnal foraging seems beneficial since visual predators are disadvantaged. To test this hypothesis, we collected three common macroalgal species and affiliated mesograzers, approximately 3 h before and after sunset. All associated mesofauna were counted and densities recorded. Amphipod densities were significantly decreased during the night on the chemically defended Desmarestia menziesii, while significantly increased on the palatable Iridaea cordata. Additionally, the amphipod Gondogeneia antarctica was found in significantly higher densities at night on Palmaria decipiens, a species shown to be readily eaten by G. antarctica and omnivorous fish. We believe that chemically defended macroalgae act as a refuge for mesograzers during the day, while more widespread foraging occurs at night.