Food Source Or Refuge: What Is Behind Amphipod Choices For Seaweeds?

To assess whether amphipods' habitat choice and feeding preference for a specific seaweed species are based on nutritional value or structural complexity, and to evaluate the role of chemical cues in the amphipod choice, we carried out laboratory experiments using amphipods (Ampithoe marcuzzii) and seaweeds from different divisions (Chlorophyta, Ochrophyta, and Rhodophyta). To evaluate feeding preference, we carried out a multiple-choice experiment offering six species of seaweeds to amphipods. To assess whether habitat choice by amphipods is guided exclusively by seaweed complexity, we conducted an assay offering seaweeds and complexity-like mimics (artificial plants) to amphipods, quantifying their choices. To evaluate whether seaweeds' secondary metabolites acted as chemical cues affecting seaweed choice by the amphipods, we used the same plastic mimics, one of which was brushed with crude extracts from the most-consumed seaweeds and a control. The consumption of Gracilaria cervicornis by amphipods was around three times higher than that of the other offered species, likely due to its paucity of chemical and physical defenses. The highest contents of nitrogen and crude proteins in this species may have contributed to its high consumption. Amphipods also used G. cervicornis as a shelter more than its mimic, but exhibited no preference between other seaweeds and their mimics, indicating that a more palatable refuge is preferred. This idea was corroborated when amphipods were able to detect chemical cues of G. cervicornis painted over a plastic mimic of similar structure and preferred it over the control mimic, which had the same complexity but no chemicals. Our results suggest that amphipods use chemical cues to identify high-quality habitat regarding refuge (complexity) and food source (nutritional value).