Strong consequences of diet choice in a talitrid amphipod consuming seagrass and algal wrack

Deposits of detached macrophytes, known as wrack, are a common feature on shore lines and can represent an important subsidy of organic material from subtidal systems to low productivity intertidal and terrestrial systems. On beaches, these support high densities of consumers that have an important role in wrack decomposition. The feeding behavior of wrack consumers is poorly known relative to the marine herbivores that consume marine macrophytes in situ. To understand how feeding behavior relates to the quality of wrack for an abundant wrack consumer, the talitrid amphipod Notorchestia sp., we test for habitat preferences, differences in feeding rates, and survival among four species of macroalgae and seagrasses in an estuary in New South Wales, Australia. Notorchestia displayed strong preferences for Sargassum sp. and Zostera capricorni as habitat, but consumed only Sargassum in feeding experiments, and only this alga supported high survival in a longer term performance assay. The differences in food quality, as measured by survival over 30 days, did not translate to differences in the abundance of amphipods colonizing each food resource in the field. Our results suggest that feeding by Notorchestia will result in the rapid loss of Sargassum in the wrack, and that other consumers or microbial degradation may be more important in the decomposition of seagrass.