Pereopod specialization in pelagic amphipods

Abstract In the midwater, earth’s largest habitat by volume, the evolution of animal bodies is shaped by unique selective pressures associated with navigating through and feeding in a dimly-lit habitat with few places to hide. Holopelagic hyperiid amphipods (suborder Hyperiidea H. Milne Edwards, 1830), which are often found associated with gelatinous zooplankton, possess a strikingly varied array of appendage morphologies, much greater than do their benthic relatives, yet the functions of most of these structures remain unknown. We attempt to systematically categorize and describe variation in appendage morphology across hyperiids and other midwater amphipod groups, including Lysianassoidea Dana, 1849; Aristioidea Lowry & Stoddart, 1997; Stegocephalidae Dana, 1852; Pardaliscidae Boeck, 1871, and others. We hypothesize functions for specific appendage types and features, with the paired goals of 1) connecting in situ and laboratory observations of swimming, predation, and host association behavior to morphology, and 2) inferring the ecology and behavior of new and poorly known species based on pereopod morphology. Increasing our understanding of the function of these structures will help us to address broader questions about the evolution and diversification of amphipod morphology as it relates to locomotion and trophic interactions, and how animals more broadly have adapted to moving and feeding in midwater habitats.