Deep-sea amphipod community structure across abyssal to hadal depths in the Peru-Chile and Kermadec trenches

Deep-sea necrophagous amphipods were sampled from 5 stations across the abyssal and hadal zones (4602 to 8074 m depth) of the Peru-Chile Trench (SE Pacific Ocean) and combined with comparative data taken from 7 stations at corresponding depths (4329 to 7966 m) in the Kermadec Trench (SW Pacific Ocean) to investigate the diversity and structure of the amphipod communities in the South Pacific Ocean. Four distinctive community groups were identified and their relationships with environmental factors were examined using a total of 6 variables (latitude, longitude, hydrostatic pressure, primary productivity, temperature, sediment characteristics), of which pressure (i.e. depth) and longitudinal (i.e. geographic isolation or dispersal distance) gradients best explained the observed variation in the amphipod assemblage structure. The composition of the abyssal community was dominated by cosmopolitan species belonging to the genera Paralicella, Abyssorchomene and Eurythenes. The 2 most dissimilar groups corresponded to the sites at deeper, hadal depths in both trenches: the hadal Kermadec sites (6890 to 7966 m), dominated by Hirondellea dubia, and the hadal Peru-Chile sites (7050 to 8074 m), characterised by the presence of E. gryllus and 3 undescribed Hirondellea species. The number of amphipod species decreased significantly with increasing depth across all the sampling stations, but the decreasing trend diverged markedly between the 2 hadal trench communities, possibly due to the stark contrast in overlying surface productivity between the 2 regions. Thus the environmental forcing exerted by the pressure and longitudinal gradients on the scavenging amphipod community structure is likely to be further influenced by the surface production and associated flux of food material to the trenches.