Nematode communities in sediments of the Kermadec Trench, Southwest Pacific Ocean

Hadal trenches are characterized by environmental conditions not found in any other deep-sea environment, such as steep topography and periodic disturbance by turbidity flows, which are likely responsible for the distinct nature of benthic communities of hadal trenches relative to those of the abyssal plain. Nematodes are the most abundant metazoans in the deep-sea benthos, but it is not yet clear if different trenches host distinct nematode communities, and no data are yet available on the communities of most trenches, including the Kermadec Trench in the Southwest Pacific. Quantitative core samples from the seafloor of the Kermadec Trench were recently obtained from four sites at 6000-9000 m depth which allowed for analyses of meiofauna, and nematodes in particular, for the first time. Nematode community and trophic structure was also compared with other trenches using published data. There was a bathymetric gradient in meiofauna abundance, biomass, and community structure within the Kermadec Trench, but patterns for species richness were ambiguous depending on which metric was used. There was a change in community structure from shallow to deep sites, as well as a consistent change in community structure from the upper sediment layers to the deeper sediment layers across the four sites. These patterns are most likely explained by variation in food availability within the trench, and related to trench topography. Together, deposit and microbial feeders represented 48-92% of total nematode abundance in the samples, which suggests that fine organic detritus and bacteria are major food sources. The relatively high abundance of epigrowth feeders at the 6000 and 9000 m sites (38% and 31%, respectively) indicates that relatively freshly settled microalgal cells represent another important food source at these sites. We found a significant difference in species community structure between the Kermadec and Tonga trenches, which was due to both the presence/absence of species as well as differences in relative abundances of shared species. The cluster and SIMPROF analyses of nematode genus community data across Pacific and Atlantic trenches identified two statistically significant natural groupings: the first group comprised all three Puerto Rico Trench samples, and the second comprised all remaining trenches (South Sandwich, Atacama, Tonga, and Kermadec). Our analyses show that differences in nematode between the adjacent Kermadec and Tonga trenches are observable when analyses are conducted with species-level identifications, but genera-based and trophic structure analyses revealed only limited heterogeneity among trenches. The present study contributes to the growing amount of information on hadal trench environments, which ultimately will build a greater understanding of these rarely sampled deep-sea habitats.