Our study is the first to describe the diversity patterns of polynoid assemblages along a 1400 km transect crossing a productivity gradient in the CCFZ with the aim to assess the factors possibly driving alpha and beta diversity. Polynoids are a very diverse group of scale worms living at water sediment interface that can only be accurately sampled in the abyss with an epibenthic sledge (EBS). Although the EBS may suffer from methodological biases, our findings are surprisingly contrasting with previous studies examining mostly sedentary polychaetes in that we found high abundance and species richness in the most oligotrophic site. We suggest that during their radiation in the deep sea, macellicephalins have adapted to the exploitation of meager resources at the benthic boundary layer. Our findings contribute to understanding of how food-limitation could drive diversification in the deep-sea by accessing the water column and thus exploiting new benthopelagic niches.

The patterns observed in the composition of polychaete assemblages were attributed to variations in food supply at the regional scale and nodule density at the local scale. The high levels of species replacement were mainly driven by rare species, leading to regional species pool estimates between 498 and 240 000 species. The high proportion of singletons seems reflect an under-sampling bias that is currently preventing the assessment of potential biodiversity loss due to nodule mining.

This is the first study within the CCFZ describing polynoids species (17 new species and four new genera). Also we provide a reclassification of the subfamilies and comments about polynoids evolution within the deep-sea.