Niche partitioning among abyssal deposit-feeding echinoderms is linked to mobility and gut microbiota

Deposit-feeding echinoderms are dominant megafauna on abyssal plains, where they consume organic detrital material at the base of the benthic food web. However, the strategies they use to survive on irregular pulses of poor-quality detritus remain poorly understood in many regions. Using compound-specific stable isotope analysis of amino acids, we found that deposit-feeding holothurians and echinoids in the oligotrophic North Pacific Subtropical Gyre are secondary rather than primary consumers of detritus, consistent with earlier findings from the productive California Current Ecosystem suggesting they consume gut microbial biomass or its products. At both sites, gut microbial communities were dominated by Actinobacteria , Proteobacteria , and Planctomycetes , and in some species, by the ammonia-oxidizing archaea Nitrosopumilales . Many of the more mobile species, including swimming holothurians, also contained high proportions of Cyanobacteria in their guts during high-flux seasons, demonstrating that fast-moving taxa can consume fresher phytodetritus. Using a mixing model based on δ 15 N values of source amino acids, we found that deposit feeders capable of swimming consume a higher proportion of larger and fresher particles than obligate benthic species, directly linking swimming behavior to feeding selectivity for the first time. Differences in the gut microbiota of deposit feeders on abyssal plain ecosystems, characterized by both high- and low-flux regimes, correspond to niche partitioning based on detritus of differing nutritional qualities. We show that abyssal deposit-feeding echinoderms use multiple adaptations, including swimming behavior and gut microbial communities, to consume a variety of detrital food sources.