Award: OCE-1129612

The deep sea (>200 m in depth) is the largest ecosystem on Earth, and harbors a highly diverse unique bottom fauna. However, much of the deep ocean remains unexplored, and our understanding of the structure and function of its communities is recent and incomplete. The deepest and most remote regions of the deep sea are the vast abyssal plains below 4000 m. Where food supply is low, abyssal plains show depressed animal abundance and species diversity. In this case, the abyssal fauna is largely a subset of the shallower bathyal fauna (<4000 m) that has larval dispersal ability. There is little abyssal endemism. We hypothesized that these impoverished abyssal assemblages were sinks maintained by source-sink dynamics; constituent species were too sparsely distributed to be reproductively viable, and were maintained by continued larval dispersal from larger bathyal source populations. Where food supply is higher, abyssal assemblages should be more species rich and show higher endemism through adaptation to abyssal conditions. This proposal was designed to test this by comparing species makeup and life histories of bathyal and abyssal assemblages among basins that varied in food supply from particulate organic carbon flux to the seafloor. Our findings supported the hypothesis. As predicted, predatory gastropods in the abyss form an ordered subset of bathyal species assemblages, and the degree of nestedness is a significant function of the proportion of species with dispersing larvae, reflecting immigration, and faunal density, reflecting extinction potential in the abyss. In the eastern North Atlantic, where food supply and diversity remain relatively high compared to the western Atlantic, the abyssal fauna is highly endemic. In the west, abyssal diversity is depressed and the bivalve abyssal fauna is largely a subset of the bathyal fauna. Similarly, echinoderms in the eastern North Atlantic show high abyssal diversity and endemism. Most compelling, polychaete worms in the impoverished western Gulf of Mexico show reduced abyssal diversity and little endemism. The significance of these findings is that faunal diversity and composition in the deep sea are modulated by food supply. The effect of food supply on species diversity has been recognized for some time. The discovery that food supply also affects faunal makeup in predictable ways may lead to a more unified theory of deep-sea biodiversity. The findings also have important implications for conservation of deep-sea species diversity and projections of future faunal change due to climate change since species diversity and composition vary with food supply. We hope that the success of these analyses will inspire similar studies on other taxa and ocean basins to determine the generality of these biogeographic patterns. All data for this study have been deposited in the Biological and Chemical Oceanography Data Management Office Website (http://www.bco-dmo.org/dataset/565318) Last Modified: 12/22/2016 Submitted by: Michael A Rex