A deep-sea isopod that consumes <i>Sargassum</i> sinking from the ocean’s surface

Most deep-ocean life relies on organic carbon from the surface ocean. While settling primary production rapidly attenuates in the water column, pulses of organic material can be quickly transported to depth in the form of food falls. One example of fresh material that can reach great depths across the tropical Atlantic Ocean and Caribbean Sea is the pelagic macroalgae <i>Sargassum</i>. However, little is known about the deep-ocean organisms able to use this food source. Here, we encountered the isopod <i>Bathyopsurus nybelini</i> at depths 5002-6288 m in the Puerto Rico Trench and Mid-Cayman Spreading Center using the Deep Submergence Vehicle <i>Alvin</i>. In most of the 32 observations, the isopods carried fronds of <i>Sargassum</i>. Through an integrative suite of morphological, DNA sequencing, and microbiological approaches, we show that this species is adapted to feed on <i>Sargassum</i> by using a specialized swimming stroke, having serrated and grinding mouthparts, and containing a gut microbiome that provides a dietary contribution through the degradation of macroalgal polysaccharides and fixing nitrogen. The isopod's physiological, morphological, and ecological adaptations demonstrate that vertical deposition of <i>Sargassum</i> is a direct trophic link between the surface and deep ocean and that some deep-sea organisms are poised to use this material.