Amphipods associate with epibiotic filamentous bacteria at a shallow-water hydrothermal vent

In the deep ocean, hydrothermal vents sustain remarkable animal–microbial symbioses, in which bacteria colonizing animal external or internal tissues use reduced chemicals from venting fluids to perform chemosynthesis and nourish their host. Hydrothermal vents also occasionally occur within the photic zone, but the prevalence of photosynthesis within these settings is considered to override the development of chemosymbiotic interactions; animal–microbial associations such as those observed at deep-sea vents have rarely been recorded at shallow vents. During 2019 sampling of the Strýtan hydrothermal vent site, Eyjafjörður, Iceland (16–70 m depth), amphipods in the genus Ischyrocerus were opportunistically collected from near the top of the tallest vent chimney. A subset of these were found to have microbial mats specifically colonizing their antennae, with prevalent bacteria in these mats demonstrating genetic similarity to deep-sea hydrothermal vent bacteria, including ectosymbionts of vent-endemic crustaceans. These findings demonstrate the importance of vent productivity to the development of unique associations between animals and putatively chemosynthetic bacteria even in environments where photosynthesis dominates, highlighting a pathway through which more intimate vent symbioses may arise, as well as the influence of vent environments in animal evolution throughout ocean depths.