The 16th Deep-Sea Biology Symposium was held in Brest, France, and online from the 12th to the 17th of September 2021. The first DSBS hybrid symposium brought together scientists, students, managers, policymakers, and industry specialists who presented advances in deep-sea research. Themes of the symposium, and of this Research Topic, include: - Conservation and stewardship: natural/anthropogenic impacts, conservation, governance. This includes but it is not limited to: deep-seabed mining, pollutants and debris, climate change impacts; marine spatial planning; stewardship of the deep ocean; - Biodiversity and ecosystem functioning: biodiversity patterns, species distribution, function; from polar to temperate regions, mesopelagic to hadal, microbes to large pelagic; - Life-history traits and population connectivity: reproductive ecology, larval development and dispersal, and population connectivity; - Adaptations of deep-sea organisms: from molecules to organisms: how life adapts to extreme conditions, including for instance bioluminescence and vision in the deep-sea; - Access to the deep sea: technological and methodological advances to access and investigate deep-sea life, including observatories and cutting edge technologies –e.g. A.I. and omics; - Deep-sea biomimicry: discovery of new technologies inspired by deep-sea biological solutions; - Science communication in the deep including innovative approaches to increase ocean literacy (merging “arts & sciences”).