Biodiversity, Ecological Roles and Threats to Deep-Pelagic Fauna: Insights from the Southwestern Tropical Atlantic

Deep-pelagic ecosystems, spanning depths of approximately 200–5000 meters, host a remarkable diversity of life, including fishes, crustaceans, squids, salps, and cnidarians. Considering fish only, 1,554 species are classified as deep-pelagic, with many more likely yet to be discovered. Beyond their contribution to global biodiversity, deep-pelagic species play a critical role in connecting pelagic layers and trophic levels, acting as prey for numerous commercially exploited and threatened species, including sea lions, penguins, whales and tuna. However, the growing interest in deep-water ecosystems reflects an increasing awareness of their critical ecological roles but is offset by significant knowledge gaps and escalating threats. For example, the massive biomass of deep-pelagic species is part of the largest global animal migration and pivotal to oceanic carbon sequestration, naturally transporting huge amounts of carbon to the deep ocean at no economic cost. This group faces multiple threats, including pollutants, the offshore oil and gas industry, and potential resource exploitation through deep-sea mining and fishing. In the southwestern Tropical Atlantic (SWTA), more than 200 species of deep-pelagic fish were identified, with over 60 new records and 12 new fish species, indicating a rich yet unknown deep-pelagic fish diversity. This little-described biodiversity faces many threats. Among them, microplastics (MPs) are harmful to the marine community and of major concern for human health, as they can adsorb pollutants and potentially bioaccumulate and biomagnify into the food web. A high MP contamination in fishes and cephalopods was observed in the SWTA region. At greater depths, smaller polymers have been reported, suggesting that these particles undergo physical weathering and/or biological degradation through biofouling and intestinal digestion processes. Moreover, increased interest in exploiting deeper waters could jeopardize key species, decreasing biomass and diversity. This decline may exacerbate climate change effects, highlighting the need for these organisms as critical but often overlooked allies. This worrying scenario underscores the urgency of improving our understanding of deep-pelagic ecosystems and taking immediate action to protect them. Much of the deep-water biodiversity remains undiscovered, especially below 1000 m, where there are still substantial gaps, particularly in taxonomy, ecological roles, population dynamics and species interactions.