Rich and underreported: First integrated assessment of the diversity of mesopelagic fishes in the Southwestern Tropical Atlantic

Mesopelagic fishes play critical ecological roles by sequestering carbon, recycling nutrients, and acting as a key trophic link between primary consumers higher levels. They are also an important food source for harvestable economically valuable fish stocks shallow deep-sea ecosystems. Despite their relevance, mesopelagic ecosystems increasingly threatened direct indirect human activities while representing some of the largest least understood environments on Earth. The composition, diversity, other aspects most basic biological features numerous groups still poorly known. Here, we provide first integrative study biodiversity southwestern Tropical Atlantic (SWTA), based two expeditions in northeastern Brazil 2015 2017. A full list region is provided, including rare species new records Brazilian Exclusive Economic Zone indication potentially such Stomiiformes Beryciformes. Key diversity were assessed, considering different depth strata diel periods. At 200 species, 130 genera, 56 families, 22 orders Teleostei one shark ( Isistius brasiliensis , Dalatiidae, Squaliformes) recorded, eight (4%) 50 (25%) waters. Five families accounted 52% 88% specimens collected, 66% total biomass: Stomiidae (38 spp., 8% specimens, 21% biomass), Myctophidae (34 36%, 24%), Melamphaidae (11 2%, 7%), Sternoptychidae (9 26%, 10%), Gonostomatidae (7 16%, 4%). During day, richness at lower depths (500–1000 m), with contributions typically bathypelagic likely associated seamounts oceanic islands. night, increased epipelagic depths, indicating ascension several (e.g., myctophids sternoptychids) that can endure temperature ranges up to 25°C. Information geographic distribution worldwide provided.