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Lanternfishes (Myctophidae) are among the most abundant, widespread, and diverse fish groups in the world ocean. They account for a significant part of oceanic fish biomass and play crucial roles in various ecosystem processes, including carbon sequestration and nutrient recycling. However, despite the increasing risks they face (e.g. global warming, plastic pollution, and exploitation of deep-sea resources), many aspects of the lanternfishes ecology still remain poorly known. Here, we investigate the species composition, vertical migration, and trophic ecology of lanternfishes in the Southwestern Tropical Atlantic (SWTA) and the influence of physicochemical factors on their horizontal structuring. We show that lanternfishes are a highly diverse and an abundant fish family of the SWTA, comprising at least 33 species and contributing 40% of all fish collected (in number). We reveal that some of these species may differ in their patterns of prey composition and migratory behaviour, leading to multidimensional niches, underestimated trophic links (e.g. gelatinous organisms), and several mechanisms to avoid competitive exclusion. At least 73% of the lanternfish species reported here seem to migrate to the surface to feed at night. Additionally, they are a central food source for mesopelagic and bathypelagic predators, thereby connecting shallow and deep-sea ecosystems. Finally, we show that the structure of lanternfish assemblages is not strongly affected by environmental conditions analysed here (i.e., throughout the thermohaline structure and current systems), leading to weak horizontal assemblage separation.

Length‐weight relationship parameters were calculated for twelve mesopelagic fish species from the western Tropical Atlantic: Diretmus argenteus, Melamphaes polylepis, Bolinichthys distofax, Diaphus lucidus, Diaphus splendidus, Electrona risso, Hygophum taaningi, Taaningichthys bathyphilus, Melanolagus bericoides, Winteria telescopa, Diplophos taenia, Astronesthes similus. Data was collected off northeastern Brazil from April 9th to May 6th, 2017. Hauls were conducted during day and night at 47 stations by using a micronekton trawl (body mesh: 40 mm, cod‐end mesh: 10 mm) from 10 to 1,113 m depth. The material was fixed in a 4% formalin solution for 1 month and then preserved in a 70% alcohol solution for proximally 6 months before processing for length (nearest 0.1 cm of standard length) and weight (nearest 0.01 g of total weight). A new maximum standard length for Winteria telescopa is also provided.

Species of the family Sternoptychidae (hatchetfishes) occur worldwide and play critical roles by sequestering carbon, recycling nutrients, and acting as a key trophic link between epipelagic primary consumers and higher trophic levels in marine ecosystems. Nevertheless, basic knowledge on their ecology is still lacking and their functional ecology remains understudied with respect to composition, organization, functions and environment interactions. Here we integrated comprehensive information collected in the western Tropical Atlantic on the diversity, abundance, distribution and trophic ecology of hatchetfishes, including physicochemical features of their habitats and extensive carbon and nitrogen stable isotope data on its main prey groups. On this basis we defined five functional groups of hatchetfishes with different diet preference, isotopic composition, and vertical abundance peaks and reveal a possible high resource partitioning. Additionally, these species might have a different feeding tie chronology. Hence, hatchetfishes segregate in different ecological groups responding differently to environmental constraints including oxygen concentration and presenting diverse functional roles. As deep-sea species that migrate to epipelagic waters, hatchetfishes may play a key role in the transfer of sub-surface photoassimilated carbon to deeper waters, a pathway through which the effects of climate change at the surface are transferred to the deep ocean. Moreover, as consumers of gelatinous organisms, these species convert “gelatinous energy” into “fish energy” readily usable by higher trophic levels, including endangered and commercially important species. This is a crucial trophic relationship that has been historically underestimated due to methodology limitations (e.g., quickly digested gelatinous organisms were probably underestimated in previous studies, based solely on stomach contents). Considering in ecosystem models this trophic relationship, as well as th