The spatial and vertical distribution of larvae, juveniles, and adults of meso- and bathypelagic fish during the dark period of the day was analyzed at three areas in the tropical part of the Mid-Atlantic Ridge, located north and south of the Cape Verde Fault in the zone of the Russian exploration area. The material was collected at four standard layers of 250–0, 700–0, 1500–0, and 2500–0 m. In the study area, 127 species of juveniles and adult fish, belonging to 29 families, and 81 species of larvae, representing 32 families, were recorded. Species abundance and total abundance of deep-sea fish communities at different areas did not differ statistically, regardless of the trawling layer. In the night, the species diversity and average abundance of the fishes gradually increased ( from 25.9 to 531.6 sp./100 m²) with an increase of fishing depth from the surface up to a zone of 1500 m. The maximum fish abundance was observed nearby the summits and slopes of seamounts at a depth range of 700–1500 m. No signs of nighttime peaks in the abundance of deep-sea fish in the 250–0 m layer in the tropical part of the Mid-Atlantic Ridge were found. With increasing of the depth of fishing, the number of warm-water species of lanternfishes (Myctophidae), belonging to tropical and equatorial habitats, decreases, whereas the number of species with bicentral and central-peripheral ranges on the contrary increases. At greater depths there are also appear the subtropical species, preferring less warm water masses, and the species, inhabiting the continental slope and seamounts.

A new species of bathypelagic whalefishes of the genus Cetomimus is described; its two specimens were caught over the central tropical part of the Mid-Atlantic Ridge at the layers of 1500–0 and 2680–0 m. The species is characterized by the presence of 31/2 gill arches (with a small slit behind the third gill arch) and cavernous tissue developed only around the anus and above the bases of the first to sixth rays of the anal fin; large pores in the lateral line canal, which are approximately as wide as this canal; the absence of large, valve-shaped triangular outgrowths along the margin of septa between pores in the posterior part of the lateral line; 42–48 vertebrae; and 20, 17–18, and 18 rays in the pectoral, dorsal, and anal fins, respectively; 17–18 pores in the lateral line canal between the upper margin of the gill cover and the end of the caudal peduncle, as well as by some other diagnostic features. The first molecular genetic data for the described species (mtDNA COX-1 sequences, or DNA barcodes) and analysis of interspecific divergence in the genus Cetomimus are given.

New morphometric data on the rare mesobathypelagic species Rondeletia bicolor (Rondeletiidae) caught over the central part of the Mid-Atlantic ridge in the depth range 2500–0 m are presented. An analysis of the phylogenetic relationships of Rondeletia bicolor, based on new genetic data, was performed, the monophyly of the genus Rondeletia, as well as its place in the Stephanoberycoidei suborder system, was shown.

—A review of the composition and structure of deep-sea pelagic ichthyocenes in the Bransfield Strait and northern Weddell Sea (Powell Basin) in the Atlantic sector of the Southern Ocean is presented. The material was obtained in summer 2020 and 2022 at 26 stations at layers from 120–0 to 2200–0 m. Nine hundred one specimens of juveniles and adult fishes belonging to 19 species from 10 families were collected. Statistical analysis made it possible to identify four significantly different regions in which the dominant ichthyocenes were located in waters of different origin. These ichthyocenes are poor in the number of identified species, but often quite numerous. Relatively shallow-water representatives of the families Bathydraconidae, Channichthyidae, Liparidae, and Nototheniidae, endemic to the shelf and slope of Antarctica, dominated in the area of the Antarctic Strait and on the shelf of the Antarctic Peninsula. At the same time in the Bransfield Strait, the abyssal basin of the Powell Basin, and in the pelagic zone above deep-sea ridges and trenches bordering it from the north, meso- and bathypelagic fish from the families Mycthophidae, Bathylagidae, Gonostomatidae, Paralepididae, and Scopelarchidae prevailed. Near the northern boundary of the Powell Basin, the number of mesobatypelagic species and their average abundance were higher than in its central part (11 vs. 7 species and 63.5 vs. 84.1 sp./100 m²). In the central part of the basin, Bathylagus antarcticus dominated, with an average share of 40% in catches and 28.8 sp./100 m², while in the northern part of the basin, Electrona antarctica dominated, with an average share of 33%, 27.4 sp./100 m².

Ophthalmolycus lacrimegalos sp. nov. is described from a single specimen collected off the South Orkney Islands, Weddell Sea, Southern Ocean at a depth of 1,495 m. The new species differs from its congeners by the combination of extensive squamation covering the abdomen, predorsal area, and pectoral axil; an enlarged lachrymal bone, a wide bony interorbital space, and other characters (cephalic pore configuration, meristics, body proportions, coloration, etc.) in combination. This is the sixth species of the genus in the Antarctic waters, bringing the total number of known species in the genus to ten. However, the distinction of Ophthalmolycus in its current composition from the species-rich genus Lycenchelys requires elucidation. A revised key to the species of Ophthalmolycus is provided.

The composition and structure of deep-sea pelagic fish communities and their relationship with oceanographic factors were analyzed on a transect between 25° N and 43° N (Eastern Atlantic) conducted in summer 2024. Samples were collected from the 0–200 and 0–1000 m water layers during the day and night using a non-closing Samyshev–Aseev–modified Isaacs–Kidd midwater trawl (SA-IKMT). A total of 1,878 juvenile and adult individuals of mesopelagic fish belonging to 100 species from 18 families were recorded in the catches. The families Myctophidae (39 species), Stomiidae (22), Gonostomatidae (11), and Melamphaidae (7) were represented by the largest number of species. Two groups of stations were identified: southern and northern; the boundary between them located along 33°–35° N and was associated with the zone of influence of the Azores Current. Total abundance and species diversity were nearly the same in the catches in the southern and northern parts of transect, but the composition of the dominant species had noticeable differences. The data obtained are consistent with the biogeography of the mesopelagic zone, which distinguishes two ecoregions here. However, the boundary between them located more southwards than indicated earlier. This may be due to both the interannual latitudinal variability of deep-sea ichthyofauna affected by the changes in oceanographic conditions and to the existence of smaller latitudinal subregions within the Central North Atlantic ecoregion.