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Bony fishes are extremely successful in the marine environment, having evolved into nearly every ocean habitat. However, bony fishes do not seem to inhabit the ocean’s deepest depths, likely due to constraints of pressure adaptation. How deep do bony fishes live? Relatively few studies have examined the deepest living vertebrates, because sampling in hadal environments, depths 6000–11,000 m, is technologically challenging. Here, we review the literature on records of the deepest living bony fishes. Current depth records are held by the hadal snailfish Pseudoliparis swirei (family Liparidae) in the Mariana Trench, collection depth 7966 m, filmed to 8178 m, and the cusk eel Abyssobrotula galatheae (family Ophidiidae) in the Puerto Rico Trench, collection depth 7965 m. Observations of abyssal and hadal fish communities suggest that hadal snailfishes are endemic to trenches but occasionally cross into abyssal areas. On the other hand, cusk eels dwell on the abyssal plains, but can extend their ranges into the trenches. These habitat differences allow both snailfishes and cusk eels to occupy distinct niches in the greatest ocean depths. We then comment on the ecological and physiological significance of these two major hadal families and present recommendations for future research.

We tested the hypothesis that deep-sea fishes have poorly mineralized bone relative to shallower-dwelling species using data from a single family that spans a large depth range. The family Liparidae (snailfishes, Cottiformes) has representatives across the entire habitable depth range for bony fishes (0 m–> 8000 m), making them an ideal model for studying depth-related trends in a confined phylogeny. We used micro-computed tomography (micro-CT) scanning to test three aspects of skeletal reduction in snailfishes (50 species) across a full range of habitat depths: 1) reduction of structural dimensions, 2) loss of skeletal elements, and 3) reduction in bone density. Using depth data from the literature, we found that with increasing depth, the length of the dentary, neurocranium, and suborbital bones decreases. The ventral suction disk decreases width with increasing maximum habitat depth and is lost entirely in some deeper-living taxa, though not all. Although visual declines in bone density in deeper-living taxa were evident across full skeletons, individual densities of the lower jaw, vertebra, suction disk, hypural plate, and otoliths did not significantly decline with any depth metric. However, pelagic and polar taxa tended to show lower density bones compared to other species in the family. We propose that skeletal reductions allow snailfishes to maintain neutral buoyancy at great depths in the water column, while supporting efficient feeding and locomotion strategies. These findings suggest that changes in skeletal structure are non-linear and are driven not only by hydrostatic pressure, but by other environmental factors and by evolutionary ancestry, calling the existing paradigm into question.

The hadal zone, Earth's deepest oceanic region, is defined by distinct geological features and hosts a variety of endemic species, including the Liparidae Gill, 1861 (snailfishes). Ecological understanding of snailfishes dwelling at depths greater than 6000 m remains limited due to challenges in physical specimen collection and preservation. This study employs molecular tools to assess the phylogenetic relationships and distribution patterns of hadal snailfishes by analyzing three mitochondrial DNA markers (16S, Cyt‐B, COI) and incorporates 20 new specimens from the Japan and Tonga trenches (Pacific Ocean) and the Diamantina Fracture Zone (Indian Ocean). The phylogenetic hypotheses and species delimitation assessments were tested among a framework of six taxonomic units —Pseudoliparis swirei Gerringer and Linley, 2017, Pseudoliparis belyaevi Andriashev and Pitruk, 1993, Notoliparis kermadecensis Nielsen, 1964, Notoliparis stewarti Stein, 2016, and Paraliparis selti Linley, Gerringer, and Canto‐Hernández, 2022. The results revealed wider geographic distributions than previously thought, particularly for Notoliparis c.f. stewarti. Further, the molecular data support the hypothesis that Notoliparis Andriashev, 1975 should be treated as a subjective junior synonym of Pseudoliparis Andriashev, 1955. Our findings do emphasize the challenges and limitations of using DNA barcoding solely to distinguish closely related or recently diverged species. Together, this study advances the biogeographic understanding of hadal snailfishes and highlights the importance of expanding sampling efforts. Hadal snailfish are not endemic to single deep‐sea features, as evidenced by multiple documented range extensions of hadal snailfish species. Our data supports the recommendation that Notoliparis should be considered a subjective junior synonym of the genus Pseudoliparis and highlights the importance of expanding sampling efforts.

The taxonomic validity of Northeast Indian endemic Mahseer species, Tor progeneius and Neolissochilus hexastichus, has been argued repeatedly. This is mainly due to disagreements in recognizing the species based on morphological characters. Consequently, both the species have been concealed for many decades. DNA barcoding has become a promising and an independent technique for accurate species level identification. Therefore, utilization of such technique in association with the traditional morphotaxonomic description can resolve the species dilemma of this important group of sport fishes. Altogether, 28 mahseer specimens including paratypes were studied from different locations in Northeast India, and 24 morphometric characters were measured invariably. The Principal Component Analysis with morphometric data revealed five distinct groups of sample that were taxonomically categorized into 4 species, viz., Tor putitora, T. progeneius, Neolissochilus hexagonolepis and N. hexastichus. Analysis with a dataset of 76 DNA barcode sequences of different mahseer species exhibited that the queries of T. putitora and N. hexagonolepis clustered cohesively with the respective conspecific database sequences maintaining 0.8% maximum K2P divergence. The closest congeneric divergence was 3 times higher than the mean conspecific divergence and was considered as barcode gap. The maximum divergence among the samples of T. progeneius and T. putitora was 0.8% that was much below the barcode gap, indicating them being synonymous. The query sequences of N. hexastichus invariably formed a discrete and a congeneric clade with the database sequences and maintained the interspecific divergence that supported its distinct species status. Notably, N. hexastichus was encountered in a single site and seemed to be under threat. This study substantiated the identification of N. hexastichus to be a true species, and tentatively regarded T. progeneius to be a synonym of T. putitora. It would guide the conservationists to initiate priority conservation of N. hexastichus and T. putitora.

Joining the ranks of vertebrates that glow is the Pacific Spiny Lumpsucker, Eumicrotremus orbis, a subtidal species widely distributed across the North Pacific Ocean. Aside from their charismatic appearance, the Pacific Spiny Lumpsucker is known for its ventral suction disc that is used to stick to substrates amid changing currents and tides. Here we show that red lumpsuckers, which are usually male and a deep red color under broad-spectrum light, fluoresce bright red under ultraviolet (UV) light and blue light (360–460 nm), while green color morphs (usually female) do not. In all color morphs, the suctorial disc glows green-yellow. The red glow of the males matches the red glow of encrusting algae in their nesting areas, while the suctorial disc may be a signaling system. The green and red fluorescence observed in red lumpsuckers is the rarest fluorescent pattern and is only seen in 17 families of marine fishes. Pacific Spiny Lumpsuckers are cryptically colored under broad-spectrum light; our observed fluorescence suggests a potential avenue of communication and camouflage in an environment where red light is absent or rare.

— The paper presents the up-to-date information on the composition of the White Sea ichthyofauna, updated from literature sources and the results of regular studies conducted by the Russian Federal Research Institute of Fisheries and Oceanography, Northern Branch in 1980–2023. During more than 40 years of trawl and coastal surveys, 64 species (subspecies) of pisciformes and fishes have been recorded in catches. Five species are listed for the White Sea for the first time: the freshwater species, the silver bream Blicca bjoerkna and burbot Lota lota ; marine species, the pale eelpout Lycodes pallidus , the snakeblenny Lumpenus lampretaeformis and northern wolffish Anarhichas denticulatus . The presence of the merling Merlangius merlangus in the White Sea was confirmed. Taking into account literature data, the White Sea ichthyofauna includes 86 species and subspecies from 33 families (54 marine species, 14 anadromous and semi-anadromous species, and 18 freshwater species entering brackish waters). The marine ichthyofauna is basically represented by 17 families: Petromyzontidae, Clupeidae, Osmeridae, Coregonidae, Salmonidae, Gadidae, Gasterosteidae, Cottidae, Agonidae, Cyclopteridae, Liparidae, Zoarcidae, Stichaeidae, Pholidae, Anarhichadidae, Ammodytidae and Pleuronectidae. Representatives of the other families are few or rare, or occur in low salinity estuarine areas. The catches of some species have not been recorded for half a century. The group of commercial fish species includes 23 species; Clupeidae, Osmeridae, Coregonidae, Salmonidae, Gadidae, and Pleuronectidae form the basis of fishery.

Snailfishes are among the most rapidly radiating families of marine fishes, resulting in a global distribution from the coastal intertidal to deep subduction trenches. The true diversity and distribution of deep-water snailfishes, particularly at hadal depths (>6000 m) and in the Southern Hemisphere, remain uncertain due to the rarity of samples. Here, we present the snailfish diversity at near-hadal and hadal depths in the Atacama Trench, which runs along the southwest coast of South America. Using free-fall baited cameras and traps, we documented at least three species of hadal snailfishes between 5920 and 7608 m based on distinct morphologies. One snailfish specimen was recovered from 6714 m, which we describe herein as Paraliparis selti sp. nov., based on a combined morphological and molecular taxonomic approach (16S, COI, and Cyt-b). Paraliparis selti sp. nov. is morphologically distinct from described snailfishes due to a combination of high number of; vertebrae (65) particularly the abdominal vertebrae (12), dorsal fin rays (60), anal fin rays (52), and caudal fin rays (8); comparatively low number of pectoral fin rays (18) which forms a deep notch with two widely spaced non-rudimentary rays. Micro-CT was used to minimise dissection of the specimen and to provide a digital holotype. Paraliparis selti sp. nov. highlights the importance of the Liparidae at hadal depths and provides evidence for at least two independent radiations of snailfishes into the hadal zone.

The complete mitogenome of the Atlantic spiny lumpsucker (Eumicrotremus spinosus) was generated using the PacBio Sequel II HiFi sequencing platform. The mitogenome assembly has a length of 19,281 bp and contains 13 protein-coding sequences, 22 tRNA genes, 2 rRNA genes, one control region containing the D-loop (2383 bp) and a duplicate control region (1133 bp) Phylogenetic analysis using maximum likelihood revealed that E. spinosus is closely related to the Siberian lumpsucker (E. asperrimus). The mitogenome of the spiny lumpsucker will be useful in population genomics and systematic studies of Cyclopteridae, Liparidae, and Cottidae.

Phylogenetic morphological, and molecular analysis of lumpsuckers of the family Cyclopteridae is carried out. The cladograms reveal the validity of three subfamilies within the family Cyclopteridae: Cyclopterinae, Aptocyclinae, and Eumicrotrematominae. Five valid genera ( Cyclopterus , Aptocyclus , Cyclopsis , Lethotremus, and Eumicrotremus ) are revealed, and the genus Cyclopterus is accepted as the most generalized within the family Cyclopteridae. The genus Lethotremus is separated from the species of the genus Eumicrotremus , and, thus, the generic status of the former genus is restored. The taxonomic position of E. soldatovi is discussed. Based on the presence of a large number of monotypic genera among generalized clades of lumpsuckers, they could represent the remains of the former widely distributed fauna with a weak development or absence of external armament. The forms of the genus Eumicrotremus with a good armament represent a comparatively young group of weakly differentiated species. Several heterochronies of the paedomorphic and peramorphic types are revealed in the evolution of the family Cyclopteridae.

Hadal snailfishes are the deepest-living fishes in the ocean, inhabiting trenches from depths of ∼6,000 to 8,000 m. While the microbial communities in trench environments have begun to be characterized, the microbes associated with hadal megafauna remain relatively unknown. Here, we describe the gut microbiomes of two hadal snailfishes, Pseudoliparis swirei (Mariana Trench) and Notoliparis kermadecensis (Kermadec Trench), using 16S rRNA gene amplicon sequencing. We contextualize these microbiomes with comparisons to the abyssal macrourid Coryphaenoides yaquinae and the continental shelf-dwelling snailfish Careproctus melanurus. The microbial communities of the hadal snailfishes were distinct from their shallower counterparts and were dominated by the same sequences related to the Mycoplasmataceae and Desulfovibrionaceae. These shared taxa indicate that symbiont lineages have remained similar to the ancestral symbiont since their geographic separation or that they are dispersed between geographically distant trenches and subsequently colonize specific hosts. The abyssal and hadal fishes contained sequences related to known, cultured piezophiles, microbes that grow optimally under high hydrostatic pressure, including Psychromonas, Moritella, and Shewanella. These taxa are adept at colonizing nutrient-rich environments present in the deep ocean, such as on particles and in the guts of hosts, and we hypothesize they could make a dietary contribution to deep-sea fishes by degrading chitin and producing fatty acids. We characterize the gut microbiota within some of the deepest fishes to provide new insight into the diversity and distribution of host-associated microbial taxa and the potential of these animals, and the microbes they harbor, for understanding adaptation to deep-sea habitats.