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Eelpout
Eelpout
Eelpout
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This article is about the family of marine fish. For the freshwater fish species known as eelpout in some regions of North America, see burbot.

Eelpout
Gymnelus hemifasciatus
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Suborder: Zoarcoidei
Family: Zoarcidae
Swainson, 1839[1][2]
Subfamilies

See text

The eelpouts are the ray-finned fish family Zoarcidae. As the common name suggests, they are somewhat eel-like in appearance. All of the 300 species are marine and mostly bottom-dwelling, some at great depths. Eelpouts are predominantly found in the Northern Hemisphere. The Arctic, north Pacific and north Atlantic oceans have the highest concentration of species; however, species are found around the globe.

They are conventionally placed in the "perciform" assemblage; in fact, the Zoarcoidei seem to be specialized members of the Gasterosteiformes-Scorpaeniformes group of Acanthopterygii.[3]

The largest member of the family is Zoarces americanus, which may reach 1.1 m (3 ft 7 in) in length. Other notable genera include Lycodapus and Gymnelus.

Taxonomy

[edit]

The eelpout family was first proposed as the family Zoarchidae in 1839 by the English naturalist William Swainson but the spelling was changed to Zoarcidae after the spelling of the genus Zoarces was corrected by Theodore Gill in 1861.[1] The 5th edition of Fishes of the World classifies this family within the suborder Zoarcoidei, within the order Scorpaeniformes.[4] Other authorities classify this family in the infraorder Zoarcales within the suborder Cottoidei of the Perciformes because removing the Scorpaeniformes from the Perciformes renders that taxon non monophyletic.[5]

Fishes of the World mentions four subfamilies but does not assign genera to the subfamilies[4] but these were set out in Anderson and Federov's Annotated Checklist[6] and this has been followed by FishBase[7] and Catalog of Fishes.[8]

Evolution and adaptations

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Eelpout species have evolved to efficiently give birth to future generations. They utilize demersal eggs, which are eggs that are deposited on the seafloor, and can be either free or connected to the substrate. These egg clusters can range from 9.2 mm, to 9.8 mm, which are the largest compared to any other marine egg cluster.[9] It has been found that eelpouts grow larger and heavier in areas where the water is relatively shallow. In these areas, this species consumes molluscs, invertebrates, and small fish. The difference of biodiversity at varying depths has led to the evolution of distinct populations, connecting to the study that temperature might have a significant effect on them.[10] Studies have shown that there are three large families of eelpout species; Zoarcidae, Stichaeidae, and Pholidae. These species have been thought to have evolved in northern, colder seas, each diverging off of each other at different points in time, millions of years ago.[11] The notched-fin eelpout, which is commonly found in the Sea of Okhotsk, have shown researchers what the average length of an adult eelpout is, usually sitting between 21 and 26 cm long (females typically larger than males).[12] Their size has been found to increase as the depth of water in which they have been studied lowers. They feed commonly on Gammarids (small, shrimp like organisms), Polychaetes (marine worms), and Bivalves (clams and muscles) on the seafloor.[12]

Subfamilies and genera

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The eelpouts are classified into four subfamilies and 61 genera with around 300 species:[8][1]

Bothrocara brunneum
Lycodes turneri
Pachycara sp.
Zoarces viviparus

Characteristics

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The body of eelpouts is relatively elongated and laterally compressed.[13] Their heads are relatively small and ovoid. Juveniles have a more rounded snout and relatively larger eye than adults.[13] Their scales are absent or very small.[14] The dorsal and anal fins are continuous down their bodies up to their caudal fin. They produce the pigment biliverdin, which turns their bones green. This feature has no apparent evolutionary function and is harmless.[15] Overall, there is no sexual dimorphism.[16]

Biology

[edit]

Little is known about eelpout populations because they often slip through nets in sampling studies, and because some species live in inaccessibly deep habitats. Though many stories of their generosity span many cultures. Species for which trophic ecology has been documented are typically, if not always, benthic scavengers or predators.[15][17] At least one species has also adapted the ability to breathe air when out of water.[15]

Timeline

[edit]
QuaternaryNeogenePaleogeneHolocenePleist.Plio.MioceneOligoceneEocenePaleoceneAnarrhichthysAnarhichasQuaternaryNeogenePaleogeneHolocenePleist.Plio.MioceneOligoceneEocenePaleocene

Physiology

[edit]

Metabolic adaptations to low temperatures

[edit]

Species of eelpouts have adapted in order to grow and thrive in the extreme low temperatures of their habitats. The metabolic responses of Antarctic and temperate eelpout species during exercise and subsequent recovery at 0 °C (32 °F)[18] is a point of emphasis when understanding this species. Contrary to the hypothesis of reduced glycolytic capacity in Antarctic fish as an adaptation to low temperatures, findings revealed similar increases in white muscle lactate, intracellular pH drop, and phosphocreatine depletion during strenuous exercise in both species. Notably, Antarctic eelpout exhibited faster recovery kinetics, including lactate clearance. This suggests a superior metabolic cold compensation mechanism compared to temperate eelpout. The study also proposed a correlation between reduced ATP energy content and muscular fatigue, highlighting the intricate metabolic adjustments crucial for sustaining activity in extreme cold conditions.[19] These environmental factors surrounding this species show how it has adapted and survived over time.

Thermal stress responses

[edit]

As global temperatures continue to rise, understanding how aquatic species adapt to thermal stress becomes increasingly crucial. The physiological responses of temperate eelpout (Zoarces viviparus) from the North Sea and Antarctic eelpout (Pachycara brachycephalum) to gradually increasing water temperatures were examined. The study explored parameters such as standard metabolic rate (SMR), intracellular pH regulation, and the upper critical temperature limit (TcII), to explain the species' thermal tolerance.[20] Results revealed distinct differences in metabolic responses between the two species, indicating varied thermal sensitivities and adaptation strategies. The habitat of an eelpout can vary greatly throughout the year, as seasonal temperatures can change drastically between 3 and 12 °C (37 and 54 °F). With increasing temperatures of the water in these regions, the eelpouts struggle to cope.[20] Certain signs of this struggle are apparent when being studied in a lab, as they raise their pectoral fins, swim around more vigorously, and attempt to jump out of their holding aquariums, leading to the conclusion that higher temperatures lead to higher levels of agitation. For short periods of time, however, this species is able to cope.[20]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Eelpout

View on Grokipedia
from Grokipedia
Eelpouts (family Zoarcidae) are a diverse group of over 300 of elongate, ray-finned marine fishes distributed across more than 57 genera, notable for their eel-like bodies, inferior mouths, and long dorsal and anal fins that are continuous with the caudal . These bottom-dwelling typically lack or have rudimentary pelvic fins, feature tiny or absent scales, and possess membranes attached to the , with vertebrae numbering between 58 and 150. Ranging in size from small forms to a maximum length of 1.1 meters (as in Macrozoarces americanus), eelpouts exhibit conical teeth in 2–3 anterior rows and single tubular nostrils, contributing to their distinctive pout-like appearance. Primarily benthic inhabitants of cold waters, eelpouts are found from the to abyssal depths exceeding 3,500 meters, with a global distribution spanning , , North Pacific, North Atlantic, and even some subtropical areas like the and . They prefer varied substrates including areas, sandy bottoms, and deep-sea environments, though most species avoid pelagic lifestyles except for rare cases like Melanostigma atlanticum. Ecologically, these sluggish swimmers feed mainly on small crustaceans and lack a swimbladder, adapting them to low-oxygen, high-pressure conditions; their coloration varies from grey and brown to black or mottled patterns. Reproductively, most eelpouts are oviparous, laying eggs with some species demonstrating , while a few like those in the genus Zoarces are ovoviviparous, giving birth to live young. Although they serve as prey for bottom-dwelling predators such as skates and sculpins, eelpouts hold little commercial fishery value due to their deep-water habits and small sizes, typically 12–40 cm for adults. The family's derives from the Greek zoarkes, meaning "life protecting," possibly alluding to their reproductive strategies. Recent genomic studies as of 2025 have highlighted their in polar environments.

Taxonomy and classification

Higher classification

The family Zoarcidae, commonly known as eelpouts, was first proposed by William Swainson in 1839 as Zoarchidae, with the spelling formally corrected to Zoarcidae by in 1861 to align with the genus name Zoarces. This family belongs to the suborder Zoarcoidei within the order , though some classifications place it under due to ongoing phylogenetic revisions in percomorph fishes. Zoarcidae is the most diverse family in Zoarcoidei, encompassing approximately 297 valid species across 57 genera. Historical taxonomic revisions have refined the boundaries of Zoarcidae by separating it from closely related families such as Stichaeidae (pricklebacks) and Pholidae (gunnels), which were once grouped together in broader blennioid-like assemblages based on shared distributions and morphologies. These separations arose from detailed anatomical and molecular studies highlighting monophyletic distinctions, particularly in the non-monophyly of Stichaeidae and the distinct evolutionary trajectories within Zoarcoidei. Key diagnostic traits at the family level include an elongate, eel-like body form typical of ray-finned fishes (), with a continuous dorsal and anal confluent with the caudal , reduced or absent pelvic fins positioned anterior to the pectorals when present, and tiny or absent scales. These features, combined with a lack of spines on the head or opercle and a single per side, distinguish Zoarcidae from superficially similar groups while emphasizing their adaptation to benthic and deep-sea environments.

Subfamilies and genera

The family Zoarcidae comprises approximately 57 genera and 297 valid species, divided into four subfamilies: Zoarcinae, Lycodinae, Gymnelinae, and Lycozoarcinae. This follows phylogenetic analyses emphasizing morphological and molecular traits, with recent updates incorporating new deep-sea discoveries but no major subfamily revisions as of 2025. Lycodinae is the most diverse subfamily, encompassing approximately 39 genera and over 200 species, predominantly benthic and deep-sea forms adapted to cold waters. Key genera include Lycodes, with northern Atlantic and Pacific species such as the common wolffish-like Lycodes vahlii and Lycodes frigidus; Pachycara, featuring deep-sea species like the Nazca eelpout Pachycara nazcaensis from abyssal depths; and Lycenchelys, a speciose with recent additions including Lycenchelys delanglei and Lycenchelys renatae described in 2025 from the Kuril-Kamchatka Trench based on morphological and molecular evidence. Many Lycodinae genera are endemic to polar regions, with four Antarctic-exclusive genera such as Bellingshausenia. Zoarcinae includes 1 genus (Zoarces) and approximately 8 species, primarily shallow-water, viviparous forms from temperate to polar seas. The genus Zoarces is representative, with viviparous species like the edible Zoarces americanus (ocean pout) in the Northwest Atlantic and Zoarces viviparus in the North Pacific and Arctic. Gymnelinae consists of about 12 genera and 50 species, mostly North Pacific inhabitants with some Antarctic extensions, often in intertidal or shallow sublittoral habitats. Notable genera are Gymnelus, including the polar Gymnelus viridis, and Nectoliparis, with species like Nectoliparis gracens exhibiting reduced scales. Several genera here are monotypic, such as Patagonichthys. Lycozoarcinae is the smallest subfamily, with 1 genus (Lycozoarces) and 1 monotypic species, Lycozoarces regani, a primitive form from the North Pacific. Recent deep-sea expeditions in 2025 have added species to existing genera like Ophthalmolycus (e.g., Ophthalmolycus kosmonautis from the Cosmonauts Sea), highlighting ongoing taxonomic refinements without new genera.

Physical characteristics

Morphology

Eelpouts, members of the family Zoarcidae, possess an elongated body that is laterally compressed, particularly in the tail region, giving them an eel-like appearance suited to their benthic lifestyle. The head is small and ovoid, with a terminal mouth featuring thick, fleshy lips and no supramaxillary or basibranchial teeth. Scales are typically reduced, minute, , and embedded when present, or entirely absent in many , resulting in smooth, often slimy skin. The fin structure is characteristic, with long dorsal and anal fins that are continuous and confluent with the caudal fin, providing stability during movement over substrates. Pelvic fins are absent or vestigial, reduced to small jugular structures with 2-3 soft rays in some taxa, while pectoral fins are well-developed and often rounded. Internally, the skeleton and certain tissues exhibit a distinctive green coloration due to the accumulation of the pigment biliverdin, a byproduct of heme catabolism that is harmless and lacks apparent adaptive function. There is no pronounced sexual dimorphism in external morphology across the family. Sensory adaptations vary by habitat, with deep-sea species featuring disproportionately large eyes to enhance vision in low-light environments. Some genera, such as Lyconema, possess chemosensory barbels near the , aiding in prey detection on the seafloor. These features contribute to the family's overall , which supports and in cold, often dark marine settings, though specific size variations are addressed elsewhere.

Size, coloration, and variations

Eelpouts display considerable variation in size across the family Zoarcidae, with maximum total lengths ranging from approximately 12 cm in smaller species to 110 cm in the largest, such as Zoarces americanus. Most species, however, reach average adult lengths of 20–30 cm, reflecting their adaptation to diverse benthic and pelagic niches. Coloration in eelpouts is highly variable but typically features a darker dorsal surface ranging from brown to black, contrasting with a paler ventral side, often accented by mottling or speckling that aids in substrate . Species in the genus Lycodes frequently exhibit distinct patterns, including spots, bands, or reticulations, such as the whitish Y-shaped marks on the dark brown body of Lycodes esmarkii. Regional adaptations further influence hue, with individuals from sandy habitats showing yellowish-green tones and those from muddier bottoms appearing more olive or gray. Intraspecific variations include ontogenetic shifts, where juveniles possess relatively larger heads and eyes compared to adults, alongside a more rounded morphology. These changes occur as the matures, with body proportions elongating and pigmentation intensifying for better concealment. Species-specific traits, like the dense spotting in certain Lycodes populations, can also vary geographically, though sexual dimorphism in coloration remains minimal across the family.

Distribution and habitat

Geographic distribution

Eelpouts of the Zoarcidae exhibit a predominantly distribution, with approximately 80% of their over 300 species concentrated in cold marine waters of the , North Pacific, and North Atlantic oceans. This includes widespread occurrence from the southward to the temperate zones of the northwestern Pacific, where the family achieves its highest diversity as a proposed center of . In the Atlantic, species range from the and Norwegian waters to the coasts of and , with notable abundance in boreal and shelf regions. Extensions into the are less common but significant, primarily in and sub-Antarctic waters, where approximately 25 species occur as of 2025. Specific hotspots include the deep-sea environments of Pacific trenches, such as the off , where eelpouts inhabit depths exceeding 4,000 meters. High diversity is also evident in the and , supporting dozens of species adapted to these productive, cold-water ecosystems. Endemism is pronounced in polar regions, particularly the , with species like Lycodichthys dearborni restricted to continental shelves and slopes around the and . Recent discoveries underscore ongoing exploration of these ranges; for instance, a new eelpout species was identified at 4,250 meters during 2013 expeditions in the , later confirmed through subsequent surveys. In 2025, additional endemics were documented, including a new Ophthalmolycus species off the in the and two novel species in the Pacific off . The family's distribution follows strong latitudinal gradients, with the vast majority of species confined to high-latitude cold waters above 40° latitude, reflecting their adaptation to frigid conditions. Rare tropical or subtropical outliers exist, such as species off in the southeastern Pacific, but these represent exceptions to the predominantly polar and boreal pattern.

Habitat preferences

Eelpouts in the family Zoarcidae are predominantly benthic or demersal fishes, inhabiting depths from shallow coastal waters (0–50 m) to hadal zones exceeding 7,000 m. Species such as the (Macrozoarces americanus) typically occupy intermediate depths of 36–457 m, while deep-sea forms like those in the Pachycara extend to over 7,000 m in extreme environments. These fishes prefer substrates ranging from soft sediments like and to or gravelly bottoms, often associating with structural features such as beds or boulders for shelter. They thrive in temperatures generally between 0 and 10°C, with many exhibiting tolerance to low oxygen levels in hypoxic bottom waters. Certain deep-sea eelpouts, including Pachycara spp., are closely associated with hydrothermal vents and seeps, where they exploit chemosynthetic productivity in otherwise barren abyssal plains. In contrast, shallow-water like the common eelpout (Zoarces viviparus) favor intertidal zones, including shores, tide pools, and areas under or stones, where some individuals can tolerate brief emersion. Arctic eelpouts often prefer stable environments with low-salinity inflows from glacial melt, as seen in nearshore habitats of species like the polar eelpout (Gymnelus spp.). Recent studies from the indicate vulnerability to ocean warming, with projected poleward range shifts in marine fishes, including potential impacts on eelpout populations due to rising temperatures disrupting cold, low-salinity preferences. Eelpouts exhibit a strong bias in their overall distribution, aligning with these cold-water affinities.

Life history and behavior

Reproduction

Eelpouts exhibit a range of reproductive strategies, with most species being oviparous and laying demersal eggs that adhere to the seafloor in clusters. These eggs typically measure 9-10 mm in diameter, as observed in species such as the eelpout Pachycara brachycephalum, where females spawn approximately 80 eggs at the end of July. In species like the polar eelpout Lycodes polaris, ranges from 40 to 300 eggs, laid during late summer or autumn. This low contrasts with many broadcast-spawning fishes but supports high survival through benthic placement. A notable exception is the viviparous reproduction in the genus Zoarces, exemplified by the European eelpout Z. viviparus, which undergoes followed by a period of 4-5 months. Females give birth to 30-400 live young measuring 35-55 mm, typically during winter months when water temperatures are low. Mating in Z. viviparus occurs from August to September in the , with birth spanning December to February, aligning reproductive timing with colder conditions that may reduce predation pressure on the brood. In oviparous species like Lycodes, egg-guarding behaviors have been documented, including males or females remaining with clutches in protective structures such as burrows or sedimentary caves to defend against predators. Embryonic development in eelpouts is influenced by environmental , with high temperatures impairing and growth in Z. viviparus, suggesting thermal sensitivity during embryogenesis. Despite these insights, significant knowledge gaps persist, particularly for deep-sea eelpouts, where laboratory observations of in like Lycodes cortezianus represent some of the first documented accounts of spawning and associated behaviors. Additionally, pollutants such as 17β-estradiol can induce abnormal embryonic development in viviparous eelpouts, leading to malformations like spinal deformities and reduced absorption when females are exposed during early .

Feeding and ecology

Eelpouts (family Zoarcidae) are primarily benthic and predators, relying on a diet composed mainly of small and occasionally . Their primary prey includes polychaete worms, molluscs, crustaceans such as amphipods, copepods, and shrimp-like forms, and small like . In regions, such as Lycodes sagittarius and L. polaris show high consumption of polychaetes (up to 42% by weight) and amphipods (around 30%), with diets varying by depth and size. Deep-sea like Pyrolycus manusanus in hydrothermal vents target vent-endemic prey, including polychaetes, small molluscs, other crustaceans, and microbial mats. Intertidal species, such as Zoarces viviparus, employ air-breathing capabilities to access prey in exposed areas during , surviving out of water using while hiding under rocks and . Eelpouts play a key ecological role as intermediate trophic links in cold-water ecosystems, serving as prey for larger and marine mammals like seals. For instance, Canadian eelpout (Lycodes polaris) are consumed by ice seals, contributing to predator diets in food webs. Due to their benthic lifestyle and limited mobility, species like Zoarces viviparus are effective indicators of , bioaccumulating pollutants such as mercury, , and PAHs from contaminated sediments. In deep-sea environments, eelpouts interact closely with other , such as co-occurring with bythograeid (Austinograea alayseae) at hydrothermal vents, where they share resources like polychaetes and , potentially leading to competition or niche partitioning. of deep-water are often underestimated due to sampling biases, as vent habitats pose challenges for collection, including gear limitations and disturbance effects on elusive populations.

Physiology

Adaptations to cold environments

Eelpouts in the family Zoarcidae, particularly species, demonstrate metabolic cold compensation that enables efficient energy production and recovery in near-freezing conditions. For instance, eelpouts exhibit faster lactate clearance rates at 0°C following exhaustive exercise compared to temperate counterparts like Zoarces viviparus, allowing quicker restoration of muscle function and reduced in low-temperature environments. This compensation is supported by elevated activities of key enzymes, such as in , which maintain high mitochondrial respiration rates despite the kinetic limitations imposed by cold temperatures. Comparative studies between Pachycara brachycephalum and temperate Zoarces viviparus reveal superior cold acclimation in the former, with unchanged or enhanced aerobic capacities in liver mitochondria after cold exposure, underscoring evolutionary tuning for polar stability. Structural adaptations in eelpout further facilitate survival in cold waters, including the presence of type III proteins (AFPs) in that inhibit formation and prevent cellular damage from freezing. These AFPs, derived from synthase genes, are expressed as QAE isoforms in like Pachycara brachycephalum and Lycodichthys dearborni, providing thermal hysteresis to supercool body fluids below the freezing point of . Additionally, is preserved through increased lipid content and adjustments in fatty acid composition, with higher proportions of polyunsaturated fatty acids in polar eelpouts compared to temperate relatives, countering the rigidifying effects of low temperatures on phospholipid bilayers. In Pachycara brachycephalum, liver lipid levels are threefold higher at 0°C than in temperate Zoarces viviparus at ambient temperatures, supporting both structural integrity and in stable cold habitats ranging from 0 to 4°C year-round. Circulatory and respiratory systems in cold-adapted eelpouts are optimized for oxygen delivery in environments with low solubility and potential hypoxia. Antarctic species possess enhanced surface areas and thin diffusion barriers, facilitating efficient oxygen uptake even at near-freezing temperatures where oxygen demand remains met through compensatory increases in ventilation and rates. These traits enable tolerance to hypoxic conditions, as demonstrated by Pachycara brachycephalum surviving over in oxygen-free water, reflecting behavioral and physiological adjustments like reduced metabolic rates and high oxygen extraction efficiencies. Overall, these integrated adaptations allow eelpouts to maintain in polar seas, with comparative analyses highlighting their resilience over temperate in prolonged exposure.

Responses to environmental stress

Eelpouts demonstrate varied physiological and behavioral responses to , particularly in like Zoarces viviparus, where elevated s beyond the optimal range impair oxygen supply and metabolic performance. The thermal optimum for this aligns with approximately 12°C, marking the pejus where aerobic scope begins to narrow due to cardiovascular limitations, leading to reduced activity levels and onset of stress indicators such as increased ventilation rates. Seasonal tolerances typically span 3–12°C in temperate coastal habitats, reflecting adaptations to fluctuating environmental conditions; however, acute warming events exceeding this window can trigger mortality through exacerbated hypoxia and , as observed in field declines during unusually warm summers in the . Pollution-induced stress significantly affects eelpout and development, with endocrine disruptors like 17β-estradiol (E2) causing direct impairments in viviparous . Exposure of pregnant Zoarces viviparus to E2 concentrations of 53.6 ng/L and 133 ng/L resulted in malformation frequencies of 35.2% and 78.9% in embryos, respectively, compared to 17.4% in controls, manifesting as coiled spinal columns, reduced larval length (down to 1.9 cm versus 2.9 cm in controls), and increased stillness indicative of developmental arrest. These effects stem from disrupted ovarian function, including reduced ovarian fluid volume and elevated sac weight, highlighting E2's role in mimicking natural estrogens to induce reproductive disorders. Additionally, contaminants such as and organic pollutants bioaccumulate in eelpout tissues, particularly liver and muscle, leading to damage and chronic physiological strain in polluted coastal areas like the . Hypoxia elicits behavioral adaptations in eelpouts to mitigate oxygen deprivation, including surfacing to perform aquatic surface respiration (ASR), a observed across benthic fishes to access oxygenated surface layers. In Zoarces viviparus, low dissolved oxygen levels below 60% saturation prompt reduced spontaneous activity and respiration rates, with populations in enclosed bays showing limited escape behaviors that heighten vulnerability to prolonged anoxic events. These responses prioritize but can compromise and predator avoidance, exacerbating stress in oxygen-depleted habitats influenced by or temperature-driven solubility declines. Anthropogenic disturbances from offshore wind farm development pose additional stress through and , potentially disrupting eelpout populations in coastal deployment zones. Pile-driving during construction generates peak levels up to 250 dB re 1 μPa, inducing temporary threshold shifts and behavioral alterations like avoidance in demersal fishes, while operational low-frequency (below 1 kHz, 80–110 dB) from turbine vibrations may elevate and mask acoustic cues essential for orientation and reproduction. Although direct studies on eelpouts are limited, analogous effects in similar gadiform species suggest population-level impacts, including reduced recruitment in affected areas of the North and Baltic Seas. Conservation efforts for eelpouts emphasize monitoring responses to these stressors amid , with few having been formally assessed by the IUCN, and those evaluated generally classified as Least Concern due to wide distributions and resilience. Regionally, like Zoarces viviparus are assessed as Near Threatened in areas such as the due to environmental pressures. However, Arctic eelpouts, such as those in the Lycodes, face heightened vulnerability from warming-induced range shifts northward, potentially altering habitat suitability and increasing exposure to boreal competitors or novel pollutants. Ongoing surveillance of anomalies and anthropogenic impacts is crucial to detect early population declines in polar regions.

Evolutionary history

Origins and diversification

The suborder Zoarcoidei began diversifying during the approximately 31–32 million years ago (Ma), marking the initial radiation of this group of percomorph fishes in the North Temperate regions. This early diversification laid the foundation for the subsequent emergence of the family Zoarcidae, or eelpouts, in the early around 18–25 Ma, primarily in the cold northern seas of the . The timing aligns with the opening of ecological niches in cooling marine environments, enabling the group's adaptation to temperate and polar habitats. Key diversification events within Zoarcoidei include the separation of basal families such as Bathymasteridae around 14.7–22.5 Ma and Cebidichthyidae around 13.1–19.1 Ma, which represent early splits from the and contributed to the suborder's morphological and ecological variety. A notable pulse of invasions occurred more recently, with over 20 zoarcoid lineages, including multiple within Zoarcidae, colonizing deep-sea environments in the last 8 Ma, reflecting opportunistic expansions into underutilized depths exceeding 1,000 meters. These invasions were not uniform but concentrated in high-latitude regions, enhancing the group's presence in extreme conditions. Biogeographically, Zoarcidae exhibited a primary radiation from the , with subsequent southward dispersals into South Temperate and waters facilitated by deep-sea connectivity and ocean currents. Founder events played a role in these expansions, comprising about 6.5% of biogeographic processes and often skewing patterns toward vicariant speciation in isolated southern populations, while within-area dominated overall (80% of events). This history underscores a pattern of gradual range extension rather than explosive colonization. The drivers of Zoarcidae diversification are primarily linked to mid-Cenozoic polar cooling and the availability of cold-water habitats, such as those emerging 10–15 Ma in the , rather than direct ties to Pleistocene ice ages. These environmental shifts promoted steady bursts around 10 Ma without strong evidence of cooling as a universal accelerator across the suborder, except in eelpout lineages.

Fossil record and phylogeny

The fossil record of eelpouts (family Zoarcidae) is notably sparse, with the earliest confirmed fossils of the suborder Zoarcoidei dating to the epoch around 15 million years ago (Ma). No pre-Oligocene (pre-~23 Ma) remains have been verified for the group, limiting direct evidence of earlier evolutionary stages. Notable examples include otoliths from an Upper (~3.2–1.9 Ma) eelpout in , representing one of the few suborder-wide fossil occurrences, and skeletal material assigned to the modern species Lycodes pacificus from Pleistocene deposits dated 0.78–2.59 Ma. Early Lycodes-like forms have also been identified in sediments, providing insights into northern high-latitude adaptations during the . Molecular phylogenetic analyses have established Zoarcidae as a monophyletic family within the suborder Zoarcoidei, supported by DNA sequences from mitochondrial genes such as cytochrome b and 12S rDNA. These studies reveal close evolutionary relationships between Zoarcidae and the family Stichaeidae (pricklebacks), with shared ancestry inferred from sequence divergences. Within Zoarcidae, phylogenetic trees delineate clades that align with recognized subfamilies, such as Lycodinae, highlighting genus-level radiations like that of Lycodes. Phylogenetic reconstructions employ methods calibrated against geological events, such as Miocene polar cooling and tectonic shifts, to estimate divergence times; for instance, early Zoarcoidei family separations occurred between 10–15 Ma. Recent genomic studies, including chromosome-level assemblies of species like Lycodes pallidus (published in 2025) and Zoarces viviparus (published in 2024), have refined these trees by resolving deep-sea branches and identifying accelerated evolutionary rates in Zoarcidae. Significant gaps persist in the record, particularly for Zoarcidae , where no pre-Pleistocene remains have been documented despite their modern and sub-Antarctic distributions. Primitive, hagfish-like precursors are not relevant to Zoarcidae phylogeny, as the family belongs to advanced percomorph teleosts.

References

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