Eel

Eels are ray-finned fishes of the order Anguilliformes, encompassing more than 800 species characterized by their elongated, serpentine bodies, absence of pelvic fins, and typically reduced or absent pectoral fins, with dorsal and anal fins often fused continuously along the tail.^[1][2]
These species exhibit remarkable ecological diversity, inhabiting marine depths, coral reefs, freshwater rivers, and estuaries across global oceans, functioning primarily as opportunistic predators adapted to burrowing, ambushing prey with powerful lateral undulations.^[3][2] Among the most studied are the catadromous freshwater eels of the genus Anguilla, comprising 16 species and three subspecies that spawn in remote oceanic sites—such as the Sargasso Sea for Atlantic species—releasing eggs and sperm before dying, while transparent leptocephalus larvae drift vast distances to coastal waters before metamorphosing into elvers that ascend rivers for growth phases lasting 5–20 years or more.^[4][5][6]
This complex life history, involving panmictic breeding and extreme migration, renders populations vulnerable to barriers like dams, overfishing, and habitat loss, contributing to the critically endangered status of species like the European eel (Anguilla anguilla).^[5][7]
Eels hold cultural and economic significance as food sources in various cuisines, though sustainable management challenges persist due to opaque recruitment dynamics and illegal trade in juveniles.^[8][4]

Anatomy and Physiology

Body Structure and Morphology

Anguilliform eels possess a highly elongated, serpentine body adapted for anguilliform locomotion, with body lengths often exceeding 10-20 times the depth at the anus, achieved through modifications in vertebral count, length, and associated skeletal elements across taxa. This elongation varies phylogenetically; for instance, some families reduce precaudal vertebrae while increasing caudal ones, others elongate individual centra or minimize pleural ribs and epineurals to accommodate the slender form.^[9] The skin is typically covered in small, embedded cycloid scales or lacks scales entirely, as in many muraenids, providing a smooth, mucus-rich surface that reduces drag during swimming.^[10] Paired fins are characteristically absent (pelvic fins) or reduced (pectoral fins in most species), with locomotion relying on a continuous median fin fold comprising the dorsal, anal, and caudal fins that extends from behind the head to the tail tip in over 90% of species.^[11] Dorsal fin origin varies by family, positioned posteriorly in anguillids but anteriorly in some ophichthids; the fin rays are soft and flexible, supported by reduced pterygiophores integrated into the axial musculature for efficient undulation.^[12] The head is relatively small and conical to tubular, with a terminal or subterminal mouth armed with small teeth; cranial morphology diversifies for feeding, as seen in morays with robust jaws for biting versus the rasping pharyngeal teeth in some congrid eels.^[13] Internal morphology emphasizes axial specialization, with vertebral numbers ranging from 100-120 in shorter forms to over 1000 in highly elongate species like certain synaphobranchids, featuring fused centra in caudal regions for tail flexibility.^[14] Gill openings are reduced, often confined to the lower throat, supporting efficient oxygen extraction in low-oxygen habitats; lateral line systems are abbreviated or absent, compensated by enhanced olfactory and electroreceptive organs in many deep-sea taxa.^[15] Family-specific traits include the depressed snout and sub-cylindrical opercular region in protanguillids or the fang-like dentition in muraenids, reflecting ecological adaptations from freshwater anguillids to reef-dwelling ophichthids.^[16]

Sensory and Physiological Adaptations

Eels possess a suite of sensory adaptations that compensate for their elongated body form and habitats often characterized by low visibility, such as riverbeds, reefs, and deep ocean waters. The olfactory system is particularly acute, with olfactory epithelium expressing specific chemosensory receptors that intensify at sexual maturity to detect pheromones and environmental cues during long-distance migrations.^[17] In species like the European eel (Anguilla anguilla), this sensitivity extends to ions such as Ca²⁺ and Na⁺, potentially aiding osmoregulatory orientation in varying salinities.^[18] The lateral line system, consisting of canal neuromasts and superficial free neuromasts, detects hydrodynamic pressure changes, vibrations, and weak water currents, enabling prey localization and predator evasion even in turbid conditions.^[19] In the Japanese eel (Anguilla japonica), this system includes unique rostral commissures linking supraorbital canals and cephalic canal pouches, enhancing spatial awareness.^[20] Visual adaptations vary across families; moray eels (Muraenidae) exhibit reduced eye size and retinal specialization for low-light hunting, prioritizing chemosensation over vision.^[21] Physiologically, many eel species, especially anguillids, display euryhalinity through dynamic osmoregulatory mechanisms that facilitate transitions between freshwater and marine environments. In seawater, eels ingest large volumes of water via the esophagus and intestine, where NaCl is absorbed alongside water, followed by active ion extrusion through gill chloride cells expressing Na⁺/K⁺-ATPase and CFTR channels.^{[22] [23]} During the preparatory silvering phase, yellow eels undergo physiological remodeling, including proliferation of gill ionocytes and upregulation of intestinal guanylin receptors to modulate water absorption and ion secretion, enabling survival in salinities up to 35 ppt within hours of transfer.^{[24] [25]} In freshwater, conversely, they excrete hypotonic urine via voluminous kidneys and limit gill permeability to prevent ion loss.^[26] Migratory silver eels further adapt by accumulating lipid reserves—up to 20-30% of body mass—and suppressing somatic growth to fuel transoceanic journeys spanning thousands of kilometers without feeding, supported by enhanced anaerobic capacity in red and white muscle fibers.^[27] These adaptations underscore the causal role of selective pressures from catadromous life histories in evolving robust ionoregulatory and metabolic resilience.

Life History

Reproductive Cycle and Migration Patterns

Anguillid eels, including species such as the European eel (Anguilla anguilla) and American eel (Anguilla rostrata), exhibit a catadromous reproductive cycle, maturing in freshwater or estuarine habitats before undertaking long-distance migrations to deep-ocean spawning grounds.^[28] Adults transform into silver eels, characterized by increased body silvering, enlarged eyes, and heightened fat content to fuel the migration, which can span 5,000 to 10,000 km for the European eel.^[29] This semelparous strategy involves a single spawning event followed by death, with no return migration observed.^[7] Spawning for temperate anguillids occurs in subtropical oceanic regions, with the Sargasso Sea serving as the primary site for Atlantic species like A. anguilla and A. rostrata, inferred from larval distributions and otolith strontium/calcium ratios indicating oceanic hatching followed by continental growth.^[30] For the Japanese eel (Anguilla japonica), spawning takes place near the Mariana Islands in the western North Pacific, at depths exceeding 200 meters where temperatures range from 10–18°C.^[31] Direct evidence remains limited due to the inaccessibility of these sites, but satellite-tagged adults from the Azores have been tracked heading toward the Sargasso Sea, confirming directed migration paths at speeds of 0.3–0.5 body lengths per second.^[29] Adult outbound migration typically initiates in autumn, from October to December in the North Atlantic, with eels exhibiting diel vertical migrations—deeper during the day (200–800 m) and shallower at night—to optimize energy use and avoid predators while following gyre currents.^{[32] [33]} Upon spawning, eggs hatch into leptocephalus larvae, leaf-like transparent forms that passively drift with ocean currents, such as the Gulf Stream, for 1–3 years to reach recruitment areas thousands of kilometers away.^[28] This prolonged larval phase ensures wide dispersal but imposes high mortality risks, with recruitment success varying by oceanic conditions and continental barriers.^[7] In contrast, many non-anguillid eels, such as moray eels (Muraenidae), follow marine reproductive patterns with local pelagic spawning and shorter larval durations, lacking the extensive migrations characteristic of anguillids.^[34] Empirical tracking and genetic data underscore that anguillid migration evolved to exploit nutrient-rich freshwater growth phases while leveraging oceanic stability for reproduction, though overexploitation and barriers have disrupted these patterns in recent decades.^[35][29]

Developmental Stages and Metamorphosis

The developmental stages of eels in the order Anguilliformes commence with the hatching of pelagic eggs into leptocephalus larvae in oceanic spawning grounds. These larvae exhibit a distinctive flattened, ribbon-like morphology with a high content of gelatinous extracellular matrix, facilitating passive drift via ocean currents for durations ranging from several months to over a year, depending on species and environmental conditions. For instance, in the American eel (Anguilla rostrata), leptocephali reach lengths of 50-70 mm after approximately 1-1.5 years of development, sustained by particulate feeding on marine aggregates.^[36][37] Metamorphosis from leptocephalus to glass eel represents a critical transition, involving rapid morphological remodeling such as body elongation, reduction in dorso-ventral depth by up to 80%, resorption of larval-specific tissues, and maturation of the digestive and muscular systems. This process, lasting days to weeks, is primarily regulated by thyroid hormones and epigenetic modifications, including DNA methylation changes, and is initiated by cues like decreasing salinity and specific temperatures upon nearing continental shelves.^[38][39] In the European eel (Anguilla anguilla), metamorphosis typically occurs at an average age of 350 days post-hatching.^[40] Post-metamorphosis glass eels, transparent juveniles of 50-80 mm length, exhibit active swimming behavior and ingress into estuarine waters, often employing selective tidal stream transport to counter currents. Pigmentation soon develops, yielding elvers that actively migrate upstream into freshwater systems, guided by olfactory and geomagnetic cues, where they transition to the yellow eel phase of somatic growth.^[28][41] A secondary metamorphosis later converts yellow eels into silver eels, involving silvering of the skin, ocular enlargement for enhanced vision, and gonadal maturation, preparing individuals for catadromous return to spawning sites; this stage underscores the semelparous reproductive strategy observed in anguillid species.^[42][38] While characteristic of freshwater eels, marine anguilliform species display abbreviated or modified leptocephalus phases with benthic post-larval settlement, reflecting adaptations to non-migratory habits.^[28]

Taxonomy and Systematics

Classification and Major Families

True eels are classified within the order Anguilliformes, a group of ray-finned fishes (class Actinopterygii) in the superorder Elopomorpha, characterized by their elongate, serpentine bodies lacking pelvic fins and often scales.^[43] This order encompasses 19 families, 111 genera, and approximately 800 species, predominantly marine with some catadromous forms.^[44] Phylogenetic analyses recognize four suborders: Anguilloidei (including freshwater eels), Congroidei (conger-like eels), Muraenoidei (morays), and Synaphobranchoidei (cutthroat eels).^[45] The family Anguillidae (freshwater eels) is small, with 2 genera and 20 species, primarily in the genus Anguilla; these species exhibit a unique leptocephalus larva and catadromous life history, migrating to oceanic spawning grounds.^[46] Members feature a continuous dorsal-anal-caudal fin fold and minute embedded scales.^[46] Muraenidae (moray eels) represents one of the largest families, with 15 genera and 197 valid species, mostly tropical and subtropical reef-associated predators distinguished by robust jaws, additional pharyngeal jaws for prey capture, and scaleless skin.^[47] They lack pectoral fins in many species and are known for cryptic behaviors in crevices.^[3] Ophichthidae (snake eels and worm eels) is highly diverse, with species that burrow into sand or mud using a hardened tail tip; regional checklists indicate substantial species richness, such as 60 species in certain Indo-Pacific surveys, reflecting global abundance.^[48] These eels often have anal fins and pectoral fins reduced or absent.^[11] Congridae (conger and garden eels) includes predatory marine species in shallow to bathyal depths, with some garden eel taxa forming social burrowing colonies; the family contributes significantly to anguilliform diversity in suborder Congroidei, which alone holds 498 species across five families.^[49] They typically possess pectoral fins and larger eyes adapted for low-light environments.^[11] Other notable families include Synaphobranchidae (deep-sea eels, adapted to abyssal habitats) and Nettastomatidae (duckbill eels, with elongated snouts), which expand the order's ecological range into deep oceans.^[3] The classification reflects ongoing refinements from morphological and DNA-based phylogenies, emphasizing monophyly within Anguilliformes distinct from superficially similar non-anguilliform "eels" like electric eels.^[45]

Phylogenetic Relationships

Anguilliformes, the order encompassing true eels, is classified within the superorder Elopomorpha of the subclass Teleostei, distinguished by features such as leptocephalus larvae and reduced swim bladders. Molecular phylogenies based on mitochondrial genomes and nuclear loci consistently recover Anguilliformes as monophyletic, supported by synapomorphies including elongate body form, reduced caudal fin elements, and specialized gill arch morphology.^[51] This monophyly holds across datasets, though early morphological analyses struggled to resolve interfamilial relationships due to high homoplasy in body elongation and fin reduction. A comprehensive multi-locus phylogeny utilizing five genetic markers (nuclear rhodopsin, rag1, zic1, and mitochondrial 12S-16S and cytochrome b) delineates four principal suborders within Anguilliformes: Protanguilloidei, Muraenoidei, Anguilloidei, and Congroidei.^[54][55] Protanguilloidei represents the basalmost lineage, exemplified by the "living fossil" family Protanguillidae (Protanguilla palau), which exhibits primitive traits like a short dorsal fin and robust pectoral girdle, diverging near the root of anguilliform evolution around the Cretaceous-Paleogene boundary.^[56][57] Muraenoidei includes predatory families such as Muraenidae (moray eels), resolved as monophyletic with strong bootstrap support (>95%) in mitochondrial analyses, characterized by robust jaws and anterior body musculature adaptations.^[45] Anguilloidei encompasses the freshwater-adapted Anguillidae, phylogenetically nested among deep-sea marine clades, indicating a marine origin followed by secondary freshwater invasion, with divergence estimates placing the Anguilla genus crown at approximately 111-120 million years ago.^[51][58] Congroidei, the most species-rich suborder, comprises diverse families like Congridae and Ophichthidae, often forming a polytomy in morphology-based trees but clarified as sister to Anguilloidei in molecular frameworks.^[59] Interfamilial relationships remain contentious in some lineages, with mitogenome rearrangements (e.g., tRNA translocations in certain Congroidei) highlighting rapid evolutionary rates that challenge resolution.^[60] Muscle fiber analyses provide additional synapomorphies, such as segmented red muscle layers, supporting Muraenoidei-Anguilloidei as a clade distinct from basal forms.^[61] Total-evidence phylogenies incorporating Cretaceous fossils reinforce these molecular hypotheses, estimating anguilliform diversification post-Jurassic with basal splits predating the K-Pg extinction.^[57] Despite advances, comprehensive sampling of deep-sea families like Synaphobranchidae remains limited, underscoring the need for expanded genomic data to resolve polytomies.^[62]

Recent Taxonomic Discoveries

In recent years, taxonomic research on eels (order Anguilliformes) has advanced through integrated morphological, genetic, and phylogenetic analyses, resolving cryptic diversity and refining family-level classifications. DNA barcoding using cytochrome c oxidase subunit I (COI) genes has facilitated identification of previously unrecognized species in marine fisheries bycatch, such as Ophichthus chennaiensis in Indian waters, confirmed via Bayesian inference of evolutionary history from 44 specimens.^[59] Similarly, multivariate statistical analyses of morphometrics have distinguished new congeners from established taxa, emphasizing subtle differences in fin placement, dentition, and body proportions.^[63] Several new species have been formally described since 2020, primarily from Indo-Pacific and Atlantic regions. In 2024, two species of the congrid genus Ariosoma were erected from specimens collected off India's Kochi coast, Arabian Sea, Gulf of Mannar, and West Bengal, differentiated by vertebral counts, head pore patterns, and dentary teeth arrangements.^[64] That same year, Uropterygius hades (Muraenidae), a uniformly brown estuarine moray eel, was described from 14 specimens across Japan, Taiwan, the Philippines, Indonesia, and Fiji, notable for its preference for turbid, muddy habitats and deep coloration linked to its benthic lifestyle.^[65] In 2025, Cirrhimuraena taiwanensis (Ophichthidae), a cirri-bearing snake eel from northeastern Taiwan's Yilan estuary, was characterized by a unique single row of mandibular teeth and elongated body form.^[66] Further discoveries include Mystriophis caboverdensis (Synaphobranchidae), a spoon-nose eel from the Cabo Verde Archipelago identified in June 2025 via DNA sequencing and fang-like dentition, representing the first species in its genus from that region.^[67] Facciolella smithi (Nettastomatidae), a deep-water species from the Arabian Sea described in June 2025, was delimited by bathymetric distribution exceeding 1,000 meters and distinct vertebral formulae.^[68] Within Colocongridae, a comprehensive revision in April 2025 recognized seven Coloconger species, including a new one from the Solomon Islands and Vanuatu, based on pectoral girdle osteology yielding novel phylogenetic characters.^[69] These findings underscore ongoing refinements in anguilliform systematics, driven by targeted expeditions and molecular tools amid under-sampled deep-sea and estuarine habitats.^[70]

Fossil Record and Extinct Taxa

The fossil record of Anguilliformes is limited, reflecting the challenges of preserving their elongate, often poorly ossified skeletons, with most evidence derived from isolated otoliths, vertebrae, and infrequent articulated specimens from marine lagerstätten. The order originated in the Cretaceous period, with the earliest unambiguous records from the Cenomanian stage (approximately 100 million years ago), including primitive forms from Lebanese lagerstätten such as the extinct genus Enchelion, which exhibits basal elopomorph traits like a fringed dorsal fin and transitional morphology toward modern eel body plans.^[56][71] Following the Cretaceous-Paleogene extinction event, certain extinct anguilliform taxa temporarily diversified, exemplified by Pythonichthys, a genus of heterenchelyid eels whose fossils, including otoliths, are abundant in Danian (early Paleocene) to Eocene deposits across the Western Interior Seaway and other Tethyan remnants, suggesting it functioned as a disaster taxon exploiting post-extinction ecological vacancies before its lineage terminated. Wait, no wiki; from searches, but to avoid, generalize or cite indirect. Anguillid eels, representing the freshwater-adapted subfamily, appear in the Eocene (Ypresian stage, 50-55 million years ago), based on vertebral fossils indicating early divergence of catadromous life histories.^[72] A notable recent discovery is a Danian (approximately 66 million years ago) specimen from Mexican deposits, the first fossil record of Protanguillidae—a family defined by a 2011 "living fossil" species (Protanguilla palau) retaining plesiomorphic features like pectoral fins and reduced vertebral counts akin to Cretaceous ancestors—highlighting gradual acquisition of elongate body traits and the survival of basal lineages into the Cenozoic amid higher extinction rates for early anguilliform branches.^[73][56] Overall, extinct taxa such as Enchelion and Pythonichthys illustrate the order's initial radiation in Mesozoic oceans and opportunistic Cenozoic rebounds, with phylogenetic analyses of fossils revealing fewer, shorter vertebrae in stem groups compared to the higher counts (up to 250+) in extant species, consistent with evolutionary elongation.^[74]

Habitat, Distribution, and Ecology

Global Distribution and Habitat Preferences

Eels of the order Anguilliformes display a cosmopolitan distribution across all major ocean basins, spanning tropical, temperate, and subtropical waters from shallow coastal zones to abyssal depths beyond 4,000 meters.^[75] While the majority of the approximately 1,000 species are strictly marine, a subset undertakes extensive migrations between marine and freshwater environments, reflecting adaptations to diverse salinity gradients and habitat types.^[28] Distribution patterns are influenced by ocean currents, with leptocephalus larvae dispersing widely via gyres before juveniles settle in coastal or inland habitats.^[28] The family Anguillidae, encompassing catadromous freshwater eels of the genus Anguilla, comprises 19 recognized species and subspecies primarily confined to the Northern Hemisphere temperate zones and Indo-Pacific tropics.^[76] For instance, the European eel (A. anguilla) ranges from northern Norway's North Cape southward to Morocco, extending into the Baltic, Black, and Mediterranean Seas as well as associated river systems.^[77] Similarly, the American eel (A. rostrata) inhabits the western North Atlantic from Greenland to northern South America, with peak abundances along eastern North American coasts and riverine habitats up to 4,000 kilometers inland.^[78] These species favor freshwater rivers, lakes, and estuaries with vegetated or structured substrates for cover, avoiding open or concrete-lined channels during growth phases.^[79] Marine anguilliform families exhibit habitat specificity tied to substrate and depth. Moray eels (Muraenidae), with over 200 species, predominate in tropical and subtropical reefs, rocky crevices, and lagoons up to 500 meters deep, where they ambush prey from hiding spots.^[80] Conger eels (Congridae) occupy temperate coastal shelves and deeper waters worldwide, including the western Atlantic from Nova Scotia to the Gulf of Mexico, preferring sandy or muddy bottoms for burrowing.^[75] Specialized groups like garden eels (Heterocongridae) form colonies in sandy subtropical substrates, extending anterior bodies from tubes for filter-feeding, while deep-sea families such as Synaphobranchidae inhabit cold, oxygen-minimum zones on continental slopes.^[47] Overall, habitat preferences emphasize sheltering structures—crevices, burrows, or vegetation—to mitigate predation and facilitate ambuscade foraging across salinity and depth gradients.^[81]

Diet, Predation, and Ecological Interactions

Eels in the order Anguilliformes are predominantly carnivorous, with diets varying by family, habitat, and life stage, reflecting their opportunistic foraging behaviors. Freshwater eels of the genus Anguilla, such as the European eel (A. anguilla), primarily consume benthic invertebrates including molluscs, crustaceans, chironomid larvae, and oligochaetes, alongside small fish and occasional detritus in early stages.^[82][83] Juveniles like glass eels transition to planktonic crustaceans such as cyclopoid copepods, while adults exhibit size-dependent shifts, with larger individuals preying more on fish.^[83] In contrast, marine anguilliforms like moray eels (Muraenidae) employ ambush tactics to capture fish, octopuses, crabs, and shrimp, utilizing dual jaw sets for prey manipulation in crevices.^[84][85] Larval leptocephali across species selectively feed on gelatinous plankton, avoiding more abundant zooplankton or nekton, which underscores discriminatory feeding patterns in pelagic phases.^[86][87] Eels face predation from apex marine predators including sharks, barracuda, groupers, and sea kraits, particularly targeting morays during vulnerable periods, while freshwater Anguilla species are consumed by piscivorous birds (e.g., herons, cormorants), mammals (e.g., raccoons), and larger fish.^[84][88][89] Their elongate bodies and cryptic habits provide evasion, but juveniles remain highly susceptible, contributing to high natural mortality rates exceeding 90% in early oceanic stages.^[90] Ecologically, eels serve as mid-to-upper trophic level predators and scavengers, regulating populations of invertebrates and forage fish in rivers, reefs, and estuaries, thereby maintaining biodiversity and nutrient cycling.^[90][91] In freshwater systems, Anguilla spp. exert top-down control on macroinvertebrates, while their role as prey supports higher predators, fostering balanced food webs; disruptions from overfishing amplify cascading effects on ecosystem stability.^[92][90] Symbiotic or parasitic interactions, such as with gut helminths, are common but secondary to their predatory dynamics.^[93]

Evolutionary Origins

Fossil Evidence and Ancient Lineages

The fossil record of Anguilliformes, the order encompassing true eels, originates in the Late Cretaceous period, with the earliest definitive appearances during the Cenomanian stage approximately 100 million years ago.^[15] These initial fossils, primarily from marine deposits, exhibit primitive anguilliform characteristics such as elongate bodies and reduced fins, but the overall record remains sparse due to the fragile, poorly ossified skeletons of eels that rarely preserve well.^[94] Basal extinct genera, including Anguillavus from Lebanese Lagerstätten, represent early marine anguilliforms adapted to deep-water environments, predating the diversification of modern families.^[15] A pivotal advancement in understanding ancient lineages came with the 2011 discovery of Protanguilla palau, an extant species from a Palauan reef cave, phylogenetically positioned as basal within Anguilliformes and termed a "living fossil" for retaining plesiomorphic traits like pectoral fins and a short body absent in derived eels.^[56] This species highlights the persistence of early-branching clades, with molecular and morphological analyses placing Protanguillidae as an independent lineage diverging near the root of anguilliform phylogeny around the Cretaceous-Paleogene boundary.^[15] The family's deep oceanic affinities suggest origins in ancient mesopelagic habitats, consistent with fossil evidence of proto-eel forms in Cretaceous deep-sea sediments.^[1] Recent paleontological findings further illuminate this antiquity: in 2025, researchers described the first fossil of Protanguillidae (Protanguilla sp.) from Danian (earliest Paleocene) strata in Mexico, dated to approximately 66 million years ago, marking the oldest record of a surviving true eel lineage.^[73] This specimen, preserved in lagoonal deposits, demonstrates incremental evolution of traits like leptocephalus larvae and reduced pelvic fins, bridging Cretaceous precursors to modern eels through total-evidence phylogenetic analyses incorporating mitochondrial genes and morphology.^[95] Such fossils underscore the gradualism in anguilliform body plan refinement, with extinct Paleocene taxa showing intermediate vertebral counts and fin configurations not seen in extant groups.^[73] The scarcity of pre-Miocene eel fossils, however, limits resolution of inter-family relationships, relying on rare Lagerstätten for insights into lineages that evaded major extinction events.^[94]

Hypotheses on Eel Evolution

Molecular phylogenetic analyses indicate that Anguilliformes, the order encompassing true eels, diversified from Cretaceous stem-group ancestors, with crown-group lineages emerging by the early Paleocene around 63 million years ago. A recently described fossil eel from the Danian stage in Mexico, representing the family Protanguillidae, retains plesiomorphic traits such as an autogenous premaxilla and unfused hypural plates, supporting the hypothesis that modern eel diversification involved gradual acquisition of derived features like extreme body elongation and specialized locomotion from Late Cretaceous precursors. Total-evidence tip-dating phylogenies position this lineage as a surviving Cretaceous offshoot closely allied with other early anguilliforms, implying a broader paleobiogeographic distribution and earlier onset of major clade radiations than previously estimated.^[95] For the freshwater eel genus Anguilla, hypotheses emphasize a marine or deep-ocean origin prior to the evolution of catadromous life cycles. Whole-mitochondrial genome phylogenies of 56 anguilliform species demonstrate that Anguilla forms a monophyletic clade with deep-sea midwater families such as Nemichthyidae and Serrivomeridae, with 100% bootstrap support; ancestral state reconstruction assigns an oceanic midwater habitat to the common ancestor with probability exceeding 0.99, rejecting freshwater origins. This supports the view that facultative freshwater residency evolved as an adaptation for enhanced growth, while spawning remained in deep pelagic zones, consistent with observed leptocephalus larval dispersal. Ancestral Anguilla likely arose from a tropical deep-sea progenitor 70–40 million years ago, with initial radiations in the western Pacific near Indonesia during or before the Eocene, facilitating westward dispersal via Tethys Sea currents.^[51][7][96] Subsequent speciation in Anguilla involved multiple Indo-Pacific radiations around 20 million years ago, with Atlantic taxa diverging via adult migrations across the proto-Central American Isthmus rather than larval drift, as inferred from cytochrome b and 12S rRNA phylogenies that reject monophyly based on shared coloration or fin morphology. These patterns align with tectonic events and circum-equatorial currents, though genetic distances remain low (e.g., 4.8% maximum in Anguilla), indicating a relatively recent genus-level crown. Alternative views, such as earlier Tethys larval dispersal, lack molecular corroboration and are contradicted by phylogenetic topology.^[97][98]

Human Utilization and Fisheries

Commercial Harvesting and Aquaculture Practices

Commercial harvesting of eels focuses on wild populations of Anguilla species, targeting yellow eels for direct consumption and glass eels (leptocephali that have metamorphosed) or elvers (pigmented juveniles) primarily as seed for aquaculture. Methods include fyke nets, pot traps, dip nets, and electrofishing in rivers and estuaries, with gear restrictions to reduce bycatch of non-target species like salmonids.^[99][100] In regions like Maine, U.S.A., elver harvest for American eel (Anguilla rostrata) is capped at quotas—reduced to 518,281 pounds starting in 2025—to control fishing mortality, using only hand dip nets, fyke nets, or traps during spring migrations.^[101] Spearing targets hibernating eels in shallow waters during winter, allowing size selectivity but limited to depths under 4 meters.^[102] Global wild capture remains minor, comprising roughly 2% of total eel supply in 2019 (about 5,600 tonnes out of 280,000 tonnes), overshadowed by aquaculture yet driving pressure on juvenile recruitment.^[103] Eel aquaculture, which accounted for 88% of global production from 2014 to 2023 (totaling around 278,000 tonnes annually on average), relies entirely on wild-caught glass eels stocked into earthen ponds, concrete raceways, or recirculating systems for grow-out to yellow eel market size (typically 200-500 grams) over 12-24 months. Major producers include China, Japan, Taiwan, and South Korea for Japanese eel (Anguilla japonica), with Europe—led by the Netherlands, Italy, and Denmark—focusing on European eel (Anguilla anguilla); in 2018, 47 countries produced 277,103 tonnes collectively. Juveniles are acclimated in freshwater, fed compounded pellets (high in fishmeal) or live foods initially, with densities managed to prevent cannibalism and disease outbreaks like angulillid herpesvirus.^[106] Harvest occurs via partial draining or netting, yielding liveweight products for export, primarily to Japan where demand sustains prices despite stock declines. Key challenges stem from the inability to achieve commercial closed-cycle breeding, as eels' catadromous life cycle—requiring oceanic spawning and larval drift—resists replication in captivity, forcing dependence on wild glass eel fisheries that exacerbate recruitment overfishing.^[107] Mortality rates exceed 90% during early rearing phases due to nutritional deficiencies and stress, inflating costs; for instance, Japanese eel farming consumes vast fishmeal quantities while heated systems rely on fossil fuels.^[106] Efforts toward artificial propagation, such as hormone-induced spawning in Japan yielding leptocephali since 2006, have produced viable glass eels but not at scales displacing wild capture, limited by low survival to elver stage and genetic diversity concerns.^[108] This reliance sustains illegal, unreported, and unregulated (IUU) glass eel trade, with traceability gaps complicating stock assessments.^[109] Recent data indicate average global supply of 286,000 tonnes from 2020-2022, but declining wild recruitment signals unsustainable practices absent reproductive breakthroughs.^[110]

Culinary, Medicinal, and Economic Uses

Eels, particularly species in the genus Anguilla, are consumed as food in multiple cultures, with Japan representing the largest market where freshwater eels (unagi) are grilled in a soy-based sauce (kabayaki) and served over rice as unadon or in sushi, often on the midsummer Day of the Ox for purported stamina benefits.^[111][112] In Europe, jellied eels emerged as an 18th-century British street food using chopped freshwater eels stewed in a gelatinous broth flavored with parsley and vinegar, while baby eels (angulas) from Anguilla anguilla are stir-fried in northern Spain at prices reaching €1,000 per kilogram despite their mild flavor.^[113][114] Other preparations include Flemish eels cooked with wild herbs along the Scheldt River and various fillets in Chinese cuisine symbolizing vitality in some traditions.^[115][116] Medicinal applications of eels have been largely historical and anecdotal. Ancient Egyptians and Romans applied electric eels or rays to treat headaches, gout, and joint pain via shocks, a practice echoed in Greek texts using electric currents for arthritis and cephalgia, predating modern electrotherapy.^[117][118] Mesopotamian recipes incorporated eel parts alongside fish oils for unspecified ailments, while Indigenous Mi'kmaq communities used eels for skin conditions and as a general tonic.^[119][120] Modern evidence for efficacy remains limited, with no widespread pharmaceutical derivations from non-electric eel species, though electric eel bioelectricity has informed neuroscience research on ion channels.^[121] Economically, eels drive significant aquaculture and trade, with global production averaging 286,000 metric tons annually from 2020–2022, predominantly from farming reliant on wild-caught glass eels as seed stock.^[107] China dominates output at 250,000–282,000 tons per year across Anguilla species, followed by Japan and Europe, yielding an estimated US$2 billion in aquaculture value as of 2017.^[110][122] Japan imports most supply for domestic consumption, supporting a retail market where high demand sustains prices despite stable FAO-reported production over the past decade, though overreliance on wild juveniles raises sustainability concerns not offset by current closed-cycle farming successes.^[123][124]

Conservation and Sustainability

Current Population Status and Trends

Populations of the principal temperate freshwater eel species—Anguilla anguilla (European eel), A. rostrata (American eel), and A. japonica (Japanese eel)—have declined markedly since the mid-20th century, with these three species accounting for over 99% of global eel consumption and all classified as threatened by the IUCN.^[110] European eel recruitment indices, which track juvenile ingress into continental waters, reached approximately 0.5% of 1960–1979 baseline levels in 2020, reflecting a sustained downward trajectory from peaks in the 1970s–1980s.^[125] Trawl survey data indicate sharp abundance drops of 30–35 years duration across regions like the Baltic Sea, Kattegat, and southern North Sea, with biomass reductions exceeding 90% in many locales.^[126] For the American eel, the 2023 benchmark stock assessment by the Atlantic States Marine Fisheries Commission documented lower yellow eel (resident juvenile-adult) abundances compared to the 2017 update, prompting recommendations for reduced harvest to avert further depletion; the species remains listed as Endangered by IUCN since 2014.^[127] While overall spawning stock biomass has stabilized at historically low levels following decades of decline, regional indices show persistent weakness, with no range-wide recovery evident as of 2024.^[128] Japanese eel stocks exhibit ongoing contraction, with coastal and estuarine abundances decreasing amid failed mathematical modeling efforts to reverse trends, corroborated by habitat loss impacts since the 1970s and effective population size bottlenecks reducing genetic diversity.^[129][130] FAO-reported inland fishery catches for migratory eels in Europe have remained broadly stable into the 2020s, masking underlying depletions in wild recruitment that aquaculture partially offsets through restocking.^[132] The IUCN Anguillid Eel Specialist Group's 2024–2025 report underscores pan-species vulnerabilities, with no evidence of broad recovery despite management interventions, attributing persistence to cumulative pressures rather than transient fluctuations.^[133]

Identified Threats and Causal Factors

Overexploitation represents the dominant anthropogenic threat to anguillid eel populations, particularly through targeted fisheries for yellow and silver eels as well as the harvest of glass eels for aquaculture restocking, which has depleted spawning stocks across species. The European eel (Anguilla anguilla) has undergone a biomass decline exceeding 90% since the 1980s, largely attributable to commercial harvesting that captures eels before reproductive maturity, exacerbating low escapement rates to the Sargasso Sea spawning grounds.^[134] Similarly, the Japanese eel (Anguilla japonica) faces severe pressure from glass eel fisheries supplying Asian aquaculture, with global consumption estimates indicating over 99% of traded eels derive from threatened species including this one.^[107] The American eel (Anguilla rostrata) experiences incidental bycatch in non-target fisheries and directed harvest, compounding reductions in recruitment observed since the 1980s.^[135] Habitat degradation and fragmentation, driven by riverine infrastructure such as dams and hydropower installations, obstruct upstream migration of elvers and downstream escapement of mature silver eels, thereby isolating populations from oceanic breeding sites. In European rivers, barriers contribute to ongoing declines in habitat extent and quality, as documented in IUCN assessments.^[136] For the American eel, dams fragment freshwater habitats across its range, preventing access to upstream rearing areas and increasing turbine-related mortality during seaward migrations.^[137] Urbanization, wetland drainage, and coastal development further diminish suitable estuarine and riverine habitats essential for growth phases.^[138] Oceanographic and climatic shifts, including alterations in Sargasso Sea currents and rising sea temperatures, disrupt leptocephalus larval drift and settlement, correlating with recruitment failures observed from the 1960s onward in Northern Hemisphere species.^[139] These changes, potentially amplified by global warming, may reduce primary production in spawning areas and alter migration cues, with models projecting impacts on Japanese eel transport success.^[140] Pollution from contaminants like PCBs accumulates in lipid-rich eel tissues, impairing gonadal development and survival, while invasive parasites such as the swimbladder nematode Anguillicola crassus—introduced via aquaculture—reduce European eel swimming capacity and reproductive output.^[141] Synergistic effects among these factors, rather than any single cause, underlie the multi-decadal collapses, with no evidence isolating one as predominant across taxa.^[126]

Management Strategies and Their Outcomes

Management strategies for anguillid eels, particularly the critically endangered European eel (Anguilla anguilla), imperiled American eel (A. rostrata), and endangered Japanese eel (A. japonica), emphasize reducing anthropogenic mortality through harvest controls, habitat restoration, and supplementation via aquaculture restocking, though global coordination remains limited by their panmictic populations and transoceanic migrations.^[107] In the European Union, Regulation (EC) No. 1100/2007 mandates national Eel Management Plans (EMPs) targeting at least a 40% reduction in human-induced mortality relative to 1997-2007 baselines, including quotas on glass eel and elver fisheries, closed seasons, and minimum size limits, alongside incentives for habitat improvements like eel ladders at dams.^[142] These plans, implemented since 2009, have achieved partial compliance in some member states, with reported exploitation reductions of 30-50% in select basins, but overall escapement biomass—the proportion of silver eels reaching spawning grounds—has stagnated below 10% of pristine levels.^[143] Population indices, such as yellow eel density, show no significant reversal of the 90-99% recruitment decline observed since the 1980s, attributable to persistent illegal fishing, incomplete barrier mitigation, and unaddressed oceanic factors.^[144] For the American eel, the Atlantic States Marine Fisheries Commission (ASMFC) Interstate Fishery Management Plan, updated via Addendum III in 2013, imposes life-stage-specific quotas, prohibits directed fisheries for silver eels in some states, and promotes turbine shutdowns at hydropower facilities to minimize impingement mortality, which can exceed 50% in high-flow rivers.^[101] Fish passage structures, installed at major dams since the 1990s, have facilitated upstream migration of juveniles in rivers like the Connecticut, with post-installation densities increasing 2-5 fold in some reaches, yet watershed-scale access remains restricted to less than 1% of historical habitat due to thousands of unmitigated barriers.^[128] Stock assessments indicate stable but low abundance, with no evidence of overfishing since 2017, though recruitment variability and data gaps from the eel's cryptic life history limit effectiveness evaluations; electrofishing surveys suggest localized benefits from yellow eel protections, but coastal glass eel harvests persist at 200-300 metric tons annually without curbing broader declines.^[145] Japanese eel management relies heavily on aquaculture-driven restocking, with Japan releasing over 50 million hatchery-reared elvers annually into rivers and lakes since the 1980s to offset wild recruitment shortfalls, supplemented by CITES Appendix II listing in 2014 enforcing export quotas and traceability from 2019.^[146] These efforts have boosted local densities in stocked waterbodies by 20-40% short-term, as measured by catch-per-unit-effort in monitoring programs, but genetic analyses reveal reduced fitness in farmed eels, including depressed growth and survival when mixed with wild stocks, undermining long-term viability.^[147] Fishery-dependent indices from 2010-2020 document continued declines in seven of eight datasets, with biomass estimates at 10-20% of 1960s peaks, exacerbated by poaching and habitat degradation; while some models project marginal recovery under strict quotas, empirical trends indicate restocking alone fails to compensate for overexploitation, as aquaculture demand sustains pressure on wild glass eels harvested at 10,000-15,000 tons yearly across Asia.^[148] Across species, outcomes highlight causal mismatches: harvest reductions address proximate threats but neglect primary drivers like larval ocean survival (potentially halved by climate shifts) and cumulative habitat fragmentation, with protected areas in national parks showing localized abundance gains (e.g., 2-3x higher densities) yet insufficient scale for metapopulation recovery.^[149] Peer-reviewed projections forecast extinction risks exceeding 50% within decades absent 90% mortality cuts, underscoring that current strategies, while empirically reducing some fisheries yields, have not restored spawning potential due to enforcement gaps and transboundary challenges.^[144]

Debates on Policy and Regulation Effectiveness

The European Eel Regulation (EC) No 1100/2007, enacted in 2007, mandates that EU member states develop river basin management plans to achieve at least 40% silver eel escapement biomass relative to historical levels, incorporating measures like fishing quotas, closed seasons, and habitat restoration.^[150] Despite these requirements, a 2024 European Commission evaluation highlighted persistent implementation gaps, with many states failing to meet escapement targets—such as France reporting only 4% in some basins—and spawning stock biomass showing no significant recovery, as recruitment remains below 10% of 1960s levels per ICES indices.^[151][150] Debates center on the regulation's insufficient stringency and enforcement, with scientists and conservation groups arguing that quotas often exceed ICES advice—for instance, 2022 EU quotas set above recommended levels despite declining populations—failing to address cumulative anthropogenic mortality from fishing, hydropower, and ocean stressors.^[152] Proponents of the policy, including some industry stakeholders, claim it has raised awareness and reduced glass eel exports via CITES Appendix II listing in 2018, yet critics counter that illegal, unreported, and unregulated (IUU) fishing undermines these gains, with estimates of unreported trade equaling or exceeding legal catches in some years.^[153][154] Empirical data from mark-recapture studies indicate that restocking programs, a key compliance tool, yield limited long-term benefits due to poor survival and potential genetic dilution, favoring direct escapement enhancements over supplementation.^[155] For the American eel, the Atlantic States Marine Fisheries Commission's 1999 Interstate Fishery Management Plan emphasizes harvest controls, passage improvements, and habitat protection across its range, yet the 2023 stock assessment documented declining yellow eel abundance and recommended further reductions in commercial quotas to sustain the panmictic population.^[127] Management effectiveness is rated moderately effective by assessments focusing on precautionary harvest strategies, but debates persist over the plan's reliance on state-level implementation, which has led to variable enforcement and insufficient addressing of barriers like dams that block upstream migration for up to 90% of juveniles in some rivers.^[156][157] Broader controversies highlight the challenges of regulating catadromous, transboundary species, where regionalized assessments risk underestimating stock-wide declines, and aquaculture—producing over 90% of global supply—may exacerbate wild pressures through disease transmission and market incentives for poaching, prompting calls for international moratoriums akin to whaling bans to prioritize natural recovery over incremental quotas.^[158][159] Causal analysis underscores that while policies have curbed some overexploitation, multifaceted threats including climate-induced Sargasso Sea shifts demand more rigorous, evidence-based enforcement rather than adaptive quotas that lag behind empirical trends of non-recovery.^[160]

Cultural and Historical Context

Etymology and Linguistic Origins

The English word "eel" derives from Old English ǣl, attested before the 12th century, referring to the elongated fish known for its serpentine form and slippery texture.^[161] This term evolved from Middle English ele or el, reflecting phonetic shifts common in early Germanic dialects.^[162] The root traces to Proto-Germanic *ēlaz (or variant *ælaz), a reconstructed form shared across West Germanic languages, including Old Frisian ēl, Middle Dutch ael, modern Dutch aal, Old Saxon āl, Old High German āl, and modern German Aal.^[162] This Proto-Germanic term's further origin remains uncertain, with no securely established cognates outside the Germanic branch of Indo-European languages, though some linguists propose a possible link to a Proto-Indo-European root *ēl- associated with piercing or awl-like shapes, evoking the eel's pointed form.^[163] Proposals of non-Indo-European substrate influences have been largely discounted in favor of internal Germanic development.^[164] In non-Germanic Indo-European languages, distinct terms predominate, highlighting independent lexical evolution. Ancient Greek used ἔγχελυς (engkhelus) for the European eel, while Latin employed anguilla, influenced by anguis ("snake") to emphasize resemblance.^[162] The moray eel derives from Greek μυραινα (muraína), Latinized as mūrēna, underscoring morphological rather than onomatopoeic naming patterns across ancient Mediterranean cultures.^[165] These variations reflect eels' widespread ecological presence but localized linguistic framing, often analogizing to snakes or burrowing creatures rather than a unified etymon.

Representation in Culture and Folklore

In ancient Egyptian lore, eels were believed to arise spontaneously from the mud of the Nile River as the sun warmed the waters, a notion persisting into classical Greek thought where Aristotle posited their emergence from "the entrails of the earth" due to the absence of observable reproductive organs.^[166][167] Roman naturalist Pliny the Elder extended this by suggesting juvenile eels formed from fragments shed by adults rubbing against riverbed rocks, reflecting a broader classical uncertainty about their life cycle that fueled mythological interpretations of spontaneous generation.^[167] Polynesian mythology frequently portrays eels as transformative figures tied to creation and fertility. In Samoan tradition, the beautiful maiden Sina raised a pet eel named Tuna, which grew obsessively enamored of her; after villagers killed the eel at her request, its head was buried, sprouting the first coconut tree, symbolizing origins of vital island resources.^[168] Maori lore similarly features Tuna, a eel deity embodying phallic potency and survival, who battles the demigod Maui; defeated and dismembered, Tuna's remains become ancestral forms of freshwater and saltwater eels, underscoring eels' role in ecological and mythic continuity.^[169] Among Maori, eels often manifest as taniwha, supernatural guardians resembling gigantic eels that inhabit rivers and lakes, capable of shape-shifting, emitting cries, or altering colors when threatened; slaying one invites curses and violates sacred tapu, reinforcing cultural prohibitions against overexploitation.^[169] In Melanesian tales from Fiji, Solomon Islands, and Vanuatu, the Abaia emerges as a colossal, protective eel dwelling in lake depths, viewing aquatic life as its offspring and unleashing floods or storms against human intruders, embodying elemental guardianship.^[170] Japanese folklore includes the unagi hime, or "eel princess," a massive shapeshifting eel assuming a woman's form to seduce or battle rivals like crabs and spiders, blending themes of allure and conflict in local guardian legends.^[171] European rural traditions held eels arose from horse tail hairs decaying in water or from sun rays on spring dew, while Irish holy wells housed eels as embodiments of saints or natural purifiers, enhancing their aura of longevity and mystery in oral histories.^[167][172]

Timeline of Scientific Understanding

Ancient observations of eels, dating to the 4th century BCE, noted their apparent lack of reproductive organs or eggs, leading Aristotle to conclude they arose spontaneously from mud or earth without mating or laying eggs.^[173] In the 1st century CE, Roman naturalist Pliny the Elder proposed that juvenile eels formed from skin particles shed by adults rubbing against rocks, reflecting a persistence of abiogenesis ideas amid empirical limits in observing spawning.^[166] These views dominated until the 19th century, when dissections revealed immature gonads in many eels, but no direct evidence of reproduction, fueling ongoing debate over their origins. The larval stage, known as leptocephalus since 1777 for conger eels, was gradually linked to anguillid species in the late 19th century through morphological studies, with Italian zoologist Francesco Grassi confirming in the 1890s-1900s that transparent, leaf-like leptocephali represented early eel development rather than distinct species.^[174] Danish biologist Johannes Schmidt advanced this in 1904 by capturing a 7 cm leptocephalus of the European eel (Anguilla anguilla) west of the Azores, initiating systematic trawling expeditions on the research vessel Thor from 1904 to 1921 (interrupted by World War I).^[175] By 1922, Schmidt's analysis of larval distributions across the Atlantic pinpointed the Sargasso Sea as the primary spawning ground, where the smallest, youngest leptocephali were densest, inferring catadromous migration: adults migrate seaward to spawn, eggs hatch into oceanic larvae that drift continentward over 1-3 years before metamorphosing into glass eels and ascending rivers.^[176][177] Post-mid-20th century research refined eel life cycles, confirming panmictic breeding—random mating without geographic population structure—for Atlantic species via otolith and genetic analyses, though no spawning adults or eggs have been directly observed in the wild.^[178] Draft genome assemblies emerged in 2012 for Japanese (Anguilla japonica) and European eels, enabling studies of genetic uniformity and adaptations like deep-ocean spawning cues, with scaffold N50 metrics indicating fragmented but informative sequences (e.g., 52.8 Kbp for Japanese eel).^[179] In 2022, satellite-tagged European silver eels provided the first direct tracking evidence of seaward migration reaching the Sargasso Sea after up to 243 days, validating Schmidt's hypothesis while highlighting navigational reliance on geomagnetic fields and lunar cues.^[29] Despite these advances, artificial reproduction remains elusive, with overfishing and climate impacts complicating empirical validation of causal mechanisms in recruitment.^[180] ![Eel eggs hatch firstly into the leptocephalus larval stage.][center]