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Notacanthidae
Notacanthidae
from Wikipedia

Notacanthidae
Temporal range: Santonian–Recent [1]
Snubnosed spiny eel, Notacanthus chemnitzii
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Notacanthiformes
Family: Notacanthidae
Rafinesque, 1810[2]
Genera

see text

Notacanthidae, the deep-sea spiny eels, are a family of fishes found worldwide below 125 m (410 ft), and as deep as 3,500 m (11,500 ft).

Their bodies are greatly elongated, though more tapered than in true eels. The caudal fin is small or nonexistent, while the anal fin is lengthy, as long as half of the total body length. They feed on animals attached to or living on the sea floor, such as sea anemones, echinoderms, molluscs, and worms.[3]

Although not true eels, these fish do have a similar leptocephalus larval form. However, while the larvae of true eels are about 5–10% of the length of the adult, those of deep-sea spiny eels can grow considerably larger than the adult, and shrink when they develop into their final form. Thus, while adults range from 20 cm (7.9 in) to 1.2 m (3.9 ft) in length, larvae of up to 1.8 m (5.9 ft) have been recorded.[3]

Genera

[edit]

Notacanthidae includes the following extant genera, although Tilurus is known only from leptocephalus larvae:[4]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia
from Grokipedia
Notacanthidae, commonly known as deep-sea spiny eels, is a family of ray-finned fishes in the order Notacanthiformes, characterized by an elongated, eel-like body bearing a series of prominent dorsal spines along the anterior portion of the back, a positioned posterior to the , and small scales arranged in over 50 longitudinal rows per side. These benthic marine fishes inhabit deep waters globally, typically at depths ranging from 125 to 3500 meters, where they adopt a head-down foraging posture to feed primarily on polychaetes, coelenterates, and small nektonic crustaceans. The family comprises four genera—Notacanthus, Polyacanthonotus, Lipogenys, and Tilurus—encompassing approximately 15 valid species, many of which are distributed across continental slopes, seamounts, and abyssal plains in all major ocean basins. Notacanthids exhibit , with females generally outnumbering males in populations, and they reproduce oviparously as nonguarders, producing large eggs that develop into distinctive giant larvae—transparent, leaf-like forms that can exceed the size of adults and drift in surface waters before metamorphosing. is positively correlated with body size, and ovaries often contain two batches of eggs, with spawning occurring seasonally, such as from March to May in some Atlantic populations, where ova diameters range from 1.35 to 2.0 mm. As part of the superorder Elopomorpha, closely related to true eels (Anguilliformes), notacanthids have a fossil record dating back to the upper , highlighting their ancient lineage in deep-sea ecosystems.

Taxonomy

Etymology and history

The genus name Notacanthus derives from the Greek words nōton (back) and akantha (spine or thorn), alluding to the isolated dorsal-fin spines positioned along the back of these fishes. The family name Notacanthidae was established by in 1810 in his work on Sicilian , distinguishing these deep-sea fishes as a separate group based on their unique morphology. The genus Notacanthus was first described by in 1788 within his comprehensive Naturgeschichte der ausländischen Fische, where he introduced the type species Notacanthus chemnitzii based on specimens from the North Atlantic. Early descriptions often led to confusion with true eels (order Anguilliformes) owing to the highly elongate, snake-like bodies of notacanthids, despite fundamental anatomical differences such as the presence of dorsal spines and the absence of a true caudal fin. Key advancements in the included Pieter Bleeker's 1874 description of the Polyacanthonotus, named for the numerous (26–41) dorsal-fin spines along the back (poly meaning many in Greek), which expanded recognition of diversity within the family. In 1895, George Brown Goode and Tarleton Hoffman Bean introduced the Lipogenys in their revision of deep-sea fishes, highlighting species with a notably reduced lower (lipo meaning lacking in Greek), further refining the taxonomic framework. A pivotal milestone occurred in 1853 when Rudolf linked the enigmatic larvae to adult notacanthids by describing the genus Tilurus for a larval form (Tilurus gegenbauri), demonstrating from the leaf-like, transparent leptocephali to the elongate adults. This connection, based on specimens from , resolved long-standing mysteries about the life cycle of these deep-sea fishes and solidified their recognition as a distinct family within Elopomorpha by the mid-19th century.

Classification

Notacanthidae is classified within the superorder Elopomorpha, order Notacanthiformes, as one of two families in this order alongside Halosauridae. In older taxonomic systems, such as those outlined in , Notacanthidae was placed as the suborder Notacanthoidei within the broader order Albuliformes, reflecting historical groupings based on shared morphological traits like reduced caudal fins. However, molecular phylogenies from the early have elevated Notacanthiformes to ordinal status, confirming its distinct position as a basal elopomorph lineage sister to Anguilliformes. Phylogenetically, Notacanthidae forms the to Halosauridae within Notacanthiformes, with the order's strongly supported by both morphological synapomorphies—such as an elongate body, reduced , and specialized spines—and molecular data from mitochondrial and nuclear genes. Studies using multi-locus analyses, including 12S rRNA and mitogenomic sequences, have reinforced this relationship, tracing the divergence of modern notacanthid genera to the Paleocene-Eocene boundary around 55-60 million years ago. These 21st-century investigations, building on earlier morphological work, highlight Notacanthiformes as an ancient, deep-sea adapted within Elopomorpha, distinct from more derived groups like Anguilliformes. The family comprises four genera—Lipogenys, Notacanthus, Polyacanthonotus, and Tilurus (the latter known primarily from larvae)—encompassing approximately 15 valid species as of 2025. Recent taxonomic revisions have expanded this diversity, including the description of Notacanthus laccadiviensis from the in 2023, Notacanthus arrontei from the northeastern Atlantic in 2024, based on morphometric and genetic analyses distinguishing it from congeners like N. bonaparte, and Lipogenys hyalinumvelum from Portuguese waters in 2024, identified by unique membranes on its and meristic differences from L. gillii. These additions reflect ongoing efforts to resolve cryptic diversity in deep-sea notacanthids using integrated morphological and molecular approaches.

Description

Body structure

Notacanthids possess an elongate, cylindrical body that tapers gradually toward a pointed , giving them an overall eel-like appearance adapted to navigating deep-sea environments. The head is small, depressed, and features an inferior mouth positioned ventrally to facilitate bottom-feeding. Unlike many teleosts, they lack a true caudal ; the posterior end of the body instead terminates in a slender filament, which enhances flexibility in confined benthic habitats. The skin is tough and leathery, covered by small, embedded scales arranged in numerous longitudinal rows—typically more than 50 per side—that contribute to the body's streamlined profile and protection against abrasion on the seafloor. These scales form subtle longitudinal ridges along the body, aiding in structural reinforcement without impeding movement. Pectoral fins are present, though reduced in certain , reflecting variations in locomotor demands. Internally, notacanthids exhibit adaptations suited to their deep-sea lifestyle, including a reduced that provides limited control under high hydrostatic pressures. The intestine is simple and elongated, optimized for processing benthic with minimal digestive complexity. Skeletal features emphasize flexibility, with an exceptionally high number of vertebrae—often exceeding 200, and up to 239 in some —allowing the body to bend and maneuver effectively in low-light, sediment-rich conditions.

Fins and sensory features

Members of the Notacanthidae family exhibit a specialized fin morphology that reflects their to deep-sea benthic environments. The is single, short-based, and composed exclusively of 6–15 strong, isolated spines without any soft rays, a feature that originates the vernacular name "spiny eels" due to the prominent, thorn-like dorsal elements. These spines are stout and posteriorly directed, providing structural support rather than flexibility, and are typically positioned posterior to the for balanced locomotion in a head-down posture. The pectoral fins are small and positioned high on the body sides, with ray counts varying by species but generally fewer than 12, facilitating subtle maneuvering near the seafloor. Pelvic fins are reduced or absent; when present, they consist of 1–3 spines and 5–7 soft rays located ventrally and closer to the anus than the pectoral fins. The anal fin is elongate and low-profile, spanning much of the ventral posterior body with 92–136 soft rays, enabling undulatory propulsion alongside the elongate body form. The caudal fin is rudimentary or entirely absent, with the tail often terminating in a pointed or filamentous extension that supports regeneration but minimal steering function. Sensory adaptations in Notacanthidae are tuned for the dim, structured deep-sea habitat. The eyes are notably large relative to head size, enhancing sensitivity to bioluminescent cues and residual light in low-illumination conditions. The system is prominent and well-developed, running along elevated body ridges high on the lateral profile rather than in cavernous canals, which allows detection of subtle hydrodynamic disturbances from prey or currents despite the absence of protective scale tubes. This configuration, combined with the family's elongate body, optimizes navigation and prey localization in visually obscured environments.

Distribution and habitat

Geographic range

Notacanthidae, commonly known as deep-sea spiny eels, display a in temperate to polar marine waters across the world's major ocean basins. The family is recorded in the Atlantic Ocean (including the ), the region, the , and Arctic regions. This widespread occurrence reflects their adaptation to deep benthic habitats in these areas. The Atlantic Ocean hosts the greatest diversity of Notacanthidae species, with several taxa concentrated in the North Atlantic, such as Notacanthus chemnitzii, which ranges from the western North Atlantic to the eastern Atlantic and Mediterranean. In contrast, the features species like Notacanthus indicus, restricted to the western including the and . Representation in the eastern Pacific is limited, with only rare occurrences, exemplified by Notacanthus spinosus in the eastern central Pacific. The includes records of larvae from waters, potentially associated with the Tilurus. Endemism within Notacanthidae is evident in certain species confined to specific basins or geological features, such as Lipogenys gillii, which is primarily restricted to the western North Atlantic from to and occasionally reported from Australian waters. Other taxa show localized distributions on seamounts and continental slopes, contributing to regional patterns without broad overlap across oceans.

Depth preferences and ecology

Members of the Notacanthidae family primarily inhabit deep-sea environments, with a vertical distribution ranging from upper bathyal depths of approximately 125 m to abyssal depths exceeding 3,000 m, and a maximum recorded depth of around 3,500 m. This range encompasses bathyal (200–2,000 m) and abyssal (2,000–4,000 m) zones globally, though some species occur on upper continental slopes at 100–500 m. They are predominantly found in marine settings, exhibiting a worldwide oceanic presence across continental slopes, seamounts, and abyssal plains. These fishes are benthic or demersal, closely associated with soft bottoms consisting of or , where they maintain a head-down orientation while hovering or swimming near the substrate. In regions like the , they occupy middle and lower slope habitats with stable sedimentary environments, showing adaptations to low-oxygen conditions through elevated activity that supports anaerobic (3.72–8.75 µmol/min/mg protein in key ). Their distribution and behavior are influenced by bottom currents and sediment stability, which facilitate the vertical of essential for deep-sea ecosystems. Ecologically, Notacanthidae play roles as infaunal predators and within benthopelagic communities, contributing to nutrient cycling on the seafloor despite their generally low abundance. Populations remain stable in pristine or lightly impacted areas, with higher densities at preferred depth centers (e.g., around 900–1,700 m in the Mediterranean), though females significantly outnumber males across the family. Their presence underscores resilience in oligotrophic deep-sea habitats, where they form a consistent, albeit minor, component of demersal assemblages.

Biology

Feeding and diet

Members of the Notacanthidae family primarily consume benthic , with diet composition varying by and reflecting their to deep-sea environments. Polyacanthonotus rissoanus preys on small epibenthic and suprabenthic crustaceans, including mysids, isopods such as Munnopsurus atlanticus, and amphipods like Rhachotropis caeca, alongside polychaetes from families such as (e.g., Harmothoe spp.). Priapulids, gastropods, and foraminiferans also contribute to its diet, particularly at depths exceeding 1400 m where resources are scarcer, leading to greater dietary diversification. In contrast, Notacanthus bonapartei targets sessile and low-mobility benthic fauna, with echinoderms (e.g., ophiuroids like Penilpidia ludwigi and asteroids such as Hymenodiscus coronata) forming a dominant component, supplemented by actinians, sponges, bryozoans, and uniquely, polyps of corals (Isididae, e.g., Isidella elongata). Larger individuals occasionally incorporate motile prey such as amphipods or small fish, though infaunal and epifaunal dug from sediments remain central. Gut content analyses across notacanthids highlight a consistent emphasis on polychaetes and small crustaceans, often comprising the majority of in sampled specimens. Feeding strategies are opportunistic, with prey selection influenced by depth and seasonal flux of from surface productivity; for instance, gut fullness in both P. rissoanus and N. bonapartei correlates with concentrations in overlying waters. Their protrusible mouths equipped with small, pointed teeth enable grasping and extraction of infaunal prey from burrowed positions in soft sediments, supporting a largely benthic mode. Trophically, notacanthids function as mid-level predators in deep-sea ecosystems, with estimated trophic levels around 3.9 based on dietary composition. Low metabolic rates, evidenced by reduced activity (e.g., 3.72–8.75 µmol/min/mg protein in N. bonapartei), imply infrequent feeding intervals suited to sparse resources. Stable isotope analyses further affirm their heavy reliance on benthic food webs, with δ¹³C and δ¹⁵N signatures indicating incorporation of sediment-derived over pelagic inputs.

Reproduction and development

Notacanthidae are oviparous, producing pelagic eggs that undergo in the water column. They exhibit batch spawning, with ovaries containing two distinct cohorts of eggs released over a protracted season, allowing for multiple spawning events per reproductive cycle. is relatively low for deep-sea fishes, typically ranging from 1000 to 5000 eggs per female, and shows a strong positive correlation with body weight, as demonstrated by analyses in studies of species such as Notacanthus bonapartei and Polyacanthonotus rissoanus. Sexual maturity varies by species, with males and females showing differences in size and age at maturity; for example, in Notacanthus chemnitzii, the minimum length for mature females is approximately 55 cm (at 18 years), while males reach maturity around 66 cm (at 14 years). Spawning occurs in deeper waters, often below 1000 m, and varies by region: year-round in tropical areas due to stable conditions, but seasonal in temperate zones, such as in the Mediterranean for N. bonapartei. This aligns with environmental cues like gradients in habitats. The life cycle features a distinctive larval stage, characterized by a transparent, leaf-like body adapted for a prolonged pelagic existence lasting several months to over a year before into juveniles. These larvae, including forms like Tilurus, Tiluropsis, and giganteus (the latter confirmed as the larva of N. chemnitzii), drift in upper ocean layers, feeding on particulate matter and growing slowly due to the nutrient-poor deep-sea environment. Post-, individuals exhibit slow overall growth, with lifespans estimated at 10–20 years or more, as indicated by otolith-based age assessments showing generation times around 10 years in N. chemnitzii.

Genera

Notacanthus

Notacanthus is the of the family Notacanthidae, established by in 1788. It currently includes eight valid species of deep-sea spiny eels, distributed benthopelagically on continental slopes and seamounts across the Atlantic, Indian, and Pacific Oceans. The is Notacanthus chemnitzii Bloch, 1788, commonly known as the snubnosed spiny eel, which exemplifies the genus's elongate, eel-like body covered in small cycloid scales and a series of isolated dorsal spines. Species in this genus typically reach maximum body lengths of up to 120 cm total length (N. chemnitzii), though most are smaller, and they inhabit depths from about 200 to 2,500 meters. Prominent species include Notacanthus bonaparte Risso, 1840, the shortfin spiny , which is endemic to the and eastern Atlantic, attaining up to 26 cm total length with characteristically six dorsal spines. Notacanthus indicus Lloyd, 1909, occurs in the , particularly the western , where it specializes in burrowing into soft sediments; it grows to about 20 cm total length. In the North Atlantic, Notacanthus chemnitzii is widespread and attains the largest sizes in the , up to 120 cm, while Notacanthus sexspinis Richardson, 1846, the spiny-back , is found in waters of the Atlantic, Indian, and Pacific Oceans, reaching 60 cm with 5–9 dorsal spines. Recent additions include Notacanthus laccadiviensis Bañón et al., 2023, and Notacanthus arrontei Gomes et al., 2024, both described from the and Atlantic, respectively, highlighting ongoing taxonomic discoveries in the . Distinguishing features of Notacanthus include a variable number of dorsal spines (typically 5–11, isolated and without soft rays), a short snout, and a long anal fin base; Atlantic species such as N. chemnitzii and N. bonaparte tend to be larger-bodied compared to Indo-Pacific congeners. Historically, species like N. chemnitzii have been caught as bycatch in deep-sea trawl fisheries, particularly in the North Atlantic, though they are not primary targets due to their deep-water habits and low commercial value.

Polyacanthonotus

Polyacanthonotus is a of deep-sea spiny eels within the family Notacanthidae, established by Bleeker in 1874 with Notacanthus rissoanus as the . The genus currently includes four recognized : P. africanus, P. challengeri, P. merretti, and P. rissoanus. These fishes are adapted to bathyal and abyssal environments, generally occurring at greater depths than congeners in Notacanthus, with records extending to over 3000 m and occasionally deeper, such as 4560 m for P. challengeri. Maximum recorded length reaches 60 cm total length, though some like P. rissoanus are smaller, attaining only 9.5 cm. Distinctive features include a slender, elongate body and head, a long preoral , and 26–40 isolated dorsal spines extending along much of the back, far exceeding the spine count in Notacanthus (6–15 spines). Like other notacanthids, in this genus exhibit burrowing behavior using their robust anal fin spines. Key species within the genus highlight its diversity in abyssal adaptations. Polyacanthonotus challengeri, distributed across the (including near and ), North Pacific, and Southeast Atlantic, possesses a notably elongate body and one of the highest vertebral counts among notacanthids, ranging from 242 to 255. This species typically inhabits depths of 2000–3000 m. Polyacanthonotus merretti, known from the western Atlantic (e.g., off ), is an abyssal form restricted to bathydemersal habitats up to at least 1409 m, though genus-wide records suggest potential for deeper occurrences; it reaches 27 cm in length. These species, along with P. africanus (700–3000 m, eastern and western Atlantic) and P. rissoanus (500–2875 m, widespread Atlantic including Mediterranean), demonstrate circumglobal but patchy distributions tied to deep continental slopes and basins. Compared to Notacanthus, Polyacanthonotus species are more slender overall, with finer scales and a less robust head, reflecting enhanced adaptations for navigating soft abyssal sediments. Larval stages remain poorly studied across the , with limited records due to the challenges of sampling at extreme depths. Captures are rare, attributed to the inaccessibility of habitats below 2000 m, resulting in sparse specimens and ongoing taxonomic refinements based on few individuals.

Lipogenys

Lipogenys is a of deep-sea spiny eels within the family Notacanthidae, established by Goode and Bean in 1895 to accommodate with a notably short . The currently includes two recognized , both characterized by their small size, reaching a maximum standard length of approximately 38 cm in L. gillii, and a pale, elongated body adapted to bathydemersal lifestyles. These fishes exhibit a semi-translucent appearance in preservative, with a slender, tapering lacking a caudal , and an inferior, sucker-like suited for benthic feeding. The type species, Lipogenys gillii, inhabits slope waters of the western North Atlantic, from the eastern of , at depths ranging from 400 to 2000 m. It features 6–8 isolated dorsal-fin spines and a body form that is less elongate than in other notacanthid genera, reflecting adaptations to mid-slope environments where it forages on small . In , a second species, L. hyalinumvelum, was described from specimens collected off the in the northeastern Atlantic, at depths of 650–796 m. This species is distinguished by a (transparent) membrane along the posterior edge of the opercular flap, a trait absent in L. gillii, along with 5–6 dorsal-fin spines and subtle genetic differences confirmed via COI and 16S rRNA sequencing. The name hyalinumvelum derives from Latin, referring to this veil-like structure. Members of Lipogenys differ from other Notacanthidae genera, such as Notacanthus and Polyacanthonotus, by possessing fewer dorsal-fin spines (5–8 versus 8–13 or more) and a relatively shorter, less attenuated body, which may enhance maneuverability in mid-slope habitats rather than abyssal depths. The is endemic to the Atlantic Ocean, with L. gillii in the western basin and L. hyalinumvelum marking the first record in the eastern basin, suggesting potential for further discoveries in under-sampled deep-sea regions.

Tilurus

Tilurus is a of leptocephalus larvae assigned to the family Notacanthidae, established by Kölliker in 1853 and currently regarded as a genus inquirendum due to its uncertain taxonomic status as either monotypic or exclusively larval. Specimens are known solely from the pelagic stages, primarily collected in the eastern North , with no confirmed adult counterparts identified to date. The genus etymology derives from tílos (fiber or shred) and ourá (tail), alluding to the thread-like tail termination observed in these larvae. The type and only recognized species, Tilurus gegenbauri, honors the anatomist Carl Gegenbaur and is described from larvae reaching 10–15 cm in total length. These leaf-shaped forms feature a distinctive short, stubby head (about 5% of standard length), round eyes, and a compressed body (depth around 8% of standard length), adapted for a pelagic lifestyle. They possess a high count of approximately 300 myomeres, including 45–46 predorsal myomeres, with the predorsal region spanning 16% of standard length and the preanal region 20%. Pigmentation is minimal, consisting of subtle spots along the body, aiding in open ocean waters. As representatives of the enigmatic notacanthiform leptocephali, Tilurus larvae exhibit a prolonged pelagic phase, but their precise affinities remain unresolved, preventing definitive linkage to adult genera like Notacanthus or Polyacanthonotus. This lack of confirmed poses ongoing identification challenges, underscoring the developmental diversity and knowledge gaps within Notacanthidae. Three distinct notacanthiform larval types are recognized (Tilurus, Tiluropsis, and giganteus), differentiated by head shape and eye morphology, yet none can be assigned below the ordinal level with certainty.

References

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