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Hexanchiformes
Hexanchiformes
Hexanchiformes
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Hexanchiformes
Temporal range: Early Jurassic–Recent[1] Possible Permian record[citation needed]
Broadnose sevengill shark (Notorynchus cepedianus)
Frilled shark (Chlamydoselachus anguineus)
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Division: Selachii
Superorder: Squalomorphi
Series: Hexanchida
Order: Hexanchiformes
F. de Buen, 1926
Families

See text

The Hexanchiformes /hɛkˈsæŋkɪfɔːrmiːz/ are a primitive order of sharks, numbering just five extant species in two families, Chlamydoselachidae and Hexanchidae. Chlamydoselachidae are also known as frilled sharks, these sharks are very rare fishes and typically reside in deeper waters. Hexanchidae are also known as cow sharks and are the lesser known of the two types of Hexanchiformes and also reside in deep waters.[2][3]

Taxonomy

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Due to their primitive anatomy, hexanchiforms were previously considered the most basal group of sharks. However, more recent phylogenetic studies indicate that while primitive, they in fact belong to the superorder Squalomorphi, which also contains dogfishes, angelsharks, and sawsharks, although they are thought to be the most basal member of the group.[4][5]

Description

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Hexanchiform sharks have one spineless dorsal fin located over or behind the pelvic fins and one anal fin. The vertebral column extends into the long dorsal lobe of the caudal fin, while the ventral lobe is either small or absent. They have either six or seven gill slits, located in front of the pectoral fins. They have a large mouth, with eyes on either side of the head. The spiracles are small and located well above and behind the eyes.[6] The eyes have no nictitating membrane.

The frilled sharks of the genus Chlamydoselachus are very different from the cow sharks, and have been proposed to be moved to a distinct order, Chlamydoselachiformes. However, genetic studies have found them to be each other's closest relatives, and they share certain derived features supporting them both being in the same order.[4][5]

Shark teeth similar to modern hexanchids and echinorhinids are known from Devonian deposits in Antarctica and Australia, as well as Permian deposits in Japan. If these are in fact hexanchids, this may be the only extant order of elasmobranchs to have survived the Permian extinction (and by extension, the oldest extant order of elasmobranchs). However, the Australian/Antarctic shark teeth, from the family Mcmurdodontidae, have also been found to lack a multilayer enameloid layer covering the tooth crown, something found in all modern sharks and most Devonian sharks, indicating that they are neoselachians of uncertain affinity or even indeterminate chondrichthyans. The occurrence of derived sharks in the Devonian is also irreconcilable with the results of all phylogenetic estimates in the group.[7][8][9]

It is debated whether the extinct families Orthacodontidae and Paraorthacodontidae belong to the Hexanchiformes or the extinct Synechodontiformes. However, the Shark-References database currently lists them as members of the Hexanchiformes.[10][11][12]

Distribution

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Species are widespread and found across most of the world. They are most common in cold deep water in the tropics, but are also found closer to the shore in more temperate regions.[6]

Reproductive biology

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Hexanchiforms are viviparous, meaning they give birth to live young. Males have two testes which are capable of producing sperm year-round and females have two ovaries and two uteri. Chlamydoselachus africana Males have two testes which produce sperm and females have two ovaries and ovulate from summer to autumn. Embryos develop only in the right uterus of a female. The research regarding the reproductive Biology of the Hexanchidae family is limited but thought to be similar, as no year-round research has been done regarding female hexanchids.[2]

Classification

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Living species

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Extinct species

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Notidanodon sp. fossil at the Geological Museum, Copenhagen
  • Suborder Chlamydoselachoidi
    • Family Chlamydoselachidae
      • Chlamydoselachus Garman, 1884
        • Chlamydoselachus balli Cappetta, Morrison & Adnet, 2019
        • Chlamydoselachus gracilis Antunes & Cappetta, 2002
        • Chlamydoselachus lawleyi Davis, 1887
        • Chlamydoselachus tobleri Leriche, 1929
      • Dykeius Cappetta, Morrison & Adnet, 2019
        • Dykeius garethi Cappetta, Morrison & Adnet, 2019
      • Rolfodon Cappetta, Morrison & Adnet, 2019
        • Rolfodon bracheri (Pfeil, 1983)
        • Rolfodon fiedleri (Pfeil, 1983)
        • Rolfodon goliath (Antunes & Cappetta, 2002)
        • Rolfodon keyesi (Mannering & Hiller 2008)
        • Rolfodon landinii (Carrillo-Briceño et al. 2014)
        • Rolfodon ludvigseni Cappetta, Morrison & Adnet, 2019
        • Rolfodon tatere (Consoli, 2008)
        • Rolfodon thomsoni (Richter & Ward, 1990)
  • Suborder Hexanchoidei
    • Family Crassodontidanidae
      • Crassodontidanus Kriwet & Klug, 2011
        • Crassodontidanus serratus Fraas, 1855
        • Crassodontidanus wiedenrothi Thies, 1983
      • Notidanoides Maisey, 1986
        • Notidanoides muensteri Agassiz, 1843
      • Notidanus Cuvier, 1816
        • Notidanus amalthei Oppel, 1854
        • Notidanus atrox Ameghino, 1899
        • Notidanus intermedius Wagner, 1862
        • Notidanus nikitini Chabakov & Zonov, 1935
      • Pachyhexanchus Cappetta, 1990
        • Pachyhexanchus pockrandti Ward & Thies, 1987
    • Family Hexanchidae
      • Gladioserratus Underwood, Goswami, Prasad, Verma & Flynn, 2011
        • Gladioserratus aptiensis Pictet, 1864
        • Gladioserratus dentatus Guinot, Cappetta & Adnet, 2014
        • Gladioserratus magnus Underwood, Goswami, Prasad, Verma & Flynn, 2011
      • Heptranchias Rafinesque, 1810
        • Heptranchias ezoensis Applegate & Uyeno, 1968
        • Heptranchias howellii Reed, 1946
        • Heptranchias karagalensis Kozlov in Zhelezko & Kozlov, 1999
        • Heptranchias tenuidens Leriche, 1938
      • Hexanchus Rafinesque, 1810
        • Hexanchus agassizi Cappetta, 1976
        • Hexanchus andersoni Jordan, 1907
        • Hexanchus casieri Kozlov, 1999
        • Hexanchus collinsonae Ward, 1979
        • Hexanchus gracilis Davis, 1887
        • Hexanchus hookeri Ward, 1979
        • Hexanchus microdon Agassiz, 1843
        • Hexanchus tusbairicus Kozlov in Zhelezko & Kozlov, 1999
      • Notidanodon Cappetta, 1975
        • Notidanodon lanceolatus Woodward, 1886
        • Notidanodon pectinatus Agassiz, 1843
      • Notorynchus Ayres, 1855
        • Notorynchus borealus Jordan & Hannibal, 1923
        • Notorynchus kempi Ward, 1979
        • Notorynchus lawleyi Cigala Fulgosi, 1983
        • Notorynchus primigenius Agassiz, 1843
        • Notorynchus serratissimus Agassiz, 1843
        • Notorynchus subrecurvus Oppenheimer, 1907
      • Pachyhexanchus Cappetta, 1990
        • Pachyhexanchus pockrandti Ward & Thies, 1987
      • Paraheptranchias Pfeil, 1981
        • Paraheptranchias repens Probst, 1879
      • Pseudonotidanus Underwood & Ward, 2004
        • Pseudonotidanus semirugosus Underwood & Ward, 2004
      • Welcommia Cappetta, 1990
      • Weltonia Ward, 1979
        • Weltonia ancistrodon Arambourg, 1952
        • Weltonia burnhamensis Ward, 1979
      • Xampylodon Cappetta, Morrison & Adnet, 2019
        • Xampylodon brotzeni (Siverson, 1995)
        • Xampylodon dentatus (Woodward, 1886)
        • Xampylodon loozi (Vincent, 1876)
    • ?Family Orthacodontidae
      • Occitanodus Guinot, Cappetta & Adnet, 2014
        • Occitanodus sudrei Guinot, Cappetta & Adnet, 2014
      • Sphenodus Agassiz, 1843
        • Sphenodus alpinus Gümbel, 1861
        • Sphenodus longidens Agassiz, 1843
        • Sphenodus lundgreni Davis, 1890
        • Sphenodus macer Quenstedt, 1852
        • Sphenodus nitidus Wagner, 1862
        • Sphenodus longidens Agassiz, 1843
        • Sphenodus planus Agassiz, 1843
        • Sphenodus rectidens Emmons, 1858
        • Sphenodus robustidens Seguenza, 1900
        • Sphenodus tithonius Gemmellaro, 1871
        • Sphenodus virgai Gemmellaro, 1871
  • ?Family Komoksodontidae Cappetta, Morrison & Adnet, 2019[10]
    • Komoksodon Cappetta, Morrison & Adnet, 2019
      • Komoksodon kwutchakutch Cappetta, Morrison & Adnet, 2019
  • ?Family Paraorthacodontidae
    • Macrourogaleus Fowler, 1947
      • Macrourogaleus hassei
    • Paraorthacodus Glückman, 1957
      • Paraorthacodus andersoni (Case, 1978)
      • Paraorthacodus antarcticus Klug, Kriwet, Lirio & Nuñez, 2008
      • Paraorthacodus arduennae Delsate, 2001
      • Paraorthacodus clarkii (Eastman, 1901)
      • Paraorthacodus conicus (Davis, 1890)
      • Paraorthacodus eocaenus (Leriche, 1902)
      • Paraorthacodus jurensis (Schweizer, 1964)
      • Paraorthacodus recurvus (Trautschold, 1877)
      • Paraorthacodus rossi Cappetta, Morrison & Adnet, 2019
      • Paraorthacodus turgaicus Glikman, 1964

Species

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Family Image Common name Genera Species Description
Chlamydoselachidae Frilled sharks 1
extant
1
extinct 		2
extant
12
extinct 		Frilled sharks contain only two extant species of deepsea creatures which are typically weakened in areas closer to the surface. The most widely known species still surviving is the frilled shark, known as a living fossil, along with the Southern African frilled shark, found along coastal areas of South Africa. Several extinct species are known.
†Crassodontidanidae †Crassodontidanidae 4 8 Extinct
Hexanchidae Cow sharks 3
extant
5
extinct 		5
extant
31
extinct 		Cow sharks are considered the most primitive of all the sharks, because their skeletons resemble those of ancient extinct forms, with few modern adaptations. Their excretory and digestive systems are also unspecialised, suggesting that they may also resemble those of their primitive shark ancestors. Their most distinctive feature, however, is the presence of a sixth, and, in two genera, a seventh, gill slit, in addition to the five found in all other sharks.[15] They range from 1.4 metres (4.6 ft) to over 5.5 metres (18 ft) in adult body length.
†Komoksodontidae? †Komoksodontidae? 1 1 Extinct
†Orthacodontidae? †Orthacodontidae? 2 12 Extinct
†Paraorthacodontidae? †Paraorthacodontidae? 2 11 Extinct

See also

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Footnotes

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hexanchiformes

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from Grokipedia
Hexanchiformes is an order of primitive elasmobranch fishes, commonly known as frilled and , characterized by six or seven slits, a single without a spine, an anal fin, the absence of a , and a small spiracle behind each eye. This order comprises two families (Chlamydoselachidae and Hexanchidae), four genera, and seven extant , all of which inhabit marine waters, with some entering brackish estuarine environments, and primarily deep waters on continental slopes and shelves worldwide. These represent one of the most basal lineages among modern elasmobranchs, retaining archaic traits that provide insights into the early evolution of dating back to the period. The Chlamydoselachidae family includes the frilled (genus Chlamydoselachus), notable for their eel-like bodies, loose skin with frilled gill slits, and predatory behavior involving eel-like lunging attacks on prey. In contrast, the Hexanchidae family encompasses and sevengill (genera Hexanchus, Heptranchias, and Notorynchus), which feature robust bodies, comb-like lower teeth for grasping prey, and ovoviviparous reproduction with litters ranging from dozens to over 100 pups per female. Species such as the (Hexanchus griseus) can attain lengths exceeding 6 meters and weights up to 600 kilograms, serving as apex predators in deep-sea ecosystems by scavenging and hunting , cephalopods, and other . Hexanchiformes species are generally elusive and poorly studied due to their deep-water habitats, often at depths of 100–2,000 meters, though some like the (Notorynchus cepedianus) venture into shallower coastal waters in temperate regions. Conservation concerns exist for several , with IUCN assessments indicating vulnerabilities from in deep-sea fisheries and limited population data as of 2012, underscoring the need for further research into their ecology and distribution.

Anatomy and Physiology

Morphological Features

Hexanchiformes, an order of primitive , exhibit a distinctive characterized by a single positioned posteriorly, an anal fin, and six or seven slits, features that distinguish them from most modern which typically possess two dorsal fins and five slits. These lack a , a protective common in other elasmobranchs, reflecting their basal evolutionary position. The body form varies by family: frilled sharks (Chlamydoselachidae, genus Chlamydoselachus) display an elongated, eel-like shape adapted for deep-sea environments, while (Hexanchidae, genera , Heptranchias, and Notorynchus) have a more robust, build with a broad head and underslung jaws. Spiracles are present but small and positioned well behind the eyes. Dentition in hexanchiforms is highly specialized and primitive, featuring multiple rows of small, spike- or hook-like upper teeth for grasping, contrasted with large, comb-like lower teeth bearing multiple cusps for slicing prey. Tooth counts vary, with frilled sharks having 24-27 upper and 21-26 lower teeth, all tricuspid, while sixgill sharks (Hexanchus) possess around 20 upper and 13 lower teeth, and sevengill sharks (Notorynchus) have 15-16 upper and 13 lower. This heterodonty, with weak differentiation between anterior and lateral teeth, underscores their archaic morphology. Adult sizes range from 1.4 to over 4.8 meters in total length, with frilled sharks (Chlamydoselachus anguineus) reaching up to 2 meters, sharpnose sevengill sharks (Heptranchias perlo) about 1.4 meters, broadnose sevengill sharks (Notorynchus cepedianus) up to 3 meters, and bluntnose sixgill sharks (Hexanchus griseus) exceeding 4.8 meters. The skin is loose and covered in dermal denticles that range from coarse and spiky to smooth, aiding in camouflage and protection; some species, like the bluntnose sixgill, feature denticle-free patches near the pectoral fins. Coloration is typically dark brown to black or gray above, paler ventrally, often with spots in shallower-water species like Notorynchus, enhancing deep-sea concealment.

Sensory and Locomotory Adaptations

Hexanchiform sharks possess highly specialized sensory systems adapted to the dim, low-visibility conditions of their deep-sea habitats. The , a network of electroreceptors distributed across the head, enable detection of weak electric fields generated by prey muscle contractions and heartbeats, facilitating prey location even in complete darkness. Complementing this, their olfactory rosettes—spiral arrangements of lamellae in the nasal capsules—provide acute chemosensory capabilities, allowing the sharks to track scent trails of potential prey over considerable distances in nutrient-scarce environments. Visual and mechanosensory adaptations further enhance perception in these light-limited realms. Their eyes are notably large relative to body size, featuring a —a reflective layer of crystals behind the that amplifies available light by reflecting photons back through the photoreceptors, improving sensitivity in low-light conditions by up to tenfold compared to human vision. The lateral line system, consisting of neuromasts along the body flanks, detects hydrodynamic pressure changes and vibrations from nearby organisms or environmental disturbances, aiding and ambush predation. Locomotion in hexanchiforms varies by family, reflecting their primitive morphology and deep-water lifestyles. Frilled sharks (Chlamydoselachus spp.) employ an anguilliform swimming mode, characterized by undulating body waves that propagate from head to tail, mimicking eel-like for stealthy, energy-efficient maneuvering in confined benthic habitats. In contrast, (Hexanchus spp.) primarily utilize body-caudal-fin (BCF) propulsion, where powerful beats of the heterocercal caudal fin generate thrust for sustained cruising and predatory bursts, supported by a robust, cylindrical body form. Buoyancy regulation is achieved through an enlarged liver comprising up to 25% of body mass, rich in low-density oil that provides near-neutral with minimal compressibility changes during vertical migrations, thus conserving energy in the stable deep-sea pressure regime. Their slow rates, evidenced by reduced red muscle mass and low activities for aerobic , align with the oligotrophic deep-sea , enabling prolonged fasting periods between infrequent meals while minimizing oxygen demands. Respiratory adaptations include six or seven gill slits, exceeding the typical five in modern , which collectively offer a greater total surface area for and enhance oxygen extraction efficiency in the hypoxic waters of the deep ocean. This configuration supports their low-activity lifestyle, allowing adequate oxygenation despite reduced ventilation rates.

Distribution and Habitat

Global Distribution

Hexanchiformes exhibit a across all major oceans, spanning tropical to temperate waters worldwide, with species occurring from the Atlantic and Pacific to the . This order, comprising seven extant species in two families (Chlamydoselachidae and Hexanchidae), is primarily bathydemersal, inhabiting outer continental shelves, upper slopes, and insular regions, though records are often patchy due to their deep-water preferences and infrequent sightings. Depth ranges for hexanchiforms typically fall between 100 and 2,000 meters, with maximum records extending to 2,500 meters, though some species venture into shallower coastal areas. For instance, the bluntnose sixgill shark (Hexanchus griseus) is documented from 0 to 2,500 m globally, with coastal occurrences in the Mediterranean Sea (e.g., off Turkey, Italy, and Spain) and the Pacific (e.g., northeastern Pacific off Baja California and Gulf of California). The frilled shark (Chlamydoselachus anguineus) occupies 120–1,280 m usually, but has been recorded as shallow as 0 m, with widespread but discontinuous distribution across the Atlantic (e.g., northern Norway to Namibia), Indian (off South Africa), and Pacific Oceans (Japan to New Zealand and southern California to Chile). In contrast, the broadnose sevengill shark (Notorynchus cepedianus) prefers shallower depths of 0–570 m, primarily in temperate Southern Hemisphere waters, including the southwest Atlantic (southern Brazil to northern Argentina), southeast Atlantic (Namibia to South Africa), eastern Pacific (British Columbia to Chile), and western Pacific (southern Japan to New Zealand). Certain hexanchiform populations display vertical migration patterns, remaining deeper during the day and ascending to shallower depths at night, likely for or predator avoidance. This diel behavior is particularly noted in H. griseus, where individuals descend below 500 m by day and rise to 200–350 m nocturnally in regions like the northeastern Pacific. Historical records of hexanchiforms date to the late 18th and 19th centuries, with initial scientific descriptions such as H. griseus by Bonnaterre in 1788, followed by expanded documentation through 19th-century explorations. Recent sightings have broadened known ranges, including new records of H. griseus in the tropical southwestern Atlantic (e.g., off northeastern in 1998–2020, with 23 specimens) and the northwestern Mediterranean (e.g., Gulf of in 2021), as well as C. anguineus in the South Pacific in 2025.

Habitat Characteristics

Hexanchiform sharks predominantly inhabit cold, deep marine environments, ranging from the mesopelagic to bathypelagic zones, where water temperatures typically fall between 4–10°C and light levels are minimal. These conditions prevail at depths of 200–2,000 meters, with species like the (Hexanchus griseus) recorded from 90 to 1,875 meters and the (Chlamydoselachus anguineus) from 100 to 1,300 meters. Such habitats support their ectothermic physiology, allowing efficient energy conservation in low-oxygen, high-pressure settings. Substrate preferences include soft sediments on continental slopes, submarine canyons, and seamounts, where these sharks often rest or forage demersally. They occasionally venture onto continental shelves, particularly in cooler temperate regions, but remain closely associated with structured deep-sea topography that provides shelter and prey concentrations. Water quality in these habitats features stable levels around 35 ppt and adaptations to extreme pressures exceeding 200 atmospheres at bathypelagic depths. Notably, hexanchiforms tolerate oxygen minimum zones (OMZs) through physiological adaptations such as enhanced surface area and efficient oxygen extraction, enabling survival in hypoxic waters with dissolved oxygen below 2 mg/L. Seasonal variations influence use, with some individuals undertaking diel or seasonal vertical migrations to shallower depths (as low as 3–50 meters) during cooler periods or at night, driven by prey availability or reproductive cues. For instance, frilled sharks shift deeper when surface temperatures exceed 15°C in summer months. These movements highlight their flexibility within deep-water niches. Emerging human activities, such as deep-sea mining in polymetallic nodule fields overlapping hexanchiform s, pose vulnerabilities by disrupting sediment stability and prey chains, as evidenced in 2025 assessments of elasmobranch risks.

Ecology and Behavior

Diet and Predatory Habits

Hexanchiformes, comprising primitive sharks such as the bluntnose sixgill shark (Hexanchus griseus) and others in the families Hexanchidae and Chlamydoselachidae, are opportunistic carnivores exhibiting generalist feeding strategies. Their diet primarily consists of teleost fishes, cephalopods, crustaceans, chondrichthyans, and marine mammals, supplemented by carrion in deeper waters. Stomach content analyses reveal a broad prey spectrum, with cephalopods prominent in smaller individuals (~64% by index of relative importance), followed by teleosts (~34%), while larger specimens shift toward higher proportions of teleosts, chondrichthyans, and marine mammals. For instance, H. griseus commonly preys on smaller sharks such as catsharks (Scyliorhinidae) and spiny dogfish (Squalus spp.), as well as rays like the little skate (Raja cf. clavata), hake (Merluccius spp.), and squid (Todarodes angolensis), with dietary composition varying by body size rather than clear seasonal patterns in available studies. Frilled sharks (Chlamydoselachus spp.) primarily consume teleost fishes and cephalopods via ambush predation in deep-sea habitats. These sharks employ versatile predatory tactics suited to their deep-sea habitats, often relying on approaches where they lie in wait near the seafloor before striking with rapid forward propulsion. Their distinctive comb-like lower teeth, arranged in a saw-like , facilitate gripping and tearing soft-bodied or evasive prey, while upper teeth aid in cutting during manipulation phases involving twisting or unilateral shearing motions. Feeding observations indicate a mix of ram-based strikes (predominant in mid-water pursuits), for benthic prey, and direct biting, allowing efficient handling of diverse items from live cephalopods to stranded carcasses. This adaptability underscores their dual role as active hunters and , particularly in low-productivity deep-sea environments where they opportunistically consume falls and discards. Ecologically, hexanchiforms occupy a mid-to-high of approximately 4.3 within marine food webs, positioning them as key mid-level predators that regulate populations of mesopelagic and benthic . Their scavenging behavior enhances in the deep ocean by rapidly processing , thereby facilitating the transfer of carbon and energy to lower trophic levels and supporting overall productivity. In regions like the southern African and northeastern Pacific, they contribute to stabilizing deep-sea communities through this top-down control and detrital processing.

Movement and Social Behavior

Hexanchiform , particularly within the Hexanchidae such as the ( griseus), exhibit highly resident behavior, often confining their movements to localized areas within deep-sea environments like canyons and slopes. Acoustic tagging studies have revealed that individuals maintain small home ranges, with maximum displacements up to 29 km, daily displacements averaging 0.2–3.1 km and long-term site fidelity spanning up to four years in areas such as . This residency is influenced by habitat structure, where return to specific sites seasonally, spending extended periods sedentary before shifting regions. Studies in other regions, such as the , show similar residency patterns with utilization of diverse habitats including seamounts and slopes. Migration in hexanchiforms is limited, primarily manifesting as diel vertical movements rather than extensive horizontal migrations. These typically occupy deeper waters (>100 m) during the day and ascend to shallower depths (around 50–100 m) at night, a pattern observed consistently across tagged individuals in temperate and subtropical regions. Contrasting ecologies among sympatric species, such as between H. griseus and the bigeye sixgill ( nakamurai), highlight variations in vertical excursion amplitudes, with some populations showing minimal diel shifts in stable canyon habitats while others exhibit broader oscillations tied to prey availability. Ontogenetic differences further modulate these movements, with juveniles displaying more pronounced vertical migrations than adults, though overall migratory distances remain under 30 km annually. Social interactions among hexanchiforms are predominantly solitary, with individuals rarely forming stable groups, though occasional aggregations have been noted in areas of high prey density or structural complexity like seamounts and canyons. Body scarring, including bite marks from conspecifics observed on the gills, fins, and flanks of H. griseus, suggests encounters. Remotely operated (ROV) footage from the (2007–2024 expeditions) documents cautious yet curious approaches by H. griseus to submersibles before retreating, indicating wariness in potential social contexts.

Reproduction and Development

Reproductive Biology

Hexanchiformes exhibit aplacental across all species, with embryos developing internally and nourished primarily through yolk-sac reserves during . In some taxa, such as the (Chlamydoselachus anguineus), additional uterine nourishment is provided via , where developing embryos consume unfertilized eggs released into the uterus by the mother, enhancing pup size and survival. This reproductive strategy contrasts with more advanced placental seen in other elasmobranch orders but aligns with the order's primitive morphology and deep-sea adaptations. Sexual dimorphism in Hexanchiformes is pronounced in reproductive anatomy, with males possessing paired pelvic claspers modified from pelvic fins to facilitate during copulation. Females are generally larger than males, reflecting investment in larger litters and prolonged . periods are notably extended, often lasting 2–3.5 years; for instance, in the ( griseus), it is estimated at approximately 2 years, while in the , it reaches up to 3.5 years. These long developmental times contribute to low reproductive output and vulnerability to . Mating behaviors are poorly documented due to the deep-water habits of most species, but internal fertilization is universal, achieved via clasper insertion. Limited observations suggest courtship involves physical contact, including biting by males to grasp females, as recorded in captive broadnose sevengill sharks (Notorynchus cepedianus), where such bites occur on the flanks and gills during precopulatory interactions. These behaviors may serve to stimulate or ensure mate retention, though field confirmations remain scarce. Litter sizes, or , range from 2–12 pups in the to 22–108 in the , with intermediate values of 9–20 in the sharpnose sevengill (Heptranchias perlo). In oophagous species like the , the consumption of excess eggs supports the survival of a smaller number of larger, better-provisioned pups. is attained at total lengths of 2–4 m and ages of 10–20 years, with males typically maturing earlier and at smaller sizes than females; for example, in H. griseus, males reach maturity at ~3.1 m and ~12 years, while females do so at 4.2–4.3 m and 18–26 years.

Life History Traits

Hexanchiform exhibit slow growth rates characteristic of deep-sea elasmobranchs, typically ranging from 2 to 5 cm per year in larger individuals, as modeled by the (VBGF). For the (Notorynchus cepedianus), VBGF analyses of wild populations yield growth coefficients (k) of 0.074 year⁻¹ for females and 0.123 year⁻¹ for males, indicating protracted development with asymptotic s (L∞) reaching 357 cm for females and 277 cm for males. Similarly, the (Hexanchus griseus) displays slow growth, with VBGF parameters derived from -age data supporting extended , though precise coefficients vary due to limited vertebral banding reliability. These models, fitted as Lt = L∞(1 - e-k(t - t0)), where Lt is at age t, underscore the order's to resource-scarce environments, prioritizing somatic over rapid size increase. Lifespans in Hexanchiformes span 30 to 80 years, reflecting their low metabolic demands and K-selected life history strategy. VBGF projections for N. cepedianus imply longevities exceeding 30 years, with captive studies confirming comparable field growth trajectories that extend into decades. For H. griseus, maximum reported ages approach 80 years, though aging via neural arches or vertebrae remains challenging due to poor , leading to reliance on length-based VBGF estimates. This contributes to population stability in stable deep-sea habitats but heightens susceptibility to perturbations. Population structures in hexanchiforms are shaped by low fecundity and K-selected traits, fostering slow recovery and vulnerability to . With litter sizes of 60–107 for N. cepedianus and 22–108 for H. griseus, reproductive output limits , enforcing density-dependent regulation suited to predictable, low-productivity ecosystems. High age at maturity—e.g., 15 years for N. cepedianus—further delays cohort replenishment, amplifying risks from in deep-sea fisheries. Natural mortality in Hexanchiformes is low, primarily from infrequent predation by larger conspecifics or marine mammals, given their apical position in deep-sea food webs. For H. griseus, predation pressure diminishes with size, contributing to high juvenile but overall low annual mortality rates in unexploited populations. Anthropogenic factors dominate current mortality, overshadowing natural causes. Limited tagging studies reveal high site fidelity in hexanchiforms, influencing population recovery dynamics. In H. griseus, juveniles in exhibit residency up to 4 years within core areas, with acoustic tagging showing seasonal fidelity to depths of 100–200 m. Similarly, N. cepedianus displays strong in Tasmanian and Washington estuaries, with mark-recapture data indicating limited dispersal (<50 km annually), which concentrates vulnerability but supports localized management. Energy allocation in hexanchiforms favors maintenance and longevity over rapid , adapted to oligotrophic deep-sea conditions. Low metabolic rates channel resources toward somatic growth and repair, with minimal investment in high-fecundity cycles, enhancing survival in energy-limited habitats.

Taxonomy and Evolution

Taxonomic Classification

The order Hexanchiformes was formally established by Francisco de Buen in 1926 as part of a broader revision of elasmobranch , recognizing the distinct primitive features of these , such as their multiple gill slits and single . Prior to this, in the , now assigned to Hexanchiformes were often grouped under the family Notidanidae (or Notidani), a catch-all for multi-gilled based on early morphological descriptions by anatomists like Johannes Müller and in their 1838–1841 systematic work. This early emphasized external traits like gill count but lacked phylogenetic rigor, leading to inconsistent placements within broader groups. In the 20th century, taxonomic frameworks evolved with Leonard Compagno's influential 1973 and 1984 works, which integrated comparative anatomy and systematics to refine Hexanchiformes within the superorder Squalomorphi, a clade of "squalomorph" sharks distinguished by features like short pectoral bases and a second dorsal fin in many members. Compagno's FAO Species Catalogue (1984) formalized the order's structure with two families: Chlamydoselachidae (frilled sharks) and Hexanchidae (cow sharks). The Chlamydoselachidae includes two extant species in the genus Chlamydoselachus (C. anguineus and C. africana), characterized by their eel-like bodies and frilled gill slits, while the Hexanchidae comprises five species across three genera: three in Hexanchus (H. griseus, H. nakamurai, H. vitulus), H. perlo in Heptranchias, and N. cepedianus in Notorynchus, noted for their robust builds and comb-like teeth. In 2018, Hexanchus vitulus was resurrected as a distinct species based on genetic and dental analyses. Post-2000 cladistic analyses, incorporating molecular data such as mitochondrial genomes, have supported the overall of Hexanchiformes but introduced debates regarding internal relationships, particularly the positioning of Chlamydoselachidae relative to Hexanchidae. Some molecular phylogenies suggest Chlamydoselachus as the to all other hexanchiforms or even basal to broader Neoselachii, potentially rendering traditional divisions paraphyletic, though morphological synapomorphies like the extra hypobranchial arch uphold the order's cohesion. These revisions highlight Hexanchiformes' basal position among neoselachians (modern and rays), as the to all other shark orders, reflecting their retention of plesiomorphic traits from early elasmobranch evolution.

Fossil Record and Extinct Taxa

The fossil record of Hexanchiformes extends back to the , approximately 190 million years ago, marking the order as one of the earliest diverging lineages among modern neoselachian sharks. Possible precursors from the Permian period, such as isolated teeth from deposits in tentatively assigned to hexanchid-like forms, have been reported but remain debated due to limited material and uncertain affinities. These early records suggest that hexanchiforms may represent survivors of the Permian- mass extinction, though definitive evidence for the order's origin is firmly established in the with primitive morphologies including multiple slits and single dorsal fins. Over 60 extinct species are known from the fossil record, spanning several families that highlight the order's diversification. Extinct families include Crassodontidanidae, known from deposits in with genera like Crassodontidanus exhibiting robust, cutting dentition adapted for predation on hard-shelled prey; Orthacodontidae, represented by taxa such as Sphenodus from to strata worldwide, featuring slender teeth suggestive of a piscivorous diet; and other basal groups contributing to the order's early radiation. These families document a peak in diversity during the and , with more than 30 species attributed to hexanchid lineages alone. Key fossil discoveries underscore the persistence of primitive traits, such as six or seven slits and amphistylic suspension, throughout the . For instance, Sphenodus lindsayi from the Jurassic of the preserves tricuspid teeth indicative of the order's retention of archaic features amid neoselachian evolution. In the , Chlamydoselachus balli, described from the Hornby Island deposits in , represents a species with elongated, multi-cuspidate closely resembling modern Chlamydoselachus anguineus, bridging forms to their ancestors. Such finds illustrate evolutionary conservatism, with hexanchiforms maintaining eel-like body plans and deep-water affinities over 150 million years. Hexanchiformes demonstrated resilience through major mass extinctions, including the end-Triassic and end-Cretaceous events, likely due to their adaptable predatory habits in offshore environments. However, post-Cretaceous patterns reveal significant lineage losses, with many families like Crassodontidanidae and Orthacodontidae vanishing by the Eocene, reducing diversity to the two extant families (Chlamydoselachidae and Hexanchidae) that persist today. This decline coincided with the rise of more derived shark orders, such as , which outcompeted hexanchiforms in shallow neritic habitats.

Current Species and Conservation

Living Species

The order Hexanchiformes encompasses seven extant species distributed across four genera: Chlamydoselachus, Hexanchus, Heptranchias, and Notorynchus. These primitive sharks are characterized by their archaic features, including multiple gill slits and single dorsal fins, and inhabit primarily deep or coastal waters. Chlamydoselachus anguineus, commonly known as the frilled shark, exhibits an eel-like body form with six gill slits, the first pair connected across the throat, and reaches a maximum length of approximately 2 meters. It possesses a dark brown or grey coloration, often paler ventrally, and inhabits deep-sea environments globally, typically between 120 and 1,500 meters depth. The species is distinguished by its tridentate upper teeth and fringed gill septa, contributing to its primitive appearance. Closely related, Chlamydoselachus africana, the , shares the eel-like morphology and six slits of its congener but is regionally restricted to the southeastern Atlantic off . Described in 2009 from specimens collected at depths of 400 to 850 meters, it attains a maximum recorded length of 1.17 meters and features subtle differences in and vertebral counts for differentiation. Hexanchus griseus, the , is a robust, heavily bodied with six gill slits, a broad head, and bluntly rounded , growing to lengths of up to 5.5 meters. It displays comb-shaped lower teeth in six rows and a in temperate and tropical waters, often at depths from 10 to 2,500 meters. The first originates posterior to the pectoral fin base, aiding in its identification. Hexanchus nakamurai, the bigeye sixgill shark, is more slender with proportionally larger eyes and a pointed , also featuring six slits and reaching up to 2.06 meters in length. It prefers deeper waters, typically 200 to 700 meters, in the Indo-West Pacific region, and is differentiated from H. griseus by its smaller size, eye diameter exceeding 5% of head length, and more anterior position. Hexanchus vitulus, the Atlantic sixgill shark, is a small, slender species with six slits, a narrow pointed head, large eyes, and reaches a maximum length of about 1.8 meters. It inhabits deep waters greater than 300 meters in the tropical western Atlantic, including the and , and is distinguished by its five rows of comb-like lower teeth and uniform grey coloration. Heptranchias perlo, the , has a slender body, seven gill slits, a narrow pointed head, and large green eyes, attaining lengths up to 1.5 meters. It has a in temperate and tropical waters, occurring at depths from 27 to 1,720 meters, and features small upper teeth with oblique cusps and comb-like lower teeth. Notorynchus cepedianus, the , stands out with seven gill slits, a wide head, short blunt , and body, attaining lengths of up to 3 meters. Confined to coastal and shelf waters of the , primarily temperate regions, it has small dorsal fins with the first originating over or behind the pelvic fin bases and distinctive blade-like teeth. Identification of hexanchiform species relies on key diagnostic traits such as gill slit count—six in Chlamydoselachus and Hexanchus, seven in Heptranchias and Notorynchus—along with fin positions, where the single dorsal fin's origin varies relative to pelvic fins, and body proportions like head width and eye size. Genetic markers, including mitochondrial DNA sequences, further resolve close relationships, particularly between the two Chlamydoselachus species and within Hexanchus, through differences in cytochrome b and control region haplotypes.

Conservation Status and Threats

The conservation status of Hexanchiformes species varies across the order, with assessments reflecting their deep-sea habitats and limited fisheries interactions. The (Hexanchus griseus) is classified as Near Threatened globally by the IUCN, based on inferred population reductions of 20–29% over three generations due to and habitat degradation, though regional stability is noted in areas like the Mediterranean. The bigeye sixgill shark (Hexanchus nakamurai) shares this Near Threatened status, driven by similar vulnerabilities in deep-water fisheries. The (Heptranchias perlo) is also assessed as Near Threatened due to suspected declines from targeted fisheries. In contrast, the (Notorynchus cepedianus) is assessed as Vulnerable, with documented declines in targeted and incidental catches across its temperate range. The frilled sharks (Chlamydoselachus anguineus and C. africana) are listed as Least Concern, owing to their rarity in fisheries and broad but poorly quantified distribution. The Atlantic sixgill shark (Hexanchus vitulus) is similarly Least Concern, with limited data indicating low fishery interactions. Primary threats to Hexanchiformes stem from anthropogenic activities in deep-sea environments, exacerbating their low resilience due to slow growth and late maturity. in deep-sea trawl and longline fisheries poses a significant risk, particularly for H. griseus and H. nakamurai, where post-capture mortality is high despite occasional releases. Emerging concerns include disruption from deep-sea , with a 2025 study identifying overlaps between mining zones in polymetallic nodule fields and the ranges of at least 25 deep-water elasmobranch species, including primitive forms like hexanchiforms, potentially leading to plumes that smother grounds and alter prey availability. trends indicate declines in the Mediterranean for H. griseus, with abundance following a quadratic pattern of initial increase followed by reduction, linked to intensified pressures. Primitive sharks, including those in Hexanchiformes, face elevated risks due to their ecological specialization and morphological uniqueness, as highlighted in a 2025 analysis showing unusual species are disproportionately threatened by loss and . Conservation measures for Hexanchiformes are limited but include protections within marine protected areas (MPAs), such as the Sea of the Hebrides MPA in the Northeast Atlantic, which safeguards deep-sea habitats for H. griseus. Enhanced monitoring through and acoustic tagging has revealed site fidelity in sixgill sharks, informing targeted to reduce . However, significant knowledge gaps persist, particularly for the , where basic data on population sizes, movement patterns, and fishery interactions remain scarce, necessitating prioritized research to assess true vulnerability. Recommendations emphasize expanded baseline surveys, international cooperation on deep-sea regulations, and integration of life history traits into risk assessments to address these deficiencies.

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