Hexanchiformes (Six- and Sevengill Sharks)
(Six- and sevengill sharks)
Number of families 2
Evolution and systematics
Hexanchiforms are an ancient lineage, as well-documented fossil skeletons of Notidanoides muensteri date from the late Jurassic (some 150 million years ago, or mya) Solnhofen limestones of southern Germany. Notidanoides was a large shark, up to 9.8 ft (3 m) in length, with features suggestive of modern hexanchiforms, such as a single dorsal fin. Its teeth, with multiple cusps arranged in a series, indicate that it is related closely to the Hexanchidae. Most fossil hexanchiforms are known from isolated teeth found in all continents, ranging from the early Jurassic (180 mya) to the Tertiary, which makes the order one of the longest-surviving shark lineages. Many late Cretaceous to Tertiary species are even assigned to living genera, based on fossil teeth. The hexanchiform fossil record indicates that they were never very diverse, but more so than at present, as there are only five living species.
The bizarre frilled shark (Chlamydoselachus anguineus) was first described by the American zoologist and chondrichthyan taxonomist Samuel W. Garman in 1884. It has teeth that resemble those of some Paleozoic sharks ("cladodont teeth"), which led many early researchers to consider it a relic of Devonian seas. It is now well established, however, that Chlamydoselachus shares a more recent common ancestry with all living sharks and rays (the Neoselachii), only distantly related to Paleozoic forms.
Sixgill and sevengill sharks are the most basal ("primitive") members of the large group known as the Squalea, which includes the dogfishes and allies (Squaliformes), the angelsharks (Squatiniformes), the sawsharks (Pristiophoriformes), and the rays, or batoids (Batoidea). The Squalea group is characterized by numerous evolutionary specializations, such as complete hemal arches (ventral projections arising from the vertebral column) in the trunk region anterior to the tail. There has been debate as to whether the frilled shark is actually part of the Hexanchiformes or rather belongs in an order of its own, but derived features shared with other hexanchiforms support its placement within the order (e.g., the extra gill arch and more heart valve rows).
The five extant species of hexanchiforms are divided into two families: Chlamydoselachidae (Chlamydoselachus anguineus) and Hexanchidae (Hexanchus griseus, H. nakamurai, Notorynchus cepedianus, and Heptranchias perlo). The latter family also is known as "cowsharks."
The hexanchiform families are very distinct in their morphological characteristics. Chlamydoselachus is a highly modified and unique shark, with an eel-like body, an enlarged mouth, and well-delimited rows of teeth. (Its teeth are unlike those of any other living shark.) It shares with hexanchids a single, posteriorly located dorsal fin and a long caudal fin, an extra gill arch (hexanchiforms have either six or seven gill arches and gill slits on each side), small spiracles, a clearly demarcated lateral line along the trunk and precaudal tail regions, and a mouth extending posteriorly behind the level of the eyes. Hexanchids have unique teeth that are highly differentiated between the upper and lower jaws and also along either jaw. The upper jaw teeth are small and flattened, with either a single cusp or very small accessory cusps; the lower jaw teeth are very wide and flattened, with multiple prominent cusps in addition to a median (symphysial) tooth and smaller, blunt posterior teeth. Hexanchiforms are noteworthy for having mostly uncalcified vertebrae and notochords with little constriction.
Chlamydoselachus is a uniform dark brown to grayish brown in color, whereas hexanchids are mostly gray without strong color patterns. The exception is Notorynchus, which has darker spots. Notorynchus and Hexanchus griseus attain very large sizes, but the remaining species are more moderate in size, usually not surpassing 63 in (160 cm) in length.
These fishes are found worldwide in tropical and temperate waters, but most species have a spotty distribution, that is, they are known from many isolated regions without records from intermediate areas. Because most hexanchiform sharks occur along the continental slopes, their scattered distribution may be only an artifact of sampling.
Most species are deepwater inhabitants, occurring demersally along the continental slopes but sometimes venturing into more shallow pelagic or inshore waters. The primary exception is Notorynchus cepedianus, which also is a coastal species. There are shallow-water records for Heptranchias perlo and Hexanchus griseus as well, but these species are more common in waters deeper than 328 ft (100 m). Some species also have been recorded from oceanic islands.
Little is known concerning their behavior, but individuals of Notorynchus cepedianus may hunt cooperatively. Sharks are almost exclusively solitary hunters, but broadnose sevengills have been observed hunting as a pack off the coast of Namibia. Individuals circled a large fur seal (which can weigh 770 lb, or 350 kg, more than the shark) and slowly closed in by tightening the circle. After one shark initiated an attack, the remaining sharks followed suit. Hexanchiforms may migrate vertically, entering more shallow waters at night. Some species migrate vertically in the water column, remaining closer to the bottom during the day and ascending to the surface to feed at night.
Feeding ecology and diet
Sixgill and sevengill sharks feed on a variety of bony fishes, sharks, and rays as well as invertebrates. Marine mammals (seals and dolphins) also are consumed. Bony fishes include numerous benthic and demersal families, but pelagic species also are eaten. Some hexanchiform species are known to have attacked individuals of the same species that have been hooked, biting off pieces while they were being reeled or towed in. Because of their mostly obscure habitats, very little is recorded concerning their feeding ecology. At least one species may hunt in packs. Hexanchiform sharks are presumably consumed by larger sharks, including of their own species, but little data exist in relation to their predators.
All species are ovoviviparous (aplacentally viviparous), and the young derive their nourishment exclusively from the yolk sac before birth. Litters can be very large in the two largest species. (More than 100 pups may be born at once in the case of the bluntnose sixgill shark.). Only up to 20 (usually about 12) young are present per litter, however, in the three remaining species (sharpnose sevengill shark, Heptranchias perlo; the bigeyed sixgill shark, Hexanchus vitulus; and the frilled shark, Chlamydoselachus anguineus). Females of the two largest species give birth to young in shallow-water nurseries (though not exclusively for the bluntnose sixgill shark, which also gives birth in other locations). Lengths at birth range from 10.2 to 25.6 in (26–65 cm). Almost nothing is known concerning gestation periods and other details of reproductive biology.
Two of the five currently recognized species of hexanchiforms are listed by the IUCN. Hexanchus griseus is considered Lower Risk/Near Threatened, and Notorynchus cepedianus is cited as Data Deficient.
Significance to humans
Certain hexanchiform species are fished commercially but not to a significant extent. They may be used fresh, frozen, or dried/salted, and the flesh of at least one species (Notorynchus cepedianus) is said to be of good quality. The skin of these sharks is used as leather (particularly in China). Hexanchiforms are not considered to be strictly dangerous, but because of the large sizes of at least some species, they should be approached with caution; very few attacks are attributed to sharks of this order. They are not hardy aquarium sharks. Two species (Notorynchus cepedianus and Hexanchus griseus) have been seen in the wild by tourists. The former species is seen in many areas of its shallow-water range (e.g., in Humboldt Bay and San Francisco Bay, California, United States). The latter was sighted through commercial operations that (until recently) took tourists out in small submersibles to see young sharks at depths of about 656 ft (200 m) off Hornby Island, British Columbia. Diving to observe Hexanchus griseus in the Strait of Georgia (between British Columbia, Canada, and the United States) also is possible during the summer months, at depths from 79 to 138 ft (24–42 m).
List of SpeciesFrilled shark
Bluntnose sixgill shark
Broadnose sevengill shark
Chlamydoselachus anguineus Garman, 1884, Japan.
other common names
French: Requin lézard; Spanish: Tiburón anguila.
A slender, eel-like shark, with a single, low dorsal fin close to the caudal fin; long pelvic, anal, and caudal fins; six gill arches; very elongated gill slits reaching from the dorsal to the ventral side (the first gill slit is especially elongated); a huge mouth with clearly separated rows of numerous teeth (approximately 300), and ventral keels along trunk and tail. The teeth are similar in the upper and lower jaws; there are three long and very sharp, slender cusps with two minute cusplets between. Coloration is a uniform dark brown. Reaches perhaps 77.6 in (197 cm) in length, but more common at about 55 in (140 cm).
Known from many localities scattered in all major oceans but absent from the Mediterranean Sea.
A primarily deepwater species, demersal on the outer continental shelf and slope at depths from about 394 to 4,265 ft (120–1,300 m) but occasionally caught at the surface.
feeding ecology and diet
Mostly unknown, but its teeth suggest that it feeds on deepwater demersal and benthic fishes and cephalopods or other soft invertebrates. The huge gape of its mouth indicates that this species is capable of swallowing large prey items. Predators of the frilled shark are unknown.
Ovoviviparous (yolk sac viviparous), with litters ranging from eight to 12 young. Reproduces year-round off Japan. Gestation periods are not known but are estimated at between 1 and 2 years. Size at birth is about 15.7 in (40 cm); sexual maturity for males is reached at about 39.4 in (100 cm) and for females at about 53 in (135 cm). Breeds from March to June (in Japan).
Not listed by the IUCN.
significance to humans
Taken incidentally as bycatch by trawls or bottom longlines; not considered a significant food item but may be utilized for
fishmeal and human consumption. The remarkable appearance of this shark gives it a menacing and ferocious aspect, but because of its depth distribution, it is not considered dangerous. Unquestionably one of the most bizarre sharks known.
Bluntnose sixgill shark
Squalus griseus Bonnaterre, 1788, Mediterranean Sea.
other common names
French: Requin grisé; Spanish: Cañabota gris.
A stout-bodied, large shark that may reach 16.4 ft (5 m) in length, with a broad, blunt snout; relatively small eyes; six pairs of gill slits; a large caudal fin; a single dorsal fin situated close to the caudal fin, and broad and flattened teeth. There are eight to 10 posteriorly directed cusplets per tooth in the first six teeth of the lower jaw (except the symphysial tooth), and the upper teeth usually have a single cusp. Coloration is a uniform gray to dark brown.
Worldwide in temperate and tropical seas, including the Mediterranean Sea.
This species may be demersal along the continental slopes down to some 6,151 ft (1,875 m), but it also may occur pelagically at the surface. Young individuals are more common inshore, while larger adults are more common in deeper waters.
A solitary, sluggish shark but also capable of strong swimming. Apparently sensitive to light. May migrate vertically to feed at night.
feeding ecology and diet
Feeds on a wide range of fishes, including swordfishes, marlins, dolfinfishes, herrings, grenadiers, cod, hake, ling, and flounders as well as sharks (including hooked individuals of its own species) and rays. Also eats invertebrates (squids, crabs, shrimps) and seals. Predators are unknown for this species, although presumably it may be eaten by larger sharks (including those of the same species).
Yolk-sac viviparous, with large litters that range from 22 to 108 young. Gestation periods are unknown. Gravid females may give birth in shallow bays. Size at birth is about 25.6 in (65 cm). Females are sexually mature at about 177 in (450 cm) and males at slightly smaller sizes.
Listed as Lower Risk/Near Threatened by the IUCN. As Hexanchus griseus is fished for both food and sport and also taken as bycatch, it may not be able to sustain target fisheries. Regional populations are already depleted (e.g., in the northeast Pacific), but fisheries data are generally lacking. Efforts to protect this species are not yet under way.
significance to humans
Not considered a dangerous species, but because of its size, it should be approached with caution. Young individuals are known to thrash about violently when captured. It is not
consumed much, but it is captured in pelagic and bottom trawls and by line and utilized locally for meat, oil, and fish-meal. Can be seen during the summer months off Hornby Island and also Vancouver Island (British Columbia), as young individuals penetrate into the shallow waters of the Strait of Georgia (located between British Columbia, Canada and the United States). They are observed more readily at night.
Broadnose sevengill shark
Squalus cepedianus Peron, 1807, Tasmania, Australia.
other common names
French: Platnez; Spanish: Cañabota gata.
A large shark, reaching at least 9.8 ft (3 m) in length, possibly 13 ft (4 m), with a broad head; seven pairs of gill slits; a moderately tall, single dorsal fin; and large pectoral and caudal fins. The first six pairs of teeth of the lower jaw are wide and flattened, with four to five posteriorly pointed cusps (except the symphysial tooth); the upper teeth usually have single cusp. Background coloration is gray to brown but mottled with numerous, small darker spots and blotches.
A coastal species reaching depths of only some 443 ft (135 m) on the continental shelf and frequently found in very shallow water, bays, and close to shore.
Considered to be an active, powerful shark that usually is observed cruising slowly near the surface but is capable of quick bursts of speed. May be aggressive if provoked and is known to snap vigorously when captured. Speculated to coordinate its entry into shallow bays with tidal fluxes. Cooperative hunting ("social facilitation") has been recorded for this species in addition to three other predatory strategies (stealth, ambush, and bursts of speed). Spy-hopping, in which an individual raises its head out of the water in a vertical position (possibly to see the surroundings), also has been recorded.
feeding ecology and diet
Feeds voraciously on many different families of fishes (including salmon, anchovies, and sturgeon) and also on sharks (dogfishes, houndsharks, and hooked individuals of its own species) and rays (such as eagle rays). Predators are unknown for this species, although presumably it may be eaten by larger sharks (including those of the same species).
Yolk-sac viviparous, with large litters of up to 82 young. Gravid females give birth in shallow bays during the warmer months. Gestation periods are unknown. Size at birth is between 17.7 and 20.9 in (45–53 cm). Males are sexually mature at 59–71 in (150–180 cm) and females at larger than 78.7 in (200 cm). One instance of copulation was observed at around noon in the shallow waters of Namibia, in which it was clear that the male held the female with his mouth. The eggs are relatively large, measuring some 3 in (7.5 cm) in diameter.
Listed as Data Deficient by the IUCN.
significance to humans
Fished for its high-quality flesh in many areas where it occurs. Also fished for sport (in the United States, Australia, and South Africa) and for its skin (in China) for the production of leather. Owing to its large size and putative aggressive behavior, this shark should be approached cautiously. It has been implicated in several attacks on people (in California and South Africa), but these attacks may have been by other species. It is known to have attacked divers while in captivity.
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Marcelo Carvalho, PhD