The bigeyed sixgill shark (Hexanchus nakamurai) is a slim, deep-water cow shark of Indo-West Pacific slopes that reaches about 1.8 m (6 ft) and lives mainly between 90 and 600 m. It has six pairs of gill slits, very large green-reflecting eyes, one small dorsal fin set far back, and a lower jaw armed with broad comb-shaped cutting teeth.
It hunts small fishes, squid and crustaceans, rising higher at night. Females bear live young fed only on yolk, reportedly 13 to 26 pups of 40 to 43 cm. The IUCN lists it as Near Threatened as deep-water fisheries expand into its depth band. It is harmless to people and has never been linked to an incident.
- Maximum lengthAbout 1.8 m (6 ft); a 2.06 m record is listed by FishBase
- WeightAround 20 kg (44 lb) at 1.2 m; adults not reliably weighed
- LifespanUnknown; no ageing study for the species
- Depth range90–600 m (295–1,970 ft), recorded to about 700 m
- DietSmall bony fishes, squid, crustaceans
- IUCN statusNear Threatened (assessed 2019)
- Kingdom
- Animalia
- Phylum
- Chordata
- Class
- Chondrichthyes
- Order
- Hexanchiformes
- Family
- Hexanchidae
- Genus
- Hexanchus
- Species
- Hexanchus nakamurai
- Family (common)
- Cow sharks
- Also known as
- Bigeye sixgill shark, Nakamura's sixgill shark
On this page
- How to recognise a bigeyed sixgill shark
- How big do bigeyed sixgill sharks get?
- Where bigeyed sixgill sharks live
- What bigeyed sixgill sharks eat
- Behaviour, senses and parasites
- Reproduction and lifespan
- Is the bigeyed sixgill shark dangerous to people?
- Conservation status
- Similar species and close relatives
- Frequently asked questions
- References
How to recognise a bigeyed sixgill shark
Count the gill slits first. Almost every shark alive has five pairs; the cow sharks have six or seven, and six pairs on a slim, brown-grey shark under two metres puts you in a very short list. The body is spindle-shaped and comparatively light, the head narrow and flattened with a pointed snout, and the eye is the feature the common name refers to: large, oval and fitted with a reflective layer that flashes fluorescent green when a light hits it.
The fins are the second check. There is a single dorsal fin, and it sits far back on the body, behind the pelvic fins and close to the origin of the tail. The caudal fin is strongly asymmetric, with a long upper lobe carrying a deep notch near the tip and a much smaller lower lobe. Fin margins are pale, almost white, and juveniles carry a dark or black smudge at the tip of the upper caudal lobe that fades with age. The back is brownish-grey and the underside abruptly paler.
Open the mouth and the teeth settle the family. Each side of the lower jaw carries five rows of broad, comb-like teeth, each with a tall main cusp followed by a descending series of smaller ones, set edge to edge into a saw. The upper teeth are quite different, narrow and dagger-like. Nothing outside the cow sharks has this arrangement; for the usual pattern see how many teeth sharks have.
How big do bigeyed sixgill sharks get?
This is the small cow shark. Adults commonly measure 1.2 to 1.6 m, and a 1.2 m animal weighs roughly 20 kg (44 lb). The maximum usually accepted for the species is about 1.8 m (6 ft), although FishBase carries a 2.06 m record; some of the larger historical measurements come from specimens caught before the Atlantic population was separated out in 2018, so the upper end of the range should be treated with caution.
Newborns are large relative to the adult: 40 to 43 cm at birth, a third of the length of a mature female. Sexual maturity arrives late in the size range, at about 1.2 to 1.6 m in males and 1.42 to 1.78 m in females, so a female that has just started breeding is already close to the maximum length of the species. Older accounts give these maturity figures in metres rather than centimetres, which would make the shark a five-metre animal; the correct values are the centimetre ones.
Weight data barely exist. Beyond the approximate 20 kg for a 1.2 m shark there is no published series, and it would be dishonest to quote a maximum weight. The size bar below sets the 1.8 m maximum against a 1.8 m diver: a full-grown bigeyed sixgill is about as long as a person, and far thinner than its giant cousin the bluntnose sixgill.
Where bigeyed sixgill sharks live
Since 2018 the answer has been narrower than most sources say. Genetic work published that year showed that the sixgills of the Atlantic belong to a separate species, Hexanchus vitulus, so records from the Gulf of Mexico, the Bahamas, Cuba, the Caribbean, western Europe and the Mediterranean no longer belong on this page. Hexanchus nakamurai as currently understood is an Indo-West Pacific animal.
Within that ocean it is patchily distributed along tropical and warm-temperate slopes. Records come from Japan, where the species was first described, from Taiwan, the Philippines, New Caledonia and the eastern seaboard of Australia; in the Indian Ocean it turns up along the east African coast from Kenya south to South Africa, around Madagascar and at Aldabra in the Seychelles. Aldabra is frequently misprinted as an Indian island in secondary accounts; it is a coral atoll in the western Indian Ocean belonging to the Seychelles.
Habitat is consistent everywhere: the continental and insular shelf edge and upper slope, close to the bottom, with catches concentrated between 90 and 600 m and occasional records to about 700 m. It is a shark of the twilight and dark zones rather than the abyss, and it avoids the truly deep basins where the bluntnose sixgill can still be found. For the broad picture of how depth divides shark species, see where sharks live.
2 countries list the bigeyed sixgill shark in their species guide; the count is the number of incidents attributed to it there in the SharkSider database. Browse every guide on sharks around the world and sharks in the US.
What bigeyed sixgill sharks eat
Diet is known only from stomach contents, and the sample is small. What those samples show is a predator of small and medium bony fishes, with crustaceans appearing regularly and squid turning up in a minority of stomachs. There is nothing here that resembles the scavenging on whale and seal carcasses recorded for the bluntnose sixgill; this is an animal taking prey it can cut up itself.
The dental machinery explains how. The narrow upper teeth hold, while the six broad comb-toothed rows below work as a single saw blade: the shark clamps, then rolls or shakes its head so the lower jaw cuts through flesh in a sawing action. It is an old design, essentially unchanged since the Jurassic, and it works equally well on a fish, a squid or a piece of something larger.
Hunting appears to be a night job. Catch data and the few observations available point to a shark that sits near the bottom during daylight and moves up the slope, and up in the water column, after dark, following the same vertical migration that lifts squid and lanternfishes towards the surface each night. Its sensory kit fits that life: a rod-rich retina backed by the green-shining tapetum, a well-developed sense of smell and the electroreceptive pores known as the ampullae of Lorenzini scattered across the snout, which detect prey at close range where vision fails.
Behaviour, senses and parasites
Almost no behavioural observation exists for this shark in the wild, which is worth saying plainly rather than filling the gap with generalities. What can be reconstructed comes from catch records, anatomy and parasitology.
The vertical movement is the clearest behavioural signal. Sixgills of this genus are caught shallower at night and deeper by day, and the bigeyed sixgill seems to follow that pattern strongly, using the slope as a staircase. It is a slow, sinuous swimmer with a soft body and a large oil-rich liver, built for economy rather than pursuit, and there is no evidence of schooling. Sharks of this size and softness are prey as well as predator: larger sharks take them, which is one reason the adults sit deeper than the juveniles.
The species also supports a small parasite fauna that has proved useful to taxonomists. A monogenean flatworm, Protocotyle euzetmaillardi, has been described from the gills of specimens taken in the South Pacific, and host-specific parasites of this kind help confirm which sixgill population is which. Nothing about the shark's temperament is known, because nobody has watched one behave. The honest summary is that the bigeyed sixgill is a nocturnal, bottom-associated hunter of the slope whose social life has never been observed; the wider question of what preys on sharks of this size is covered under what eats sharks.
Reproduction and lifespan
Like every cow shark, Hexanchus nakamurai is a live-bearer whose embryos are fed entirely on their own yolk, with no placental connection to the mother. Older texts call this ovoviviparity; the current term is yolk-sac viviparity. The pups are born fully formed and self-sufficient at 40 to 43 cm.
Litter size is large by shark standards. Published counts run from 13 to 26 pups, and the figure of thirteen that circulates online is the bottom of that range rather than a maximum. Big litters are the family habit: the bluntnose sixgill can carry more than a hundred. Nothing is known about gestation length, the breeding interval or where the pups are born, although the smaller animals in catch records tend to come from shallower ground, suggesting the shelf edge acts as a nursery while adults stay deeper.
Age is unknown. No validated ageing study has been published for this species, so the lifespan figures that appear on aggregator sites are unsupported. Hexanchiform sharks generally grow slowly, mature late and live for decades, and the maturity data here point the same way: males do not breed until they are at least two-thirds of the maximum length, females until they are close to it. The Red List assessment rates the species' population doubling time at more than fourteen years, the slowest category there is, which is the practical meaning of the general point made under how long sharks live.
Is the bigeyed sixgill shark dangerous to people?
No. The International Shark Attack File has never attributed a bite to this species, and neither has the SharkSider database, which holds no incident anywhere in the world for the bigeyed sixgill. The reason is simple geography: the shark lives below 90 m, mostly far deeper, and there is essentially no overlap with swimming, surfing or recreational diving.
The one sixgill record in the attack database comes from Californian waters in 1985 and cannot involve this species, which does not occur there; it describes a sixgill or sevengill shark encountered by an abalone diver and almost certainly refers to a local bluntnose sixgill or broadnose sevengill. Attributing it here would be exactly the kind of guesswork that inflates shark statistics.
The realistic hazard is on deck. A live 1.5 m sixgill on a longline has a working set of saw-edged lower teeth and a body flexible enough to reach its own tail, and crews handle them accordingly.
The SharkSider database has no incident attributed to this species. The database draws on the International Shark Attack File, the Global Shark Attack File and verified local reports, and is updated weekly.
Conservation status
The IUCN Red List assesses the bigeyed sixgill shark as Near Threatened, in the assessment carried out in 2019. The species is not currently thought to be declining steeply, but it sits one step away from a threatened category because of what is happening around it: deep-water longline and trawl fleets across the Indo-Pacific are working further down the slope every decade, and the 90 to 600 m band is now routinely fished in much of the range.
There is no targeted fishery. Bigeyed sixgills come up as bycatch, and when they do the meat, fins and oil-rich liver are usually kept rather than discarded, so fishing mortality is effectively total for any shark caught. Against that, the species has a life history with almost no resilience: late maturity close to maximum size, unknown but certainly long generation times, and a Red List resilience rating in the lowest band available.
No cow shark is listed on CITES or the Convention on Migratory Species, and no country gives this species specific legal protection, so the only real management levers are depth limits and effort controls on deep-water fleets. Better catch reporting would help more than anything: because the Atlantic population was only separated in 2018, a great deal of the historical catch data for Hexanchus nakamurai actually describes two different species. The general pressures on slow-breeding deep-water sharks are set out under why sharks are endangered.
Similar species and close relatives
The closest relative is the Atlantic sixgill shark (Hexanchus vitulus), which was part of this species until DNA separated the two in 2018. They are near-identical in appearance and are told apart mainly by ocean basin, by small differences in the position of the dorsal fin relative to the pelvic fins, and by genetics. If your animal came out of the Atlantic or the Mediterranean, it is vitulus.
The bluntnose sixgill shark (H. griseus) shares the six gill slits but is a different proposition entirely: heavy-bodied, blunt-headed, with smaller eyes, six rows of comb teeth in each side of the lower jaw rather than five, and a maximum length beyond 4.8 m. Move to seven gill slits and you are among the sevengills, the broadnose sevengill of cool coastal water and the slender deep-water sharpnose sevengill.
The oddest relative is the eel-bodied frilled shark, in its own family but the same ancient order, with six gill slits whose frilly edges meet under the throat. Unrelated but often confused with a sixgill in trawl catches is the prickly shark, a bulky slope squaloid with five gill slits, two dorsal fins and skin like a rasp. The place of the cow sharks near the base of the modern shark family tree is covered in the evolution of sharks.
- Atlantic Sixgill Sharkspecies profile
Bluntnose Sixgill Shark (Hexanchus griseus)species profile
Broadnose Sevengill Sharkspecies profile
Sharpnose Sevengill Sharkspecies profile
The Frilled Sharkspecies profile- Prickly Sharkspecies profile
Frequently asked questions
How big does a bigeyed sixgill shark get?
Most adults measure 1.2 to 1.6 metres and the accepted maximum is about 1.8 metres, or six feet, although FishBase lists a 2.06 metre record. A 1.2 metre shark weighs roughly 20 kilograms. Pups are born at 40 to 43 centimetres, which is unusually large compared with the size of the mother.
Where do bigeyed sixgill sharks live?
In the Indo-West Pacific only, on shelf edges and upper slopes between about 90 and 600 metres. Records come from Japan, Taiwan, the Philippines, New Caledonia and eastern Australia, and in the Indian Ocean from east Africa, Madagascar and Aldabra in the Seychelles. Atlantic records now belong to a separate species.
Why does it have six gill slits instead of five?
Because the cow sharks kept an older arrangement. Six and seven gill slits are the ancestral condition for sharks, and the five-slit pattern of most modern species is the derived one. Hexanchiform sharks have changed remarkably little since the Jurassic, which is why they are often described as living fossils.
What do bigeyed sixgill sharks eat?
Stomach samples contain mainly small to medium bony fishes, with crustaceans appearing regularly and squid occasionally. They hold prey with narrow upper teeth and cut it with the comb-shaped lower teeth, which work as a single saw. Feeding seems to happen mostly at night, when the shark moves up the slope.
Are bigeyed sixgill sharks dangerous to humans?
No. No bite has ever been attributed to this species by the International Shark Attack File or the SharkSider database. It lives below 90 metres and reaches only about 1.8 metres, so there is no overlap with swimmers or recreational divers. The only handling risk is to fishing crews on deck.
How many pups does a bigeyed sixgill shark have?
Reported litters run from 13 to 26 pups. The young develop inside the mother on yolk alone, with no placenta, and are born fully formed at 40 to 43 centimetres. Gestation length, breeding interval and pupping grounds are all unknown, though small juveniles tend to be caught in shallower water than adults.
How long do bigeyed sixgill sharks live?
Nobody has aged one. There is no validated study using vertebrae or any other method, so published lifespan numbers for this species are guesses. What the data do show is very slow population growth: the Red List places its doubling time above fourteen years, the slowest category used, and maturity comes close to maximum length.
What is the difference between the bigeyed and Atlantic sixgill sharks?
Until 2018 they were treated as one species. Genetic work then separated the Atlantic population as Hexanchus vitulus, leaving Hexanchus nakamurai in the Indo-West Pacific. The two look almost identical, differing slightly in the position of the dorsal fin relative to the pelvic fins, so ocean basin is the practical way to tell them apart.
Is the bigeyed sixgill shark endangered?
It is listed as Near Threatened by the IUCN, assessed in 2019, rather than endangered. The concern is expanding deep-water longline and trawl effort across its depth band rather than a measured collapse. It is not listed on CITES, and no country protects it specifically, so management depends on general deep-water fishing controls.
Do bigeyed sixgill sharks come to the surface?
They move upslope and higher in the water column at night, following squid and small fishes that migrate towards the surface after dark, then drop back by day. Even so, the shallowest records sit around 90 metres, so this is a movement within the deep, not a visit to the beach.
References
A small team focused on researching sharks and making marine biology easy to understand. Every page is reviewed against current species records and the SharkSider attack database, and updated when new incidents or research appear.
Published November 12, 2022 · Last updated September 22, 2026 · No incident attributed to this species in the SharkSider attack database, which is updated weekly.
