The frog shark (Somniosus longus) is a small deep-water sleeper shark of the Pacific, reaching about 1.4 m (4.7 ft), known from a scattering of specimens taken off Japan, New Zealand and probably Chile at 120 to 1,160 m. It has the family's blunt snout, spineless dorsal fins and missing anal fin at the scale of a large dogfish.
Its diet is unstudied and assumed to be small fish, squid and crustaceans, like its Atlantic twin the little sleeper shark. It bears live young, though no litter has been described. The IUCN assessed it as Data Deficient in 2017 because too little is known to judge its status. It has never harmed a person.
- Maximum lengthAbout 1.43 m (4.7 ft)
- WeightNo reliable measurement
- LifespanUnknown; not aged
- Depth range120–1,160 m (390–3,800 ft)
- DietSmall fish, squid, crustaceans (presumed)
- IUCN statusData Deficient (assessed 2017)
- Kingdom
- Animalia
- Phylum
- Chordata
- Class
- Chondrichthyes
- Order
- Squaliformes
- Family
- Somniosidae
- Genus
- Somniosus
- Species
- Somniosus longus
- Family (common)
- Sleeper sharks
- Also known as
- Yume-zame (Japanese)
On this page
How to recognise a frog shark
Picture a Greenland shark shrunk to the size of a spiny dogfish and you have most of the frog shark. The body is a slender cylinder, the head short with a bluntly rounded snout, and the eye is proportionally larger than in the giant sleepers. The two dorsal fins are small, spineless and set well back, the second a little bigger than the first; there is no anal fin, and the tail is short with a weak lower lobe. The whole animal is a uniform dark grey-brown to black, without spots or bands.
The jaws are the family's trademark. The upper teeth are slender spikes for gripping; the lower teeth are broad, low blades with a cusp that leans only slightly sideways, described in the original account as semi-erect. That detail separates the frog shark from the little sleeper shark of the Atlantic, whose lower cusps lean further; both belong to the subgenus Rhinoscymnus, the short-snouted dwarf sleepers, and a specimen without locality data is genuinely hard to name.
In the water the frog shark could pass for a velvet dogfish or a kitefin shark of similar size and colour; the missing dorsal spines, blunt snout and two-part dentition settle it in the hand. The origin of the English name is not recorded, and the Japanese name, yume-zame, means dream shark.
How big do frog sharks get?
The frog shark is a small sleeper shark by any measure. The largest specimen on record is about 1.43 m (4.7 ft) long, and the handful of adults that have been measured cluster around 1.1 m for males and 1.3 m for females. Females appear to be the larger sex, but with so few animals in collections that is a tendency rather than a rule.
Nobody has published a weight, and we will not invent one. A dogfish-shaped shark of 1.3 m usually weighs 8 to 15 kg, but sleeper sharks carry an unusually large oily liver, so the true figure could sit beyond that band. Size at birth is unknown; the little sleeper shark, its closest relative, is born at 21 to 28 cm, and there is no reason to expect the frog shark to differ much. The size bar below sets the maximum known length against a 1.8 m diver; compare it with the 6 m Greenland shark.
Where frog sharks live
The frog shark was described in 1912 by the Japanese ichthyologist Shigeho Tanaka from specimens taken in deep water off Honshu, and Japan is still where most of the known animals come from: Suruga Bay and Sagami Bay, the deep embayments south of Tokyo whose canyons have produced the goblin shark and the frilled shark. Deep-water trawl surveys around New Zealand later turned up the same species on the slopes of both main islands and the Chatham Rise, and specimens from the continental slope off Chile have been referred to it, though the Chilean animals have not been compared genetically with the Japanese ones.
Those three regions are thousands of kilometres apart, which suggests either a sparse distribution along the slopes of the whole Pacific rim or several look-alike populations that a future study will split. Recorded depths run from about 120 m to 1,160 m (390 to 3,800 ft), with most captures between 250 and 1,000 m on soft bottoms of the upper and middle continental slope. Water at those depths off Japan and New Zealand is 4 to 10 °C, and the frog shark is probably not tied to cold water in the way its Arctic relative is, simply to depth.
No SharkSider country page lists the frog shark, because it never comes inshore.
What frog sharks eat
No stomach-content study of the frog shark has been published, so the diet has to be read from the jaws and from the better-known little sleeper shark. The dentition is a gripping-and-cutting set: the narrow upper teeth pin a prey item while the blade-like lower row shears through it. The little sleeper shark, with an identical jaw, eats small bony fishes, squid and octopus, and occasionally crustaceans and pieces of fish too large to have been caught whole. A frog shark almost certainly eats the same: lanternfish and other midwater fishes that migrate into the slope zone, small squid, and whatever carrion drifts down.
The hunting style is inferred too. Sleeper sharks are slow swimmers with soft, flexible bodies and large livers full of squalene that make them close to neutrally buoyant, so they can hover and cruise just above the bottom with very little effort. Their broad pharynx and small mouth work as a suction pump: get within a body length of something edible, open the mouth, and the prey is drawn in. Bite marks that sleeper sharks leave on deep-sea fish and squid caught in the same trawls show they will also take pieces out of animals far bigger than themselves.
Behaviour and senses
The frog shark has never been observed alive for any length of time, so its behaviour is reconstructed from its anatomy and from the few sleeper sharks that have been tagged. The body plan says slow: small fins, a weak tail, soft muscle and a liver that provides lift instead of speed. Tagged Greenland and Pacific sleeper sharks cruise at under half a metre a second and move up and down the slope over a day; a dwarf species is unlikely to be more energetic.
The senses are those of a fish that lives at the edge of daylight. At 200 to 1,000 m there is little or no sunlight, but there is plenty of biological light, and the frog shark's relatively large eye is probably tuned to spot bioluminescent flashes rather than to form sharp images (see are sharks blind?). Smell does most of the work at close range, and the ampullae of Lorenzini on the snout give it an electrical sense that finds prey buried in sediment or hidden in the dark.
Whether frog sharks are solitary is unknown; they turn up singly in trawls. Their evolutionary story is that of the sleeper sharks as a whole, a squaliform lineage specialised for cold and deep water (see the evolution of sharks).
Reproduction and lifespan
Every sleeper shark examined so far is aplacental viviparous: the eggs hatch inside the mother, the embryos live on their yolk sacs, and the pups are born alive and fully formed. The frog shark is assumed to follow the family pattern (see the four ways sharks reproduce), but no pregnant female has been described, so litter size, gestation, newborn size and breeding season are all unknown. The nearest guide is the little sleeper shark, which carries 8 to 17 pups and gives birth at 21 to 28 cm; a frog shark of the same size probably produces a litter in that range.
Maturity can at least be bracketed. The mature males that have been measured were around 1.1 m and the mature females around 1.3 m, which on a 1.43 m maximum means the frog shark matures at 75 to 90 per cent of its final length, late in life in proportional terms. Nobody has counted growth bands in its vertebrae, and sleeper sharks calcify their skeletons so poorly that band counts are unreliable anyway. Deep-water squaliform sharks of similar size, such as the Portuguese dogfish, live for decades and grow very slowly, so a lifespan of twenty to forty years for the frog shark would not be surprising (see how long sharks live), but that is an expectation, not a measurement.
Are frog sharks dangerous to people?
No. The SharkSider database has no incident attributed to the frog shark, and there is no record anywhere of one biting a person. That is geography rather than temperament: the species lives at 120 m and deeper, below any swimmer, surfer or recreational diver, and the only people who handle one are trawler crews and the scientists they pass it to. A 1.3 m sleeper shark hauled onto a deck is sluggish and easy to handle, and its teeth, though sharp, are set in a small mouth built for suction rather than for slashing.
The incidents that do occur in New Zealand and Japan involve coastal species (see the New Zealand attack record), never a deep-slope dogfish. If a frog shark poses any risk to people at all it is the same one the whole family carries: the flesh of sleeper sharks is rich in trimethylamine oxide and is unpleasant, and sometimes toxic, when eaten fresh.
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 assessed the frog shark in 2017 and placed it in Data Deficient. The assessors noted that most of its depth range lies below the deep-water fisheries of Japan and New Zealand, that it is not targeted by anyone, and that it is only rarely taken as bycatch in slope trawls, on bottom longlines and in crab pots; but with a handful of confirmed specimens, no population estimate and observers who may confuse it with the southern sleeper shark, they could not say whether it is stable, declining or secure. New Zealand's own threat classification reached the same verdict, Data Deficient, in 2018. It is not listed on CITES or the Convention on Migratory Species and has no specific legal protection in any country.
The main uncertainty is fishing effort at depth. New Zealand's orange roughy and hoki trawls and Japan's deep-water fisheries work parts of the slope where frog sharks live, and small sleeper sharks caught in a trawl from 800 m are unlikely to survive release. Deep-water dogfishes are vulnerable to exactly this kind of incidental catch because they mature late and produce small litters, which is how the Portuguese dogfish and the kitefin shark came to be depleted in the north-eastern Atlantic. The frog shark's apparent wide distribution and its depth refuge are reasons to hope it is not among the threatened sharks, but until someone counts it, Data Deficient is the only honest label.
Similar species and close relatives
The frog shark's twin is the little sleeper shark (Somniosus rostratus) of the eastern Atlantic and Mediterranean, the other member of the dwarf subgenus Rhinoscymnus. The two are the same size and shape and are separated chiefly by ocean, by the slightly more upright cusps on the frog shark's lower teeth and by small differences in fin position; some authors have questioned whether they are distinct species at all, and a genetic comparison is overdue.
The giant sleeper sharks of the subgenus Somniosus share the same jaw and fin plan at five times the length: the Greenland shark of the Arctic, the Pacific sleeper shark, whose range overlaps the frog shark's off northern Japan, and the southern sleeper shark, which overlaps it off New Zealand and Chile. A 1.3 m juvenile giant sleeper is the likeliest thing to be confused with an adult frog shark; the dwarf's proportionally larger eye is the clue. More distant relatives in the same family include the velvet dogfish and the Portuguese dogfish, both of which carry small dorsal spines that the sleeper sharks lack. A full list of the deep-water squaliforms is on our types of sharks page.
- Little Sleeper Sharkspecies profile
Greenland Sharkspecies profile- Pacific Sleeper Sharkspecies profile
- Southern Sleeper Sharkspecies profile
- Velvet Dogfishspecies profile
- Portuguese Dogfishspecies profile
Frequently asked questions
How big is a frog shark?
Small for a sleeper shark: the largest known specimen is about 1.43 m (4.7 ft). Adult males measure around 1.1 m and adult females around 1.3 m, so females are probably the larger sex. No weight has ever been published, and the size at birth is unknown, though its closest relative is born at 21 to 28 cm.
Where do frog sharks live?
On the deep continental slopes of the Pacific: off Japan, especially Suruga and Sagami bays, around New Zealand including the Chatham Rise, and probably off Chile. They are found at 120 to 1,160 m, most often between 250 and 1,000 m, and never come into shallow coastal water.
What do frog sharks eat?
Nobody has studied their stomach contents. Judging by the gripping upper teeth, cutting lower teeth and suction-feeding mouth shared with the little sleeper shark, the diet is almost certainly small bony fishes, squid and octopus, some crustaceans, and pieces of carrion or larger animals that drift to the slope.
Are frog sharks dangerous to humans?
No. There is no record of a frog shark biting anyone, and the SharkSider database has no incident attributed to it. The species lives far below diving depths, is slow and sluggish, and has a small suction-feeding mouth. The only people who ever touch one are trawler crews and scientists.
Why is it called the frog shark?
The origin of the English name was never explained in the scientific literature; it may refer to the blunt, wide head. The species was named Somniosus longus by Shigeho Tanaka in 1912, longus meaning long, and its Japanese name yume-zame translates as dream shark, in keeping with the sleeper shark family.
How do frog sharks reproduce?
Like all sleeper sharks they are assumed to be aplacental viviparous, hatching their eggs internally and giving birth to live pups. No pregnant female has been described, so litter size and gestation are unknown. The little sleeper shark, its closest relative, carries 8 to 17 pups, which is the best available estimate.
How long do frog sharks live?
Unknown. No one has aged a frog shark, and sleeper shark vertebrae are too poorly calcified for reliable growth-band counts. Related deep-water dogfishes of similar size live for decades and mature late, so a lifespan of twenty to forty years is plausible, but it remains a guess rather than a measurement.
Are frog sharks endangered?
Nobody knows. The IUCN listed the frog shark as Data Deficient in 2017 because it is known from a handful of specimens and there is no population estimate or catch record. Most of its depth range lies below commercial fishing and it is not targeted, but it is taken occasionally as bycatch in deep trawls off New Zealand and Japan. It has no CITES or national protection.
What is the difference between a frog shark and a little sleeper shark?
Very little. Both are dwarf sleeper sharks of about 1.4 m in the subgenus Rhinoscymnus. The frog shark lives in the Pacific off Japan, New Zealand and Chile; the little sleeper shark lives in the eastern Atlantic and Mediterranean. The frog shark's lower tooth cusps lean less. Some scientists doubt they are separate species.
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 October 10, 2022 · Last updated September 22, 2026 · No incident attributed to this species in the SharkSider attack database, which is updated weekly.
