Little Sleeper Shark

• Maximum length1.43 m (4.7 ft)
• WeightNo reliable published maximum; a 1.2 m adult is roughly 8–12 kg
• LifespanUnknown; no ageing study, probably several decades
• Depth range200–2,200 m (655–7,200 ft), mostly below 500 m
• DietSquid, bottom-living bony fish, some crustaceans
• IUCN statusLeast Concern (assessed 2019)

How to recognise a little sleeper shark

Most people who meet this shark meet it dead on a longline. A little sleeper shark is a soft, cylindrical, dark-brown to black fish with a short, rounded snout, a small mouth and eyes that seem too small for the head. The skin feels velvety rather than rough, because the dermal denticles are flat and widely spaced, unlike the sandpaper of a spiny dogfish.

The fins settle it. There are two small dorsal fins of nearly equal size, the first set closer to the pectoral fins than to the pelvics, and neither carries a spine; there is no anal fin, which puts the animal in the order Squaliformes; and the tail stalk has a low lateral keel on each side, a feature of the sleeper sharks. The tail fin has a longer upper lobe and a short but distinct lower lobe.

The teeth are the best character of all. The upper jaw holds narrow, spike-like teeth for holding prey; the lower jaw holds broad, low, interlocking teeth with a single oblique cusp that together form a cutting edge, the sleeper-shark blade that lets the family carve pieces from prey too big to swallow. Anyone who has seen the jaws of a Greenland shark will recognise them in miniature. Confusion is possible with the frog shark of the Pacific, which has larger eyes and a larger second dorsal fin, and with the Portuguese dogfish, which has tiny dorsal spines and rougher skin.

How big does a little sleeper shark get?

For a member of the genus that includes the Greenland shark, it is tiny. The largest reliably measured little sleeper shark was 143 cm (4.7 ft) long, and most adults from Mediterranean longline catches are between 80 and 120 cm. Females are the larger sex: mature females run from about 82 to 134 cm, while mature males are typically 70 to 100 cm.

Maturity comes at about 71 cm in males and 80 cm in females, which means a female must roughly triple her birth length before she can breed. Pups are born at 21 to 28 cm (8 to 11 in), a large size for a 1 m mother that reflects the long, slow embryonic development of the deep-water dogfish.

Weight has never been published for the species; a 1.2 m animal is probably 8 to 12 kg (18 to 26 lb), an estimate rather than a measurement. Age is a blank. Nobody has aged a little sleeper shark, and the family has no dorsal spines to section, so the usual method is unavailable; the radiocarbon dating that gave the Greenland shark an age of several centuries has not been tried on its small cousin. Given the slow metabolism and late maturity, a lifespan of several decades is the likeliest guess (see how long sharks live). Compare the 1.43 m maximum with a diver on the bar below.

Where little sleeper sharks live

The stronghold is the western and central Mediterranean. The species was described from Nice in 1827 and it is still caught on deep longlines off the Ligurian and Tyrrhenian coasts of Italy, off Provence and Corsica in France, around the Balearic Islands and along the slope of eastern Spain, with scattered records from Sicily, the Ionian Sea and Greece. In the past decade it has turned up on deep longlines off Israel, extending the known range to the Levant. In the Atlantic it is known from the Bay of Biscay, off Portugal and Madeira, and probably the Azores.

The Japanese and New Zealand records that used to be quoted for this species now belong to the frog shark, Somniosus longus, and a single old Cuban record is doubtful.

It lives on and just above the bottom of the continental slope, over mud and sand, from about 200 m down to at least 2,200 m (655 to 7,200 ft); most catches come from 500 to 1,500 m. Mediterranean deep water is unusual in staying at a steady 13 °C right to the bottom, so this is one of the warmest-living members of a family otherwise tied to near-freezing polar and abyssal water. Nothing is known of migrations; pregnant females, juveniles and adults are all taken on the same grounds, which suggests the population does not move far.

What little sleeper sharks eat

Stomachs from Mediterranean longline catches, mainly off Spain, France and Italy, give a clear answer: cephalopods dominate. Mid-water and bottom squid, including the large-eyed jewel squid Histioteuthis and various ommastrephids, make up most of the food by weight, with bottom-living bony fish such as grenadiers, hake and small codlings second, and crustaceans, mainly deep-water shrimps, a distant third. That is the same squid-heavy menu the Greenland shark eats in the Arctic, scaled down.

The teeth explain how it is eaten. The spiky upper teeth pin the prey and the blade of interlocking lower teeth slices through it, so the shark can take chunks out of a squid or a fish larger than its mouth. Fast squid in the stomach of such a slow fish are the puzzle of this species. The Greenland shark is thought to ambush sleeping seals and to scavenge; the little sleeper may do the same, taking squid that are resting or spent after spawning and any dead fish that drifts down the slope. Longline bait is clearly attractive, which supports the scavenging idea.

The sense that finds the food in darkness is almost certainly smell, backed by the ampullae of Lorenzini for the final approach. The eyes are small and the deep Mediterranean is black below 500 m; sleeper sharks in general are built to detect prey by scent and electric field, then close with a slow, unhurried lunge.

Behaviour and adaptations

The name is fair: this is a sluggish, slow-swimming shark. Sleeper sharks have soft, weakly calcified skeletons, small fins, flabby muscle and enormous oil-rich livers, all of which point to a low-energy life of cruising slowly along the bottom and letting buoyancy do most of the work. A large liver full of low-density squalene keeps the fish close to neutral buoyancy, so it can hang above the sediment without expending energy, the same trick used by the kitefin shark and the velvet dogfish on the same slopes.

The skin is smooth and the denticles are flat, which reduces drag at slow speed and may also reflect a life spent brushing against the bottom. There is no bioluminescence in this family; the light organs of the lanternsharks are absent.

Social behaviour is unknown, but the longline evidence hints at aggregation. Several animals are sometimes hooked on the same stretch of line, and catches that include pregnant females and juveniles suggest the species breeds and grows up on the same patch of slope. Nothing is known of daily movements. Other deep-water sharks rise up the slope at night to feed and retreat by day; whether the little sleeper does this is a question waiting for a tag. Its Mediterranean cousin lives in water ten degrees warmer than the Greenland shark, so its pace of life is probably somewhat faster, though still slow by any shallow-water standard.

Reproduction and lifespan

Like all sleeper sharks, the little sleeper is ovoviviparous, or aplacental viviparous in modern terms: the eggs hatch inside the mother and the embryos live on their yolk until they are born as fully formed miniature sharks. There is no placenta and no feeding on unfertilised eggs. Litters recorded from Mediterranean females run from 8 to 17 pups, each 21 to 28 cm (8 to 11 in) at birth, which is a substantial investment for a mother of 1 to 1.3 m.

Gestation has never been measured, and in a species that lives in constant 13 °C water with no seasons to speak of, there is probably no fixed breeding time. Pregnant females have been taken on the same slope grounds as small juveniles in the western Mediterranean, so birth appears to happen where the adults live rather than in a separate shallow nursery. Deep-water dogfish typically carry their young for a year or more, and the large size of the pups suggests the little sleeper is no exception.

Males mature at about 71 cm and females at about 80 cm. Neither age at maturity nor lifespan has been measured. If the pattern of other cold-water squaliforms holds, maturity takes well over a decade and the lifespan runs to several decades. That combination, late maturity and a litter of a dozen every two years or so, is what makes deep-sea sharks so slow to recover from fishing, and it is why the European ban on landing them matters more than the IUCN category suggests.

Is the little sleeper shark dangerous?

No. The SharkSider database contains no incident attributed to the little sleeper shark, and there is no plausible way one could occur: the animal lives hundreds of metres down, grows to 1.4 m, swims slowly and has a small mouth. A swimmer, diver or surfer will never encounter one alive. Only deep-water longline and trawl crews handle the species, and their only hazard is a jaw closing on a careless hand during unhooking.

The family as a whole has a reputation that belongs to the Greenland shark, which is big enough to be dangerous in theory and which Inuit folklore treats with respect, though even it has no verified attack on a human. The Mediterranean species inherits the name and none of the risk. If anything, the danger runs the other way: sleeper shark flesh is rich in trimethylamine oxide, which breaks down into a toxin, and the Greenland shark must be fermented for months before it is safe to eat.

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 and threats

The IUCN Red List assesses the little sleeper shark as Least Concern (2019). The reasoning is that the species lives mostly below the depths at which fisheries operate, that it is caught only incidentally, and that the deep-water regulations of the past twenty years have removed most of what pressure there was. Two of those rules matter. Since 2005 the General Fisheries Commission for the Mediterranean has banned bottom trawling below 1,000 m across the whole sea, which protects the deeper half of the shark's range outright. And since 2010 the European Union has set a zero catch limit for deep-sea sharks, with Somniosus rostratus named on the list, so any little sleeper caught by an EU vessel must be discarded rather than landed.

The threats that remain are real but modest. Deep longlines for hake, blackspot seabream and other slope fish still hook the species on the upper slope between 200 and 1,000 m, and discarded sharks brought up from that depth rarely survive the pressure and temperature change. Before the discard rule, small numbers were sold for meat or fishmeal, and that trade persists in North Africa, where EU rules do not apply.

The species is not listed on CITES or the Convention on Migratory Species, and no country protects it specifically. It is doing better than most deep-sea sharks because it lives in a sea where deep fishing has been curtailed, not because anyone set out to save it. Our page on why sharks are endangered explains how unusual that is.

Similar species and close relatives

The genus Somniosus has two branches. The giants are the Greenland shark of the North Atlantic and Arctic, the Pacific sleeper shark of the North Pacific and the southern sleeper shark of the Southern Ocean, all of which pass 4 m and may live for centuries. The little sleeper and the frog shark of Japan and New Zealand are the small branch, animals of 1.4 m or less that were once lumped together under the name S. rostratus and are now treated as separate species on opposite sides of the world. The frog shark has larger eyes and a bigger second dorsal fin.

On the Mediterranean and Atlantic slopes the little sleeper shares its habitat with several other dark, soft-bodied dogfish that fishermen rarely separate. The Portuguese dogfish and the velvet dogfish, both in the same family, have small dorsal spines and rougher skin; the kitefin shark has thick lips, a wider mouth and big triangular lower teeth; and the lanternsharks are smaller, with dorsal spines and light organs. A spineless dorsal fin, no anal fin and a lower jaw of interlocking blade teeth are the three checks that identify a little sleeper.

• Greenland Sharkspecies profile
• Pacific Sleeper Sharkspecies profile
• Southern Sleeper Sharkspecies profile
• Frog Sharkspecies profile
• Velvet Dogfishspecies profile
• Portuguese Dogfishspecies profile

Written and reviewed by

SharkSider Editorial Team

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 14, 2022 · Last updated September 22, 2026 · No incident attributed to this species in the SharkSider attack database, which is updated weekly.