The onefin catshark (Pentanchus profundicolus) is a small deepwater catshark of the Philippines, growing to at least 51 cm (20 in), and the only member of the order Carcharhiniformes with a single dorsal fin. It is the sole species in its genus, and it lives on the continental and insular slopes at roughly 673 to 1,070 m, in permanently dark, cold water.
Its diet has never been examined; relatives of the same size eat small crustaceans, squid and fishes. Its mode of reproduction has not been confirmed, and its lifespan has never been estimated. The IUCN lists it as Least Concern because it lives deeper than the fisheries that work the region. It has never been involved in an incident with a person, and at half a metre it could not be.
- Maximum lengthAt least 51 cm (20 in); larger adults are possible
- WeightNever recorded; well under 1 kg (2 lb)
- LifespanUnknown; age has never been studied
- Depth rangeAbout 673–1,070 m (2,200–3,510 ft)
- DietUnknown; probably small crustaceans, squid and fishes
- IUCN statusLeast Concern (assessed 2020)
- Kingdom
- Animalia
- Phylum
- Chordata
- Class
- Chondrichthyes
- Order
- Carcharhiniformes
- Family
- Pentanchidae
- Genus
- Pentanchus
- Species
- Pentanchus profundicolus
- Family (common)
- Deepwater catsharks
- Also known as
- Single-finned catshark
On this page
- How to recognise an onefin catshark
- How big does the onefin catshark get?
- Where the onefin catshark lives
- What the onefin catshark probably eats
- Behaviour and why one fin is interesting
- Reproduction and lifespan
- Is the onefin catshark dangerous to people?
- Conservation status
- Similar species and close relatives
- Frequently asked questions
- References
How to recognise an onefin catshark
One feature settles it, and you can see it from across a laboratory bench. Every other shark in the order Carcharhiniformes, the ground sharks, has two dorsal fins. This one has a single dorsal, set well back on the body over the rear half of the pelvic fins, with nothing at all on the forward part of the back. That absence is what the English name describes, and it is unmistakable.
The old version of this page called the combination of one dorsal fin and five gill slits "unique among all shark species", and that is not quite right. Single-dorsal sharks do exist outside this genus: the frilled shark, the bluntnose sixgill and the broadnose sevengill all have one. What they do not have is five gill slits, nictitating eyelids and the other carcharhiniform hallmarks. Among the ground sharks, the onefin catshark stands alone.
Everything else about it is standard small catshark. The body is slender and plain brown, the head flattened and spatula-shaped, the snout blunt and a little longer than the width of the mouth, and the eyes are narrow horizontal slits. The mouth is short with long furrows at its corners, and the jaws carry many rows of tiny teeth, each with a central point and a smaller cusp on either side. The anal fin is long and very low, running most of the distance to the tail, and the caudal fin is narrow with a notch near its tip and no crest of enlarged denticles along its upper edge.
How big does the onefin catshark get?
The honest figure is 51 cm (20 in), and it comes from one animal. That was the adult male on which the species was founded, taken in Philippine deep water during the Albatross expedition and described in 1912. A smaller specimen of about 38 cm was an immature fish. A few further individuals have been recorded from Philippine slope surveys since, none of them notably bigger.
Because the 51 cm fish was sexually mature, the species can be said with some confidence to be a genuinely small shark that matures somewhere around half a metre. That places it in the same size class as most deepwater catsharks and near the lower end of the range for sharks generally, though it is nowhere near the true miniatures on our list of the smallest sharks, which mature below 20 cm.
No weight has ever been published, which is unsurprising for a fish preserved at sea more than a century ago. A slender, soft-bodied catshark of 51 cm would weigh a few hundred grams at most; you could hold one comfortably in two hands. Whether the species grows appreciably beyond that length is unknown, and will stay unknown until deepwater surveys in the Philippines bring up a larger series. The bar below sets the largest recorded individual against a 1.8 m diver.
Where the onefin catshark lives
The whole known range of this shark is a small patch of the north-western Pacific: the deep water around Mindanao and the basins between the central Philippine islands. The type locality lies in the Mindanao Sea, and the later records come from the same tangle of deep, steep-sided seas that separate the Philippine archipelago from the Sulu and Celebes basins.
The depths recorded run from about 673 m to 1,070 m (2,200 to 3,510 ft), on or just above the insular slope. That is a long way below the reach of sunlight and of every fishery that matters in the region except the deepest longlines. Water there sits at roughly 4 to 8 °C year-round, the pressure is between seventy and a hundred atmospheres, and the bottom is fine ooze. It is one of the most stable habitats on the planet and one of the hardest to sample, which is the main reason the species is so rarely seen.
Whether the shark also occurs beyond Philippine waters is genuinely open. The Philippine deep-sea fauna shares many species with Indonesia, Japan and the seas off Vietnam, and there is no obvious barrier that would confine a slope-dwelling catshark to one archipelago. The absence of records elsewhere is far more likely to reflect where research trawls have been dropped than where the animal actually is.
What the onefin catshark probably eats
Nobody has opened the stomach of an onefin catshark and published what was in it, so anything said here is drawn from its anatomy and from its relatives. The mouth is small, the jaws are lightly built, and the teeth are tiny and three-pointed. That set is not built for cutting; it is built for seizing small, soft or thin-shelled animals and swallowing them whole. The old page's description of "sharp, pointed teeth for hunting prey" gives the wrong impression of an animal whose largest tooth is a couple of millimetres long.
Deepwater catsharks with this build eat the standard menu of the slope: shrimps and other crustaceans, small squid, polychaete worms, and small fishes including lanternfishes that drift down after dying in the water column above. Much of what reaches the sea floor at a kilometre down is falling food rather than living prey, and scavenging is as important as hunting.
At that depth there is no daylight at all. A catshark finds a meal by smell first, working up a scent trail along the bottom current, and confirms it at close range with the lateral line and with the electroreceptive pores of the ampullae of Lorenzini, which detect the tiny electric fields produced by living muscle. The narrow slit eyes are useful mainly for spotting the bioluminescent flashes of other deep-sea animals. Our page on shark anatomy sets out how those sensory systems fit together.
Behaviour and why one fin is interesting
No onefin catshark has ever been seen alive. There is no film, no submersible observation, no tagging study, and no account by anyone who watched one swim. Everything below is inference from the body.
The build is that of a slow, bottom-hugging fish. The long low anal fin and narrow tail suit gentle, eel-like sculling just above the ooze rather than open-water cruising; the soft body and weak musculature rule out any sort of pursuit; and the flattened, spatula-shaped head is a shape that suits nosing along sediment. Deepwater catsharks filmed elsewhere spend most of their time resting on the bottom and swim only in short unhurried bursts, usually to investigate a smell. There is no reason to think this one behaves differently.
The missing fin is the genuinely interesting question. In most sharks the first dorsal is a keel that resists roll and yaw during fast turns; drop it and you lose stability, which is a real cost unless you rarely swim fast. The onefin catshark appears to be an animal that gave up that stability because it never needed it, and several unrelated deep-sea sharks have reduced their fins in similar ways. It is a small reminder that the standard shark body plan people picture, set out on our page on the evolution of sharks, is a solution to a set of problems, not a rule, and that in the deep sea those problems change.
Reproduction and lifespan
The reproductive mode of the onefin catshark has never been confirmed. The previous version of this page stated flatly that it is oviparous "like other members of the catshark family", which overstates both the certainty and the uniformity of the group. Most deepwater catsharks do lay tough, purse-shaped egg capsules on the bottom, but several genera in the family bear live young from yolk sacs, and no female onefin catshark has ever been described with eggs or embryos in her.
If it does lay eggs, the pattern in the family would suggest one or two capsules at a time, anchored by tendrils to sponges, corals or debris on the slope, and a development period running to many months in water of 5 °C or so. A hatchling would emerge as a miniature adult of perhaps 10 to 12 cm. If it bears live young instead, the litter would be very small, probably two. Either way the output is a handful of young a year at best, which is the norm for small deep-sea sharks and the reason they recover so slowly from any pressure.
Age, growth rate and lifespan are all blank. Nobody has sectioned a vertebra from this species, and vertebrae of deep-sea sharks are often so poorly mineralised that the growth bands cannot be read anyway. Small slope-dwelling sharks that have been aged elsewhere typically live one to two decades, and cold water and scarce food push that upward rather than down; our page on how long sharks live explains why.
Is the onefin catshark dangerous to people?
Not in any sense. The SharkSider database holds no incident attributed to this species and no attack file anywhere mentions it. The shark is half a metre long, its teeth are a few millimetres high, and it lives at a depth no swimmer, snorkeller or recreational diver can reach; the deepest scuba dive ever recorded, 332 m, reaches barely half way to its habitat.
The realistic encounter is on the sorting table of a research vessel. A dead or dying catshark from a kilometre down poses no hazard beyond the abrasive skin common to all sharks. If you are the person holding one, the useful thing to do is photograph it properly, record the exact depth and position, and preserve it with a tissue sample, because material of this species is scarce enough that a single well-documented specimen would be a contribution to science. The species belongs comfortably among the least dangerous sharks, along with almost every other deep-slope catshark.
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 classes the onefin catshark as Least Concern, and the reasoning is depth. The species lives between roughly 673 and 1,070 m, while the fisheries that dominate Philippine waters, municipal gillnets, purse seines, hook-and-line and shallow trawls, work far above it. Most of its habitat is simply out of reach of the gear, so there is no evidence of a decline and no reason to expect one at present.
That verdict rests on very little information, and it is worth saying so plainly. Population size, trend, range limits and reproductive output are all unknown, and a Least Concern listing for a species known from a handful of specimens means "no evidence of a problem" rather than "known to be safe". The Philippines has no targeted deepwater shark fishery and no fin trade reaching a kilometre down, which is the strongest protection this animal has.
The pressures that could change that are the familiar ones. Deepwater trawling and longlining have crept deeper across South-East Asia over the past three decades as shallow stocks have been fished out, and deep-sea mineral exploration has begun to take an interest in the region's basins. Deep-sea sharks are the least resilient group in the ocean to any of this: they grow slowly, mature late and produce almost no young, as our page on why sharks are endangered explains. Small dark catsharks in a net are also almost never identified to species, so a decline could run for years unnoticed, which is why the little bycatch that reaches a laboratory matters.
Similar species and close relatives
Pentanchus contains one species, and that species gives its name to the whole family, Pentanchidae, the deepwater catsharks. Its closest relatives are therefore the slope-dwelling catsharks of the Indo-Pacific: the ghost catsharks of the genus Apristurus, such as the ghost catshark and the longnose catshark, which share the soft body, the flattened snout and the drab brown colouring but carry two dorsal fins; the broadnose catshark; and the tiny pale catshark, another Philippine deepwater species known from very few animals.
Two more are worth a mention for contrast. The filetail catshark of the eastern Pacific has the crest of enlarged denticles along the upper edge of its tail that this species lacks, a useful check when a specimen is damaged. The false catshark is a far larger, flabby slope shark that shows how differently the same habitat can be exploited. If you are handed an unlabelled brown catshark from a Philippine trawl, count the dorsal fins first: one means you are holding something very few people have ever seen.
- Ghost Catsharkspecies profile
- Longnose Catsharkspecies profile
- Broadnose Catsharkspecies profile
- Pale Catsharkspecies profile
- Filetail Catsharkspecies profile
- False Catsharkspecies profile
Frequently asked questions
Why is it called the onefin catshark?
Because it has only one dorsal fin. Almost every shark has two, and every other member of the order Carcharhiniformes, the ground sharks, has two. This species has a single dorsal fin set well back over the pelvic fins and nothing on the forward part of the back. No other ground shark anywhere in the world shares that arrangement.
Is the onefin catshark the only shark with one dorsal fin?
No, though it is often described that way. The frilled shark, the bluntnose sixgill and the broadnose sevengill also have a single dorsal fin, but they belong to different orders and have six or seven gill slits. Among the ground sharks, which have five gill slits and nictitating eyelids, the onefin catshark is the only one.
How big is the onefin catshark?
The largest confirmed individual measured 51 cm, about 20 inches, and it was a mature male, so the species matures at roughly half a metre. A second specimen of about 38 cm was immature. No weight has ever been published; a soft-bodied catshark of that length would weigh a few hundred grams at most.
Where does the onefin catshark live?
Only in Philippine waters as far as anyone knows: the deep seas around Mindanao and the basins between the central islands. It lives on the insular slope at roughly 673 to 1,070 m, in permanently dark water of about 4 to 8 °C. It may well occur more widely in South-East Asia, but nobody has sampled deeply enough to find out.
What does the onefin catshark eat?
No stomach contents have ever been published. Its small mouth and tiny three-pointed teeth are built for seizing small soft prey and swallowing it whole, so it almost certainly eats what other deep-slope catsharks eat: shrimps and other crustaceans, small squid, worms and small fishes, much of it scavenged from what sinks from above.
How does the onefin catshark reproduce?
Nobody knows. Most deepwater catsharks lay tough egg capsules on the bottom, but some genera in the family bear live young, and no female onefin catshark has ever been examined with eggs or embryos. Whichever route it takes, output will be tiny, at most a handful of young a year, as in all small deep-sea sharks.
Is the onefin catshark dangerous to humans?
No. It is half a metre long, its teeth are a few millimetres high, and it lives a kilometre down, far below the reach of any diver. No incident involving the species appears in the SharkSider database or in any attack file anywhere, and none is conceivable. The only people who handle it are scientists sorting deep trawl catches.
How many onefin catsharks have ever been found?
Very few. The species was founded on a single adult male collected in Philippine deep water and described in 1912, with a smaller immature specimen alongside it, and only a handful of further individuals have been recorded since from Philippine slope surveys. It remains one of the least-collected sharks in the Pacific.
Is the onefin catshark endangered?
The IUCN lists it as Least Concern, because it lives deeper than the fisheries that work Philippine waters. That is a statement about the absence of evidence rather than proof of safety: population size, trend and range are all unknown. Deepwater trawling creeping into the slope zone is the pressure most likely to change the assessment.
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 28, 2022 · Last updated September 22, 2026 · No incident attributed to this species in the SharkSider attack database, which is updated weekly.
