The false catshark (Pseudotriakis microdon) is one of the ocean’s more peculiar deep-water sharks, a large, flabby-bodied fish that inhabits continental slopes and seamounts at depths that can exceed 1,500 meters. Despite its size, which can reach around 3 meters, it remains one of the least-studied sharks alive. Encounters are rare enough that most of what scientists know comes from scattered specimens hauled up in deep-sea trawls or caught as bycatch, and new details about its biology and range continue to trickle in.
Why “False” Catshark
The common name sounds like an accusation, as if the animal is pretending to be something it is not. In a taxonomic sense, that is roughly the idea. True catsharks belong to the family Scyliorhinidae, a large and diverse group of mostly small, bottom-dwelling sharks. When Pseudotriakis microdon was first described in 1883 by the French zoologist Léon Vaillant, it bore enough superficial resemblance to catsharks and certain triakid (houndshark) species that its placement was unclear. The genus name Pseudotriakis literally translates to “false triakid,” reflecting that early confusion. The animal ended up in its own family, Pseudotriakidae, which for a long time contained just this one species. More recent taxonomic work has added a handful of smaller relatives, sometimes called pygmy false catsharks, to the family. A 2018 study described Planonasus indicus, a new pygmy species from the Indian Ocean, and provided a revised key to the entire family, underscoring how little-explored these deep-water lineages remain.1Marine Biodiversity. Planonasus indicus sp. n., a new species of pygmy false catshark (Chondrichthyes: Carcharhiniformes: Pseudotriakidae), with a revised diagnosis of the genus and key to the family
What It Looks Like
If you picture a typical streamlined shark, the false catshark is close to its opposite. The body is soft and somewhat gelatinous, with a long, low first dorsal fin that stretches much further along the back than in most shark species. The snout is rounded and broad, and the eyes are large, an adaptation to the dim light of its deep-water habitat. Its species name, microdon, means “small teeth,” and that is accurate: the jaws are lined with hundreds of tiny, hook-like teeth arranged in many rows, suited more for gripping soft prey than for shearing.
Coloration in life is typically dark brown to blackish, which helps the animal blend into the lightless environment of the deep slope. The skin texture is relatively smooth compared to many other sharks. Perhaps the most striking internal feature is the enormous liver, which can account for a substantial fraction of the animal’s total body weight. That liver is packed with low-density oils that provide buoyancy, compensating for the fact that the false catshark lacks a swim bladder, as all sharks do. The result is an animal that can hover and cruise slowly through deep water without expending much energy.
The Buoyancy Engine in Its Belly
A deep-sea shark’s liver is not just an organ for metabolism; it functions as a flotation device. In the false catshark, the liver is proportionally one of the largest among sharks. The oil stored inside is rich in squalene, a hydrocarbon lighter than water that provides neutral or near-neutral buoyancy. An analysis of liver oils from several deep-water shark species, including Pseudotriakis microdon, found that squalene content varied widely across species, ranging from less than 1% to over 80% by weight.2Journal of the American Oil Chemists’ Society. Capillary supercritical fluid chromatographic analysis of shark liver oils Species with higher squalene concentrations tend to be the most sluggish and the most neutrally buoyant. The false catshark fits this profile: it is not a fast pursuit predator but rather a slow cruiser or ambush feeder, drifting through the water column with minimal effort.
This physiological strategy has trade-offs. A body filled with watery, oil-rich tissue and a massive liver is not built for bursts of speed. But it is energy-efficient in an environment where food is scarce and widely scattered. Many deep-water sharks share this general design, though the false catshark takes it to an extreme that few other species match.
Where It Lives
The false catshark has been recorded across a surprisingly broad geographic range. It turns up in the Atlantic Ocean on both sides, the Indian Ocean, and the western and central Pacific. Records exist from waters off Iceland, the Azores, the eastern United States, Japan, Australia, and various other scattered locations. The species typically occupies depths between about 200 and 1,900 meters, with most records clustering between 500 and 1,500 meters along continental and insular slopes.
Despite that wide distribution, actual encounters are infrequent. The shark appears to live at low population densities and is not targeted by any fishery. Most specimens come from deep-water trawl surveys or longline bycatch. Range extensions still happen with some regularity: a 2016 paper documented the first record of the false catshark from the southeastern Atlantic, off the coast of Namibia, which expanded the known distribution of the species in that ocean basin.3Marine Biodiversity. First southeastern Atlantic record of the false catshark, Pseudotriakis microdon (Carcharhiniformes: Pseudotriakidae) Each new record like this hints that the species may be more widespread than catch data suggest, but also underlines how rarely it is seen.
Diet and Feeding Behavior
Stomach-content analyses, though limited by small sample sizes, suggest that the false catshark is a generalist feeder. It eats bony fishes, smaller sharks, squid, and various crustaceans found along the deep slope. The tiny, numerous teeth appear well suited for grasping rather than cutting, which fits a feeding style oriented around swallowing prey whole or in large chunks rather than biting off pieces.
Some researchers have speculated that the false catshark may use suction feeding, rapidly expanding its large mouth to inhale prey items. This technique is common among sluggish deep-sea predators that cannot chase down fast-moving fish. The broad, flattened head and wide gape are consistent with that interpretation, though direct observations of feeding behavior in the wild are essentially nonexistent. Most of what we infer about how this shark feeds comes from anatomy and from what shows up in its stomach.
Reproduction
The false catshark is one of the more fascinating sharks in terms of reproductive biology. It is ovoviviparous, meaning that embryos develop inside the mother and are nourished internally before being born live. What makes this species unusual is the practice of oophagy: developing embryos feed on unfertilized eggs produced by the mother during gestation. This in-utero feeding strategy is shared with a handful of other shark lineages, including some lamnid sharks like makos and threshers, but it is uncommon overall.
Litter sizes are small. Females typically carry only two pups, one in each uterus. The pups at birth are relatively large, often around 70 to 85 centimeters long, which gives them a significant size advantage over potential predators from the moment they are born. Gestation is thought to be lengthy, possibly two to three years, though pinning down the exact duration has been difficult given the scarcity of pregnant females in scientific collections. The combination of small litters, long gestation, and late maturity means the false catshark reproduces very slowly, a trait that makes any population vulnerable to sustained fishing pressure.
Genetic Clues to Population Structure
Because false catsharks are so rarely caught, building up a genetic picture of the species has been a long, slow process. A recent study assembled genetic sequences from specimens sampled across different ocean basins and combined them with all publicly available data to look for population structure. The results revealed a clear split: Atlantic and Indian Ocean populations grouped together genetically, while individuals from the western Pacific formed a distinct cluster.4Conservation Genetics Resources. Novel genetic data for conservation monitoring of the false catshark (Pseudotriakis microdon)
This kind of geographic genetic structuring matters for conservation. If Atlantic and Pacific populations are genetically distinct, they function as separate management units: losing one would not simply be offset by the survival of the other. It also raises questions about how much gene flow, if any, occurs between ocean basins for a deep-water species that is presumably not a strong long-distance swimmer. For a shark that lives along continental slopes, the vast abyssal plains between continents may act as effective barriers to dispersal, even over evolutionary timescales.
Conservation Status and Threats
The International Union for Conservation of Nature currently lists the false catshark as Least Concern. That assessment rests largely on its wide geographic range and the fact that it is not commercially targeted. But “Least Concern” can be misleading for a species about which so little is known. Population size estimates do not exist, and the slow reproductive rate means that even modest levels of bycatch mortality could cause long-term declines that would be difficult to detect until they were severe.
Deep-water trawling is the primary source of incidental catch. As fishing fleets have pushed into deeper waters over the past few decades, more deep-sea species have been exposed to gear that was historically confined to shallower zones. The false catshark, being large and slow, is poorly equipped to avoid trawl nets. It has no commercial value and is typically discarded, but survival after capture and release from deep-water gear is generally poor for sharks, given the physical trauma and the physiological stress of rapid pressure changes during retrieval.
The genetic structuring data add another layer of concern. If populations in different ocean basins are essentially isolated, localized depletion in one region would not be replenished by immigration from another. Monitoring a species this rare is inherently difficult, but the development of genetic tools for identifying specimens, even from fin clips or tissue scraps retained from bycatch, offers a practical path forward for tracking population health over time.
Unusual Observations and Color Anomalies
One of the more unusual recent findings involves a leucistic false catshark, an individual displaying abnormally pale coloration due to reduced pigmentation. Leucism, where pigment cells are present but fail to produce normal levels of color, is documented across many animal groups but is exceedingly rare in sharks. A 2025 report described the first known observation of leucism in Pseudotriakis microdon.5Marine Biodiversity. First observation of a leucistic false catshark Pseudotriakis microdon
Color anomalies like leucism and albinism (a complete lack of pigment) are interesting in deep-water species because the usual selective pressures around coloration work differently in an environment with almost no light. In shallow-water animals, abnormal coloration can increase predation risk by making the individual more conspicuous. In the deep sea, where visual predation is less important and bioluminescence often matters more than reflected light, a pale individual might face fewer disadvantages. Whether the leucistic false catshark observed was otherwise healthy or showed any behavioral differences is not yet clear, but the observation adds to the slowly growing catalog of biological variation in this poorly known species.
How the False Catshark Compares to Other Deep-Water Sharks
The deep sea hosts a surprisingly diverse community of sharks. Gulper sharks, lantern sharks, sleeper sharks, and kitefin sharks all occupy similar depth ranges and share some of the same adaptations: large livers, slow metabolism, and generally low reproductive rates. The false catshark stands out in a few ways. Its body shape is uniquely soft and elongated, more so than most other deep-water species. Its reproductive mode, with oophagous embryos in a two-pup litter, is relatively unusual among deep-water sharks, most of which produce either egg cases or larger litters of smaller pups.
Ecologically, the false catshark seems to occupy a niche as a large, slow-moving generalist predator on the continental slope. It is not as large as the sleeper sharks, which can exceed 6 meters, but it is considerably bigger than most lantern sharks and dogfish that share its depth range. Its role in the deep-sea food web is not well characterized, but its diet of fish, cephalopods, and crustaceans places it as a mid-to-upper-level predator in that ecosystem.
Why So Little Is Known
Studying any deep-water shark is expensive and logistically difficult. Deploying equipment to depths of 1,000 meters or more requires specialized vessels, and each trawl or longline set covers only a tiny fraction of the available habitat. Submersible and remotely operated vehicle surveys can observe animals in situ, but encounter rates with rare species are low. The false catshark adds another challenge: it is not aggregating around known features like hydrothermal vents or whale falls, so there is no obvious place to go looking for it.
The result is that fundamental questions remain unanswered. How long does the false catshark live? Growth-band studies on vertebrae, a standard aging technique for sharks, have not been conducted on enough specimens to produce reliable estimates, but the slow metabolism and large body size suggest a lifespan measured in decades. What triggers mating, and where does it occur? No one has observed courtship or mating behavior. How do juveniles disperse after birth? No tracking data exist. For a species first described nearly 150 years ago, the false catshark remains remarkably mysterious, a reminder that the deep ocean still holds enormous gaps in our understanding of even its larger inhabitants.