Hundreds of fish species lack conventional scales, and they span a surprisingly wide range of habitats and body plans. Catfish, hagfish, lampreys, many eels, paddlefish, and a large number of deep-sea species all have bare or nearly bare skin, while sharks and rays possess tooth-like structures that function differently from the flat, overlapping scales found on typical bony fish. Research on ray-finned fishes alone has identified over two thousand scaleless species, with the trait having evolved independently many times rather than descending from a single ancient ancestor.1Evolution Letters. From scales to armor: Scale losses and trunk bony plate gains in ray-finned fishes
Common Fish That Lack Scales
If you are looking for a straightforward list, these are the groups you will encounter most often in fishing, aquariums, and seafood markets:
- Catfish: Nearly all catfish species are scaleless. The channel catfish, blue catfish, and flathead catfish familiar to North American anglers all have smooth, bare skin. Some armored catfish (like plecos) have bony plates instead, but the typical catfish you would buy at a grocery store has no scales at all.
- Hagfish: These eel-shaped bottom-dwellers have thick, loose skin and no jaws, let alone scales. They are among the most ancient living fish lineages.
- Lampreys: Like hagfish, lampreys are jawless and completely scaleless. Their smooth skin stretches over an elongated, cylindrical body.
- Many eels: Most moray eels lack visible scales, and freshwater eels like the American and European eel have only tiny, deeply embedded scales that are essentially invisible and non-functional as armor.
- Paddlefish: The American paddlefish and its Chinese relative (now likely extinct) have largely naked skin with only scattered patches of tiny scales near the tail.
- Sculpins and blennies: Many small bottom-dwelling species in these families are partially or fully scaleless.
- Swordfish and some tuna relatives: Adult swordfish lose their scales entirely as they mature, ending up with tough, leathery skin.
- Snailfish: These deep-sea and polar species have gelatinous, scaleless bodies adapted to extreme pressure and cold.
That list covers the species most people will run into, but it barely scratches the surface. Scale loss has appeared across dozens of unrelated fish families, from tiny freshwater gobies to ocean sunfish.
Why Sharks and Rays Deserve Their Own Category
Sharks, rays, and skates are often described as “scaleless,” and that is partly right. They do not have the thin, bony, overlapping scales of a bass or a trout. Instead, their skin is covered in dermal denticles, tiny structures that are built more like teeth than like fish scales, complete with enamel-like surfaces and a pulp cavity inside. A review of elasmobranch skin structure describes the dermis as “rather unknown” compared to bony fish but notes its specialized features, including these denticles and a sophisticated sensory system embedded in the skin.2Journal of Fish Biology. Basics of skin structure and function in elasmobranchs: a review
Whether you call denticles “scales” depends on the context. In everyday language, if someone asks whether a shark has scales, the honest answer is “not the kind you are thinking of.” You cannot scrape them off with a fish scaler the way you would with a perch. But in strict biological terms, denticles are sometimes classified as placoid scales, a distinct type that evolved on a completely separate branch of the fish family tree from the scales on bony fish. For dietary and culinary purposes, sharks and rays are almost always grouped with scaleless fish.
How Catfish Lost Their Scales at the Genetic Level
Catfish are probably the most familiar scaleless fish worldwide, and researchers have traced the loss to specific missing genes. A genome study of channel catfish found that several genes belonging to a family involved in building mineralized skin structures are simply absent from the catfish genome. Two genes in particular, known by their shorthand labels SCPP1 and SCPP5, are present in armored catfish (the kind with bony plates) but missing or non-functional in channel catfish. Even fragments of those genes that remain in the channel catfish genome show no evidence of being actively used: promoter regions are gone and no gene transcripts have been detected.3Nature Communications. The channel catfish genome sequence provides insights into the evolution of scale formation in teleosts
This matters because it tells us that scale loss in catfish is not just a surface-level change. The genetic instructions for building scales have been physically deleted or degraded over evolutionary time. And once those genes are gone, scales do not come back. Across ray-finned fishes broadly, researchers have found that scale loss has happened many independent times, but scale re-acquisition is essentially nonexistent.1Evolution Letters. From scales to armor: Scale losses and trunk bony plate gains in ray-finned fishes That one-way street suggests the genes needed to build scales, once broken, are too complex to be rebuilt by random mutation alone.
The Bottom-Dweller Connection
One of the strongest patterns in scaleless fish biology is habitat. Fish that live on or near the bottom of rivers, lakes, and oceans are far more likely to be scaleless than fish that swim in open water. Statistical analysis of thousands of species found a tight link between the scaleless condition and a benthic (bottom-dwelling) lifestyle.1Evolution Letters. From scales to armor: Scale losses and trunk bony plate gains in ray-finned fishes Think about where you find most catfish, sculpins, blennies, and hagfish: they are burrowing in mud, hiding under rocks, or sitting on the seafloor.
Why would living on the bottom favor losing scales? The prevailing idea is that scales are most useful for open-water swimmers who need armor against predators and parasites. Bottom-dwellers rely more on camouflage, burrowing, or chemical defenses like slime. For a fish that spends its life squeezing into crevices or burying itself in sediment, rigid scales could be a liability, reducing flexibility and increasing friction against rough substrates. Bare skin can be thicker, more pliable, and better suited for the constant abrasion that comes with living on the bottom.
Mucus as a Replacement for Armor
If you have ever handled a catfish, you know how slimy they are. That slime is not incidental. For scaleless fish, mucus becomes a critical first line of defense against pathogens. The skin mucus of yellow catfish, for example, plays an outsized role in fending off waterborne bacteria because it does not have scales to serve as a physical barrier. Research has demonstrated that bacteriophages (viruses that attack bacteria) can persist in yellow catfish mucus for about a week even in flowing water, actively helping to kill harmful bacteria on the fish’s surface.4Aquaculture. Bacteriophage adhering to mucus provide protection against Aeromonas veronii infection in scaleless fish
Hagfish take mucus defense to an extreme. When threatened, they release enormous quantities of slime that clogs the gills of would-be predators. Their skin itself is a multilayered composite with a slimy epidermis on the outside, a dense fibrous dermis in the middle, and a fatty subcutaneous layer underneath.5The Biological Bulletin. Material Properties of Hagfish Skin, with Insights into Knotting Behaviors Even without scales, hagfish skin is comparable in tensile strength to the taut skin of other elongated fish.
Wound healing also varies between scaled and scaleless species. A study comparing healing rates across seven fish species with different skin types found that rates depended on the species and, interestingly, on the composition of bacteria living on the skin. In one species, clearing the skin microbiome with antibiotics actually sped up healing.6PubMed Central. Skin Wound Healing Rate in Fish Depends on Species and Microbiota Scaleless fish are not necessarily more vulnerable to wounds than scaled fish, but they depend on different mechanisms to close them.
Breathing Through Bare Skin
Some scaleless or lightly scaled fish use their skin as a supplemental lung. In species that can survive out of water temporarily, the skin absorbs oxygen directly from the air, and bare skin does this far more efficiently than armored skin would. Canterbury mudfish, a small freshwater species, get roughly 43% of their oxygen through the skin in both water and air. When a related species, inanga, is pulled out of water, cutaneous oxygen uptake jumps from about 38% to 63% of their total intake. Both species also excrete a significant portion of carbon dioxide through the skin, making the integument a genuine respiratory organ.7PubMed. The importance of cutaneous gas exchange during aerial and aquatic respiration in galaxiids
This partly explains why many amphibious and semi-terrestrial fish have reduced or absent scales. A thick layer of bony armor would act as a barrier to gas exchange. Fish that regularly leave the water, or that live in stagnant, low-oxygen environments, benefit from having as thin and permeable a skin surface as possible.
Scaleless Fish in the Deep Sea
The deep ocean is home to a disproportionate number of scaleless species. Snailfish, for instance, live at extreme depths, and their bodies reflect that pressure in every way. The Antarctic snailfish has no swim bladder but achieves neutral buoyancy by reducing skeletal density and expanding a watery, gelatinous layer under the skin. The skin itself is extremely thin, sometimes only 85 to 200 micrometers, and the epidermis is actually several times thicker than the dermis, the opposite of typical fish skin architecture.8PubMed. Buoyancy studies and microscopy of skin and subdermal extracellular matrix of the antarctic snailfish, Paraliparis devriesi
At depth, the main threats to survival are not the same as near the surface. Predator encounters are rarer, light is absent, and the priority shifts from armor to energy conservation and buoyancy management. Scales are heavy relative to their volume. A fish trying to stay neutrally buoyant at thousands of meters depth gains more by shedding weight than by carrying armor against predators it seldom meets.
Do Scaleless Fish Accumulate More Contaminants?
This is a practical question that comes up in food safety discussions, and the answer is less clear-cut than you might expect. One laboratory study exposed scaled carp and scaleless catfish to the same concentration of lead in their water. The catfish accumulated substantially more lead in their tissues: muscle tissue in the catfish contained about 14.6 times more lead than the carp’s muscle, and overall tissue lead levels in catfish were roughly 1.6 times higher. The researchers concluded that the scaly skin of carp provided an effective barrier against absorbing the heavy metal.9Int. J. Aquat. Biol. Empirical comparison of toxic lead accumulation between scaled (Cyprinus carpio) and scaleless (Pangasius hypophthalmus) fish species: A religious approach
However, a field study looking at trace metals in the kidneys of scaly and non-scaly fish from a Nigerian lagoon found no significant difference in metal concentrations between the two groups, suggesting that in real-world conditions, metal accumulation was not determined by whether or not the fish had scales.10Cuadernos de Investigación UNED. Heavy metal bio-accumulation in the kidneys of scaly and non-scaly fishes from Epe Lagoon, Nigeria The discrepancy likely reflects differences between controlled lab exposure and the messy realities of wild environments, where factors like diet, metabolism, water chemistry, and organ-specific accumulation patterns all muddy the picture. The safest takeaway is that scales probably offer some barrier effect, but it is not the only factor determining how much contamination ends up in the fillet you eat.
Does Losing Scales Help or Hurt Swimming?
Intuition suggests that bare skin would be smoother and produce less drag than a suit of overlapping scales. And at the level of skin texture, that appears to be true: scaleless carp have a smoother, more supple body surface than common carp with a full set of scales. But when researchers actually tested the two against each other in a swimming flume, measuring their maximum sustained swimming speed and metabolic rate at various speeds, they found no real difference. The scaleless carp did not swim faster or burn less energy.11Current Zoology. Interspecific differences and ecological correlations between scale number and skin structure in freshwater fishes
The researchers speculated that the drag reduction from losing scales might be too small to measure against the background of normal individual variation in fish fitness. Scales are thin and streamlined enough that they do not impose much hydrodynamic cost on a fast-swimming fish. If scale loss provided a major speed advantage, you would expect to see more scaleless species among open-water predators like tuna and mackerel, but those fish are heavily scaled. The correlation runs in the opposite direction: as discussed earlier, scaleless fish tend to be slow-moving bottom-dwellers rather than speed-oriented pelagic hunters.
Bony Plates and Other Scale Replacements
Not every fish that loses conventional scales ends up with bare skin. Some lineages replaced scales with heavier armor. Sturgeons, for instance, have rows of large bony plates called scutes running along their bodies. These scutes are mineralized structures with an unusual internal architecture. Microscopic examination of sturgeon scales has shown that they are composed mainly of collagen and mineralized hydroxyapatite, organized in a way that contains structural elements found in both bone and dentin (the hard tissue inside teeth).12Journal of Structural Biology. The 3D organization of the mineralized scales of the sturgeon has structures reminiscent of dentin and bone: A FIB-SEM study
Armored catfish like plecos and Corydoras went a similar route, developing bony plates that cover the body in rigid rows. These fish gave up the flexibility of bare skin in favor of heavy-duty protection. The evolutionary analysis of ray-finned fishes suggests that gaining trunk bony plates is a separate evolutionary event from losing scales, not an automatic swap. Some lineages lost scales and gained plates; others lost scales and stayed bare.
Religious and Culinary Significance
The distinction between scaled and scaleless fish carries real consequences for people who follow Jewish or Islamic dietary laws. In Jewish kashrut, a fish must have both fins and scales to be permissible. This rules out catfish, sharks, rays, swordfish (which adults lack scales), eels, and all shellfish. The definition of “scale” matters here, and rabbinical authorities have debated borderline cases for centuries. Swordfish, for example, have scales as juveniles but lose them by adulthood, and whether that counts has been argued both ways by different traditions.
Halal dietary guidelines are generally less restrictive about seafood than kashrut, and different Islamic schools of thought disagree on scaleless fish. The Hanafi school tends to permit only fish with scales, while other major schools allow all seafood. For anyone shopping with these guidelines in mind, the question of which fish have scales is not academic; it is the deciding factor in what goes on the plate.
In culinary terms, scaleless fish also handle differently in the kitchen. There is no scaling step, which saves preparation time, but the skin on catfish and similar species is often thick and tough, and many recipes call for skinning the fish entirely. Eel skin is famously leathery and is sometimes used to make small leather goods in Scandinavian and East Asian craft traditions. Scaled fish, by contrast, are more commonly cooked skin-on, with the scales removed but the thin skin left to crisp.