Swordfish hatch with small scales, but they shed every last one as they grow. By the time a swordfish reaches adulthood, its body is completely scaleless, covered instead in a tough, leathery skin studded with tiny tooth-like structures called denticles. This makes swordfish unusual even among their close relatives, the marlins and spearfish, which keep their scales throughout life. The loss is not a defect; it appears to be an adaptation that, combined with some remarkable biological engineering, helps swordfish become one of the fastest predators in the ocean.
Scales at Birth, Gone by Adulthood
Larval and juvenile swordfish do possess scales. If you examined a young swordfish under a few inches long, you would find recognizable bony scales embedded in the skin. As the fish grows, however, these scales are progressively lost. By the time a swordfish is a few feet long, the scales have disappeared entirely, and no new ones grow to replace them. This process, in which a feature present in youth vanishes with maturity, is relatively rare among bony fish. Most scaled fish keep their scales for life, and many add growth rings to them the way trees add rings to their trunks. Swordfish take a different path.
The result is that any adult swordfish you encounter, whether at a fish market or hooked on a deep-sea line, will have no scales at all. Its skin feels rough to the touch, like fine sandpaper, but that roughness comes from structures that are fundamentally different from the flat, overlapping scales found on a typical fish.
What Covers the Swordfish Instead
In place of scales, the adult swordfish body is covered in skin with small, hard, spiny projections. Researchers who have examined swordfish skin at high resolution describe these as roughness elements that are elongated in the direction of water flow and clustered in groups of up to six.
These projections are denticles, structures that are more closely related to teeth than to conventional fish scales. Denticles are made of dentine capped with enamel-like material, and they sit on a base embedded in the skin. If you have ever run your hand along a shark’s body and felt its sandpaper texture, you have felt denticles. Sharks, of course, are cartilaginous fish, not bony fish like swordfish. The presence of denticle-like structures on a bony fish is part of what makes swordfish skin so unusual. On the swordfish head in particular, the denticles are arranged around tiny pores in a pattern that serves a specific function, which brings us to perhaps the most striking feature of swordfish skin.
The Oil Gland That Lubricates the Head
In 2016, researchers using MRI and electron microscopy discovered a previously unknown organ in the swordfish head. It consists of an oil-producing gland connected to a network of capillary-like vessels that carry oil outward to tiny pores on the skin’s surface. The pores are surrounded by denticles, and together they distribute a thin film of oil over the front of the head and the base of the bill.1Journal of Experimental Biology. Lubricating the swordfish head
Chemical analysis showed the oil is a mixture of methyl esters, specifically fatty acid compounds including palmitic acid, oleic acid, myristic acid, and stearic acid. These are common biological fats, but swordfish are apparently the only fish known to produce and secrete them through a dedicated organ in this way. The oil found inside the gland matched the oil found on the skin’s surface, confirming that the gland is the source.1Journal of Experimental Biology. Lubricating the swordfish head
The researchers proposed that this oil layer, combined with the micro-roughness of the denticles, creates a super-hydrophobic surface on the swordfish’s head. A super-hydrophobic surface repels water at a microscopic level, and laboratory studies on engineered super-hydrophobic surfaces have shown friction drag reductions of more than 20%. The swordfish head meets the two requirements for this effect: the oil provides the water-repelling layer, and the denticles provide the micro-scale roughness.2Journal of Experimental Biology. Lubricating the swordfish head – Section: RESULTS AND DISCUSSION
How Scaleless Skin Helps Swordfish Swim
Swordfish are among the fastest fish in the ocean, capable of bursts estimated at speeds exceeding 60 miles per hour. Their body shape is famously hydrodynamic, from the pointed bill to the crescent-shaped tail. But body shape alone does not explain their speed. The skin itself plays a role.
Conventional fish scales create a relatively smooth outer surface that reduces turbulence at low speeds. But at the speeds swordfish reach, the physics of water flow around the body change dramatically. At high speeds, the boundary layer of water clinging to the fish’s body becomes turbulent, and the dominant source of resistance shifts to friction drag along the skin. This is where the denticle-covered, oil-coated skin becomes advantageous. The super-hydrophobic effect reduces the friction between the fish and the water flowing over it, essentially letting the swordfish slip through the ocean with less resistance than a smooth-scaled surface would allow.
Measurements of swordfish drag coefficients bear this out. In controlled experiments using preserved swordfish bodies at their natural cruising speed, the drag coefficient was measured at 0.0091 based on wetted surface area, which is extremely low and comparable to tuna, another group of fish famous for swimming efficiency.3PubMed Central. Hydrodynamic characteristics of the sailfish (Istiophorus platypterus) and swordfish (Xiphias gladius) in gliding postures at their cruise speeds At cruising speed, flow separation does not occur over the entire body, meaning water flows smoothly along the swordfish from head to tail without breaking away into drag-creating vortices.3PubMed Central. Hydrodynamic characteristics of the sailfish (Istiophorus platypterus) and swordfish (Xiphias gladius) in gliding postures at their cruise speeds
The skin surface roughness itself has attracted attention from engineers interested in biomimicry. High-resolution measurements of swordfish skin have documented the precise size, shape, and spacing of the spiny roughness elements, revealing that they are typically elongated in the direction of water flow.4Bioinspiration & Biomimetics. High-resolution measurements of swordfish skin surface roughness Understanding these patterns could inform the design of low-drag coatings for boats, submarines, or underwater vehicles. The idea is straightforward: if evolution has spent millions of years optimizing a surface for moving through water at high speed, there may be engineering lessons embedded in the result.
How Swordfish Differ from Marlins and Other Billfish
Swordfish belong to the billfish group, a collection of large, fast-swimming ocean predators characterized by elongated upper jaws. But swordfish are the only billfish species that loses its scales entirely in adulthood. Marlins, spearfish, and sailfish all retain scales as adults, and those scales show considerable variation and specialization of their own.
Detailed studies of billfish scales have found striking differences between species. In roundscale spearfish and white marlin, for example, the scales differ in shape, how tightly they overlap, and how many pointed projections they carry on their trailing edges. White marlin scales tend to be narrower and more tightly overlapping, while spearfish scales are broader and more widely spaced within the skin. Both species also have additional structures called denticular plates, which are ossified formations sitting on the outer surface of the skin above the scales.5Journal of Fish Biology. Comparative morphology of the scales of roundscale spearfish Tetrapturus georgii and white marlin Kajikia albida
This means marlins and spearfish have both scales and superficial denticular plates, a sort of double-layered external armor. Swordfish, by contrast, shed their scales and rely solely on denticles and their unique oil-secretion system. The evolutionary pressures that pushed swordfish down this different path are not fully understood, but one possibility is that the swordfish lineage, which separated from other billfish long ago, optimized for raw speed in open water to an extent that made conventional scales a liability rather than an asset. Scales add weight and may increase drag at the extreme speeds swordfish reach. Shedding them and replacing their protective function with a denticle-and-oil system could have been the better tradeoff for an animal that hunts by outrunning its prey in the open ocean.
The Kosher Question
The scaleless skin of the adult swordfish has practical consequences beyond biology. Under Jewish dietary law (kashrut), a fish must have both fins and scales to be considered kosher. Swordfish clearly have fins. Whether they have “scales” in the religious sense has been debated for centuries.
The core of the debate is that swordfish do have scales as juveniles but lose them. Some rabbinical authorities have ruled that because the fish possesses scales at some point in its life, it qualifies. Others argue that a kosher fish must have removable scales at the time it is caught and eaten, which adult swordfish do not. The Orthodox Union and most Orthodox authorities consider swordfish not kosher. Conservative Judaism’s Committee on Jewish Law and Standards, by contrast, ruled swordfish permissible in the 1950s, reasoning that the juvenile scales satisfy the requirement. This disagreement persists, and swordfish remains one of the most prominent examples of a fish whose kosher status depends on which authority you follow.
The debate hinges on a genuinely interesting biological fact: the swordfish is one of very few commercially important fish species that possesses scales in youth and completely loses them in maturity. Most fish that are borderline under kosher law have scales that are simply hard to see or hard to remove. Swordfish present a more fundamental case, because the scales are truly absent from the adult animal.
What the Skin Tells Us About Swordfish Evolution
Swordfish are classified in their own family, Xiphiidae, with only a single living species, Xiphias gladius. Their closest relatives, the marlins and sailfish, are grouped in a separate family, Istiophoridae. The fact that swordfish are the sole surviving member of their family suggests a long period of independent evolution, during which their unique skin characteristics had time to develop without influence from closely related species taking a different approach.
The developmental sequence itself is revealing. The fact that swordfish are born with scales and lose them, rather than simply never developing scales in the first place, tells us that the scaleless condition evolved from a scaled ancestor. The genetic instructions for building scales are still present in the swordfish genome and are still activated early in life. What changed over evolutionary time was the addition of a mechanism that deactivates scale maintenance as the fish matures. This is a pattern seen in other evolutionary modifications across the animal kingdom: rather than deleting an old feature outright, the developmental program that produces it gets switched off at a certain stage, and a new feature takes over.
In the swordfish’s case, what took over is a skin system unlike anything else in the bony fish world. The combination of denticles, oil-secreting glands, and a super-hydrophobic surface is, as far as current research shows, unique to this one species. Whether similar systems might exist in less-studied deep-sea fish remains an open question, but among the large pelagic species that scientists have examined closely, swordfish stand alone.
Why Swordfish Skin Interests Engineers
The drag-reducing properties of swordfish skin have made it a subject of active research in biomimetics, the field that takes design cues from biology. Friction drag is a major cost in any application that involves moving an object through water. Ships, submarines, torpedoes, and underwater drones all lose energy to friction between their hulls and the surrounding water. Even small percentage reductions in friction drag translate to meaningful fuel savings over time.
The swordfish offers two separate lessons. The first is the micro-scale surface texture: rows of elongated, spiny projections oriented in the direction of flow. Researchers have mapped these features in fine detail, characterizing their spatial distribution and variability across different parts of the body.4Bioinspiration & Biomimetics. High-resolution measurements of swordfish skin surface roughness The second lesson is the oil layer, which turns the already rough surface into a water-repelling one. Engineered super-hydrophobic surfaces have shown drag reductions exceeding 20% in turbulent flow conditions, and the swordfish achieves something functionally similar using biological materials.2Journal of Experimental Biology. Lubricating the swordfish head – Section: RESULTS AND DISCUSSION
The challenge for engineers is durability. Swordfish continuously produce oil from their glands, so the super-hydrophobic coating is constantly renewed. An artificial surface coating, by contrast, would degrade over time from fouling, abrasion, and chemical breakdown in seawater. Current research is exploring self-replenishing coatings and micro-textured materials that mimic the denticle pattern, though no commercial product has yet matched the swordfish’s integrated system. The fish has had roughly 15 million years of evolutionary refinement working in its favor, which is a head start that engineering labs are still working to close.