Is a Marlin a Tuna? Key Differences Explained

Marlins are not tunas. Despite sharing the open ocean and a superficial resemblance as large, fast, predatory fish, marlins and tunas belong to entirely different taxonomic families that split apart millions of years ago. Molecular studies have shown that billfishes, the group that includes marlins, are more closely related to flatfishes and jacks than they are to tunas. The confusion is understandable, though, because the two groups have independently evolved some strikingly similar traits, including the ability to warm parts of their bodies above the surrounding water temperature.

Why They Were Once Grouped Together

For a long time, fisheries scientists and taxonomists placed marlins and tunas in the same suborder, Scombroidei, alongside mackerels, barracudas, and swordfish. The logic seemed sound: these were all fast, powerful, open-ocean predators with streamlined bodies built for speed. Traditional classification relied heavily on body shape and lifestyle, and by those measures, a blue marlin and a bluefin tuna look like they belong in the same club.

Molecular phylogenetics changed that picture dramatically. A study analyzing nine mitochondrial and three nuclear gene regions found that billfishes, the family that includes marlins, sailfish, and spearfishes, are only distantly related to tunas and instead share strong evolutionary ties with flatfishes and jacks.¹ A separate large-scale analysis confirmed this and went further, statistically rejecting the idea that the traditional “Scombroidei” grouping represents a real evolutionary lineage.² In other words, the textbook grouping of marlins with tunas is not just imprecise; it is confidently wrong by modern standards.

Tunas belong to the family Scombridae, alongside mackerels and bonitos. Marlins belong to the family Istiophoridae, alongside sailfish and spearfishes. The swordfish, another billfish people sometimes lump in, sits in its own separate family, Xiphiidae. These families share the open ocean as a habitat, but their family trees diverge far back in evolutionary time.

The Bill Is the Most Obvious Giveaway

If you see both fish side by side, the single fastest way to tell them apart is the upper jaw. Marlins have a pronounced, elongated bill, a spear-like extension of the upper jaw that can account for a substantial portion of their total length. Tunas have nothing of the sort. Their jaws are relatively short and blunt, built for engulfing prey rather than slashing through schools of baitfish.

Research on how sailfish and marlins actually use their bills shows these are active hunting tools, not just streamlining features. High-speed video analysis of sailfish revealed that they insert their bills into sardine schools without triggering an escape response, then slash laterally at extreme accelerations to injure or stun individual fish.³ Striped marlin use a somewhat different strategy, relying more on high-speed dashes that break schools apart, often making contact with prey incidentally or tapping isolated fish with their bills.⁴ Tunas, by contrast, are pursuit predators that chase down and swallow prey whole. They rely on raw speed and endurance rather than weaponry.

Beyond the bill, there are other visible differences. Tunas have a row of small, non-retractable fins called finlets running along the top and bottom of their bodies between the second dorsal fin and the tail. These finlets are a hallmark of the scombrid family.⁵ Marlins have a tall, sail-like or rigid first dorsal fin that is dramatically different from anything you’d see on a tuna. Their pectoral fins also fold flat against the body in grooves, another feature that distinguishes their body plan.

Both Are Warm-Blooded, but in Different Ways

One of the main reasons marlins and tunas were historically grouped together is that both can raise parts of their body temperature above the surrounding water. In the cold ocean, being even a few degrees warmer than the environment is a huge competitive advantage: muscles contract faster, eyes process images more quickly, and the brain responds more nimbly. But the way each group achieves this warmth is remarkably different, a textbook case of convergent evolution where two unrelated lineages arrive at similar solutions through separate evolutionary paths.

Tunas warm their bodies from the muscles outward. Their elevated metabolic rates, combined with specialized networks of blood vessels called countercurrent heat exchangers, allow them to retain heat generated by swimming in their locomotory muscles, viscera, eyes, and brain. A bluefin tuna can keep its core muscle temperature well above the surrounding water, though its heart still operates at ambient temperature.⁶ This whole-body warming is what lets tunas thrive in cold temperate and even subarctic waters while maintaining explosive swimming performance.

Marlins take a completely different approach. Rather than warming large blocks of swimming muscle, they have a specialized heat-producing organ located beneath the brain and behind the eyes. This organ is derived from modified eye muscle tissue, specifically the superior rectus muscle, and it generates heat at the expense of the muscle’s ability to contract.⁷⁸ The heater organ can warm the brain and eyes up to 14 degrees Celsius above the ambient water temperature.⁹ The rest of a marlin’s body, including its swimming muscles, stays close to water temperature. So while a tuna is a broadly warm fish with a cold heart, a marlin is a cold fish with a warm brain.

Genome-level analysis has confirmed that these two warming strategies evolved independently. Researchers who sequenced the genomes of swordfish and sailfish found convergent amino acid changes in proteins related to heat production and the visual system in both billfishes and tunas, meaning both lineages hit on similar molecular solutions despite having no shared ancestor that was warm.¹⁰ The selective pressure of hunting in cold, deep water apparently pushed both groups toward warmth through entirely separate evolutionary routes.

Gills Built for the Fast Lane

Both marlins and tunas are ram ventilators, meaning they breathe by swimming forward with their mouths open so that water flows continuously over their gills. (This is why many tuna species must keep swimming to stay alive; they cannot pump water over their gills while sitting still.) But there are differences in how their gill systems are built. Tunas have the largest relative gill surface areas of any fish group, reflecting their exceptionally high oxygen demands. Billfishes also have gill areas that are disproportionately large compared with most other bony fishes, but they fall short of tunas in total gill surface.¹¹

This difference aligns with the broader metabolic picture. Tunas sustain high body temperatures across large muscle masses, which demands enormous oxygen throughput. Marlins, with most of their body at water temperature and only the brain and eyes heated, have lower overall oxygen needs even though they are clearly high-energy predators by any normal fish standard.

How Fast Do They Really Swim?

Popular culture has long crowned the sailfish, a close relative of the marlin, as the fastest fish in the sea, with numbers like 110 kilometers per hour circulated in books and documentaries. Marlins are typically listed not far behind. However, biomechanical research has challenged these figures significantly. An analysis based on muscle contraction times and stride lengths concluded that sailfish are likely incapable of exceeding speeds of about 10 to 15 meters per second, which translates to roughly 36 to 54 kilometers per hour.¹² Beyond that threshold, a phenomenon called cavitation would begin damaging fin tissue, so there is a physical ceiling that likely applies to all large fish regardless of species.

Tunas are also extremely fast swimmers, and the same cavitation limit constrains them. The practical difference in how the two groups use speed is more interesting than any raw top-speed comparison. Tunas are endurance machines: their warm muscles, massive gill area, and streamlined body plan let them cruise at high speeds for long periods, covering enormous distances during migration. Marlins and sailfish are better understood as sprinters, capable of extraordinary short bursts and rapid accelerations, especially when attacking prey with their bills. Neither group plausibly hits the headline speeds you’ll see in popular wildlife lists, but both are among the fastest fish in the ocean by any realistic measure.

Size and Life History

Both marlins and tunas include some of the largest bony fish alive, but they differ in how they grow and how the sexes compare. In blue marlin, females dramatically outgrow males. Research on Atlantic and Pacific blue marlin found that while males max out at around 160 kilograms, females can reach at least 540 kilograms, even though both sexes appear to be similarly long-lived at over 20 years. The size difference is driven entirely by differential growth rates, not by one sex dying earlier.¹³

In tunas, the sexual size difference is far less pronounced. Bluefin tuna can exceed 600 kilograms, but both males and females reach similar maximum sizes. The biggest tunas and the biggest marlins are in a comparable weight class, but the social and reproductive dynamics behind those sizes are quite different. For marlins, the enormous size of females likely reflects selection for fecundity: larger females produce more eggs. For tunas, whose reproductive strategy relies more on sheer volume of spawning events across wide ocean ranges, there is less pressure for one sex to dramatically outgrow the other.

Their flesh differs as well, which matters commercially. Tuna meat, particularly from bluefin, yellowfin, and bigeye species, is deep red and highly vascularized, a direct consequence of the warm-muscle physiology that keeps blood flowing through the tissue at high rates. This is the meat prized in sashimi and seared preparations. Marlin flesh is lighter in color and leaner, more comparable to swordfish. The different textures and flavors reflect the underlying metabolic differences: a tuna’s muscles are essentially red-meat engines, while a marlin’s muscles function more like those of a typical cold-bodied fish.

Why Marlin and Tuna Get Confused at the Fish Counter

If the two fish are so different, why do people keep mixing them up? Part of the answer is simply that both are large, oceanic, and sold as premium seafood. But there is also a deliberate element of substitution. A DNA barcoding study examining seafood trade in Mexico found that fish sold as “marlin” was mislabeled at extremely high rates, and the most common substitute was yellowfin tuna, which replaced marlin in over 70 percent of mislabeled cases.¹⁴ Because tuna is generally cheaper and more abundant, vendors can sell it under the marlin name for a higher price. The two species are different enough in taste and texture that an experienced cook would notice, but casual buyers often cannot tell the difference in a fillet.

This mislabeling also works in the other direction in some markets, where marlin has been sold as tuna. The practice is not just an economic fraud issue. Some marlin species accumulate higher levels of mercury and other heavy metals than many tuna species, so a consumer who thinks they are eating yellowfin tuna but is actually eating marlin may be inadvertently increasing their mercury exposure. Knowing what you are buying matters for health as well as for your wallet.

Why Convergent Evolution Makes This Confusing

The deeper reason people conflate marlins and tunas is that convergent evolution has made genuinely unrelated animals look and behave alike. Both groups evolved in the same ecological arena, the open pelagic ocean, where the same physical rules apply: water resistance favors streamlined bodies, oxygen demands favor large gills, and cold deep water favors some form of internal heating. Given enough time, natural selection sculpted two unrelated lineages into superficially similar shapes.

The pattern is not unique to fish. Dolphins and sharks share a similar body plan despite being separated by hundreds of millions of years of evolution. Bats and birds both fly on wings that evolved independently. The marlin-tuna case is a particularly clean example because genome data can now pinpoint exactly which molecular changes occurred in each lineage. Billfishes and tunas independently evolved modifications to the same types of proteins involved in heat production and vision, arriving at similar functional outcomes through different genetic routes.¹⁰ Knowing this makes both groups more interesting, not less: they are not close cousins who inherited the same traits, but distant relatives who independently invented them.

Telling Them Apart in the Wild

For anglers, divers, or anyone watching fishing shows, telling marlins from tunas is straightforward once you know the cues. Here are the clearest field marks:

Bill: Marlins have a long, spear-like upper jaw. Tunas do not.
Dorsal fin: Marlins have a tall, prominent first dorsal fin, either sail-like (in sailfish) or high and pointed (in blue and white marlin). Tunas have two relatively modest dorsal fins.
Finlets: Tunas have a distinctive row of small triangular finlets between the second dorsal fin and the tail. Marlins have finlets too, but they are fewer and less prominent.
Body shape: Tunas are more uniformly torpedo-shaped with a thick, round cross-section. Marlins are more laterally compressed, taller relative to their width, with a more pronounced taper from head to tail.
Coloring: Tunas tend to have metallic blue-black backs and silver-white bellies. Marlins often show vertical bars or stripes along their flanks that can flash vivid blue or purple when the fish is excited or stressed.

On a plate, the differences are equally clear once you know what to look for. Tuna steaks are dark red, dense, and rich. Marlin steaks are paler, firmer, and leaner. If someone hands you a bright-red fillet and calls it marlin, there is a good chance it is actually tuna, especially given the mislabeling rates documented in seafood trade studies. When in doubt, buying from a reputable fishmonger who can trace the source of their fish is the most reliable way to know what you are getting.