Reputation: Swordfish, sailfish, tuna and other large marine fish are capable of breathtaking top speeds, zipping through the water at 68mph (110km/h). They stab their prey with their long, sword-like bills.

Reality: It is physically impossible for the fish to even approach these speeds. However, at least one of them can accelerate its bill at an astonishing rate. They do not use their bills for stabbing.

It is almost unbelievable how fast swordfish and sailfish can reportedly swim.

Their hearts are three times larger than those of many fish

Look online and in some of the scientific literature, and you will learn that swordfish can reach speeds of about 60mph (97km/h). Sailfish swim even faster: their top speed is almost 68mph (110km/h). Other large marine predatory fish, such as tuna and marlin, reach similarly extraordinary speeds.

If these top speeds seem beyond belief, well, they are. In fact these predatory fish max out at speeds far below these record figures.

Still, swordfish and their kin certainly look like they have evolved for speed. They are muscular and streamlined, with powerful tails. But arguably, it is the more subtle internal adaptations that really seem to suggest they live life in the fast lane.

For one thing, the gills of these large predatory fish have a surface area many times larger than most fish, allowing them to pull more oxygen out of the water.

And, as explained by marine biologist Richard Ellis in his 2008 book Tuna: A love story, their hearts are three times larger than those of many fish, relative to their body size. Their blood also has an unusually high concentration of oxygen-carrying haemoglobin.

These fish don't just heat their eyes, but their brain too

What's more, swordfish – and other large predatory fish – can do something no other fish can: they can keep parts of their body warmer than the surrounding water. Exactly why this should be the case was unclear for many years, but a study in 2005 suggested an explanation.

Three marine biologists took eyes from dead swordfish and studied how temperature affected their ability to respond to light. They discovered that, at temperatures of about 21C, the swordfish retina can respond to very short flashes of light; up to 25 per second. However, at a chilly 6C the retinas could only respond to one flash per second.

This might be an adaptation for hunting at speed, says team member Eric Warrant of Lund University in Sweden.

"If you're in a situation where a prey item is moving erratically and changing directions often, you need to be able to spot those turns to hunt that fish," says Warrant. "Warmer tissues work more effectively than cold ones. These fish don't just heat their eyes, but their brain too, so they can process information more rapidly in the brain because it's a little warmer."

These observations all fit with the idea that swordfish and other large predatory fish are astonishingly fast. But they are not.

The most recent evidence of this comes in a paper published in August 2016 that tackles the myth head-on.

In theory, sailfish might be able to reach a speed of about 25mph

An international team of marine biologists looked at several large fish, including sailfish; close relatives of the swordfish that also sport a long, sword-like bill. They measured how quickly the muscles midway down each animal's body could twitch in response to an electrical stimulus. Then they used a few simple calculations to estimate how quickly each fish could beat its powerful tail and how fast it should be able to swim.

The results fell some way short of the widely-quoted speed records.

The scientists calculated that, in theory, sailfish might be able to reach a speed of about 25mph (40km/h). Tuna came in at about 16mph (26km/h). In reality, the fish probably reach maximum speeds significantly below this, and spend most of their time swimming at a much slower pace.

Many other biologists have found similar results.

As far back as 2007, Gil Iosilevskii at Technion – Israel Institute of Technology in Haifa and his colleagues explored the problem from a more mathematical perspective.

Some of those high-speed figures come from articles published in the 1940s and 1950s. One comes from Country Life

They found that, at swim speeds above 31mph (50km/h), a beating fish tail would begin to experience cavitation. That is, it would be moving so quickly through the water that it would create tiny bubbles as it did so. When those bubbles burst, they can cause damage to the tissue and reduce swimming performance.

This puts a fundamental limit on the speed that any fish or marine animal can swim in shallow surface waters, says Iosilevskii.

Studies in which fish have been tagged and then tracked fit with the slower speeds too. They show that large marine fish rarely swim much faster than 5mph (8km/h) – although they might reach speeds of about 18mph (30km/h) when they are hunting.

So why are they so often quoted as swimming faster?

"It's very frustrating," says Jens Krause at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin, Germany, a co-author of the myth-busting paper. "Some of those high-speed figures come from articles published in the 1940s and 1950s. One comes from Country Life, it's not even a scientific journal."

If these fish do not swim at high speed, why do they have an internal anatomy that seems so carefully optimised for speed?

Nobody seems to know why speed measurements made so long ago, by non-scientists, are still so widely quoted. It probably does not help that swordfish and other predatory fish are relatively rare animals and the open ocean is a huge place, so actually finding fish to study can be tricky.

Worse, measuring swimming speeds in fish is difficult, because the water they swim through is moving. A fish swimming at 50mph into a current of 50mph would appear stationary. A fish swimming at 5mph and going with the flow of a current travelling at 45mph would appear to be swimming at 50mph. Teasing apart actual swim speed from water speed is difficult.

Perhaps the popularity of the high-speed myth largely rests on the fact that swordfish and other large predatory fish have an anatomy and physiology that seems to suit high speeds.

But that raises an obvious question. If these fish do not swim at high speed, why do they have an internal anatomy that seems so carefully optimised for speed?

Richard Brill at the Virginia Institute of Marine Science in Gloucester Point, who co-authored the "fast swordfish eye" paper, has some ideas. The starting point is to understand a little about the way these fish live.

If they stop swimming they sink and suffocate

"Most marine fish, after they have been involved in a chase to exhaustion, will go and hide, become inactive for a while," says Brill. This gives them some valuable recovery time.

But swordfish and their ilk cannot do this. "They're living in the open ocean," says Brill. "There's no place to hide."

Even if there was a hiding place, the way these fish breathe would prevent them from settling down for some rest and recovery. "They are obligate ram ventilators," says Brill. This means they need to have water flowing over their gills in order to breathe. "If they stop swimming they sink and suffocate."

Swordfish, sailfish and tuna need to recover quickly after a sudden burst of activity, and this is why they have such large hearts and gills, says Brill. "When tunas go through a burst of activity they use their muscles anaerobically, like a 100m sprinter does. But then they need to pay back that oxygen debt, and tunas have evolved to do that very quickly."

There is at least one large marine fish that can move at truly high speeds

Their unique anatomy brings additional advantages, says Brill. For instance, it allows these predatory fish to grow very quickly. "Tunas and billfishes [swordfish and sailfish] have very high growth rates," he says.

Brill suggested these ideas in the mid-1990s and other marine biologists have largely accepted them – although they do not seem to have killed off the high-swim-speed myth yet. Perhaps the new myth-busting paper by Krause and his colleagues will be the final nail in the myth's coffin.

However, Krause's previous work does suggest there is at least one large marine fish that can move at truly high speeds.

In a study published in 2014, Krause and his colleagues studied the hunting behaviour of the Atlantic sailfish, a large predatory fish with a tall sail and a long sword-like bill.

They found that a hunting sailfish would slowly and carefully insert its long bill into a school of sardines – and then vigorously thrash its head from side to side. The sailfish's bill can be 12in (30cm) or so long, so its tip tears through the water very quickly during the head shakes.

Teeth sticking out sideways from the bill slash any sardine that happens to get in the way

Krause's team estimated that the tip accelerated at about 130m/s/s. That is among the fastest accelerations recorded by any marine animal with a backbone, although much slower than the accelerations achieve by some tiny marine creatures.

"The bill is so thin and long that it can achieve enormous acceleration," says Krause. "Faster than the fish escape response, in fact." Teeth sticking out sideways from the bill slash any sardine that happens to get in the way, leaving them injured and easy to eat.

So far, the fast swordsmanship is a hunting style that seems unique to the Atlantic sailfish. "Other billfishes don't seem to have perfected the technique," says Krause.

But even in this case, it is acceleration rate rather than top speed that makes the sailfish attack so effective. The maximum speed of the bill tip is probably only about 12mph (20km/h).

The truth is that swordfish and their like, despite appearances, are not particularly fast.

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