The one thing everyone knows about swordfish is wrong

Source: BBC

By Colin Barras

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.

Swordfish (Xiphias gladius) are little-studied (Credit: Jeff Rotman/naturepl.com)

Swordfish (Xiphias gladius) are little-studied (Credit: Jeff Rotman/naturepl.com)

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.

An Atlantic bluefin tuna (Credit: Wild Wonders of Europe/Zankl/naturepl.com)

Atlantic bluefin tuna (Thunnus thynnus) are not as fast as people say (Credit: Wild Wonders of Europe/Zankl/naturepl.com)

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.

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