Xiphias gladius:Broad-bill sword-fish
Kingdom:Animalia
Phylum:Chordatachordates
Subphylum:Vertebrata
Superclass:Gnathostomatajawed vertebrates
Euteleostomibony:vertebrates
Class:Actinopterygiiray-finned fishes
Order:Perciformes
Family:XiphiidaeSwordfish
Genus: Xiphias
Species: Xiphias gladius Broad bill fish
For a very long-time people have believed that swordfish are very fast swimmers. Not to worry even some scientists have fallen for that. Yes, they are among the fastest fish but compared to terrestrial and airborne creatures they come nowhere close. But how is it that their body is build for speed yet they aren’t as fast. Let’s look at the science behind it.
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.
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. 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.
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.
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?
Some of those high-speed figures come from articles published in the 1940s and 1950s.
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. He says most marine fish, after they have been involved in a chase to exhaustion, will go and hide, become inactive for a while. This gives them some valuable recovery time.
But swordfish 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 need to recover quickly after a sudden burst of activity, and this is why they have such large hearts and gills,
Their unique anatomy brings additional advantages. For instance, it allows these predatory fish to grow very quickly.
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.
The truth is that swordfish and their like, despite appearances, are not particularly fast.
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source : BBC earth
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