Mysteries surround tuna, the fastest swimmers in the world
The tuna and their close relatives, the billfish, because of their long, spear-like beaks, are among the most beautiful of the sea’s creatures. These fish, with their delicate colors and wonderfully streamlined bodies, have evolved to what appears to be the limit of hydrodynamic refinement.
When tuna are swimming rapidly, their fins are retracted into grooves, and even their eyes form a smooth surface with the rest of the head. It is not surprising, therefore, that tuna and billfish are the fastest swimmers in the world. Because of its stamina, and speed, tuna can be thought of as both a sprinter and a marathon runner.
Tuna are noted both for their ability to maintain speeds for long periods of time and for bursts of activity, during which they can attain remarkable speeds. It is not surprising that the tuna is one of the fastest swimmers in the world.
When they are being as languid as possible, they must move a distance equal to their own length every second, which, for a large tuna, is faster than a man can swim at top speed. Because they depend on their own motion to pass oxygen-rich water over their gills, tuna can never stop swimming.
They must swim in order to breathe, but of course, they must also breathe in order to swim. Their mode of breathing, by swimming with the mouth open, forcing a jet of water over their gills, is such that their breathing rate is closely adjusted to the speed and thus to the energy spent in swimming. This energy demand is very large, and consequently, tuna eat an enormous quantity of food – some of the species as much as 25 percent of their own weight every day.
Unlike nearly all other fish, and even their relatives, the billfish, tuna are warm-blooded. In fact, because they use so much energy, one of their principal physiological problems is to avoid overheating. Otherwise, they might become so hot that they would cook their own flesh.
Tuna are not “quite” fish, although, as far as we know, the sounds they make are not voluntary but rather are produced by swimming and feeding activities. Tuna are equipped with effective listening devices to help them to hunt prey. They also possess very sensitive chemical detectors, and like those other high-seed predators, hawks and wolves, they have binocular or stereoscopic vision.
The Greek word for Earth’s companions in the solar system was “planetai,” which means “wanderers.” The Greeks might well have applied this name to the tuna, for these continuously swimming fish are wanderers of the sea, capable of traveling across an entire ocean in a few months, and even from ocean to ocean to feed and breed. Like the Flying Dutchman of legend, they are fated to travel continuously. If they ever stopped, not only would they suffocate for lack of oxygen, but they would also sink into the depths because they are heavier than the water in which they live. They must maintain their depth by “flying” throughout the sea.
In ancient times, smaller tuna were popular food among the Mediterranean peoples and among the Polynesians and Japanese in the Pacific. According to Aristotle, however, large, old tuna were unfit even for pickling. He must have been referring to the giant bluefin tuna, which used to be called horse mackerel. In Japan, these big, old tunas, particularly when they are fat, have long been considered a great delicacy as a source of sashimi, the raw fish that is one of the most delectable of Japanese foods. In the Tokyo fish market, the fat, old tuna that make the best sashimi have sometimes sold for as much as $26,000 a ton.
The more they have learned about tuna, the greater the mysteries surrounding these remarkable fish. Some of the unresolved questions are: how do the tuna navigate over thousands of miles in the trackless wilderness of the open ocean? Why do female tuna lay such enormous quantities of eggs? On the average, only two of the millions of eggs laid by a female can survive to adulthood. Are the remaining eggs and larvae basically an evolutionary device to provide food, directly or indirectly, which will allow a few young individuals to survive and grow? How do adult tuna, living at times in the desert waters of the high seas, obtain the quantities of food they need to swim continuously, let alone to reach very large sizes, and to produce large quantities of reproductive products?
But the greatest mystery of the tuna is the mystery of life itself. Are the tuna no more than marvelously efficient swimming, feeding and breeding machines? Their environment and way of life are so inconceivably different from our own that it is hard for us to think of them as our own distant cousins.