2023-06-03 22:03:48
Children learn this from an early age: a dolphin should not be mistaken for a shark. One is rather nice and swimming alongside it is an enchanting experience; the other is probably a little more dangerous, and swimming in its waters is not recommended. One is a mammal, the end result of the evolution of a land creature that returned to the sea and breathes on the surface. The other is a fish whose ancestors never left the liquid element and has gills that can filter oxygen from the water. To fish, one of them dives while holding its breath. The other… Well, so does the other, or at least some of them. In an article published on May 11 in the academic journal Sciencea team from the University of Hawaii (USA) just revealed that scalloped hammerheads (Sphyrna lewini), one of the ten species of hammerhead sharks, stop breathing altogether during their stays in deep waters.
Every evening, these carnivores dive to depths of 600 to 800 meters to catch their favorite prey: squids. At these depths, the water temperature never exceeds 5°C, even in the tropics. Sharks are ectothermic creatures: unlike endothermic animals, commonly called “warm-blooded” animals – us humans, mammals, birds… – their temperature is not constant. Like reptiles, it varies depending on the environment. “For an animal accustomed to tropical surface waters, such cold can be dangerous,” explained Mark Royer, the article’s main author. “If the body gets too cold, metabolism declines, including vision and nerve circuits, as well as muscles and the heart.”
In response to this challenge, certain so-called “high-performance” fish, like tuna, swordfish and white sharks, have unique biological characteristics. Like a dynamo, part of their muscular energy is transformed into a heat source. Yet nothing of the sort has been observed with hammerheads. So how do they manage not only to survive but to hunt normally in such conditions? “We initially thought it might take advantage of thermal inertia and save heat,” continued the researcher. “When we discovered the data, we were stunned.”
Gills, ‘true natural radiators’
Royer and his colleagues equipped six fish with sensors capable of measuring water depth and temperature, the animal’s internal temperature, activity level and body orientation during a dive. After a maximum of three weeks, the device detaches itself, floats to the surface and emits a VHF (“very high frequency”) signal. All researchers had to do was fish it out and analyze the results. They found that the animal maintained an almost constant temperature during most of the process. Only at the end of the drill, more precisely around the middle of the ascent phase, following regarding 15 minutes in deep water, does their internal thermometer drop a few degrees… before quickly recovering once it reaches the surface.
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