As unbelievable as it sounds, scientists found a fish that holds its breath underwater. Just like their mammalian counterparts, hammerhead sharks stop breathing during deep dives.
They do have gills just like most fish, which allow the hammerheads to take in oxygen from the water and get rid of carbon dioxide. But they close off those gills, essentially holding their breath, hunting prey in the deep, a recent study published in the journal Nature found.
Breathing is not the issue for the tropical scalloped hammerhead, though, it is heat. Closing gill slits stops the cold water intake at depth, which holds in the animal's body temperature.
Muscles generate heat as every animal moves. Humans are warm-blooded, which means our bodies maintain their own temperature through their skin, sweat glands and blood vessels – the body’s heating and cooling system.
Hammerheads are cold-blooded ectotherms. Cold blood literally flows from the gills after heat exchange, according to the study.
"Ectotherms are any so-called cold-blooded animal—that is, any animal whose regulation of body temperature depends on external sources, such as sunlight or a heated rock surface," states Brittanica.com.
The hammerheads live in warm tropical oceans where temperatures hover around 78 degrees. But they frequently dive after prey, deeper than 2,600 feet, where water temperatures are around 40 degrees, the authors wrote.
"As fish move from warm to cold water, metabolic heat generated in muscle tissue is carried away by the blood and is rapidly lost to the environment at the gills," the authors wrote.
"Such deep diving is risky because body cooling can reduce visual acuity, cardiac function, and swimming muscle power, which could be detrimental for obligate ram ventilators—fish that rely on their forward movement to force water across their gills for respiration—such as scalloped hammerhead sharks," the study continued.
After attaching instrument packets to adult hammerheads and taking video, the researchers found that the sharks close their gill slits to water. That action only allowed their muscle temperatures to drop a fraction of the water temperature around them. Their body temperature only dropped about 4 degrees, while the water temperature dropped almost 40 degrees.
The study says that other deep-feeding fish like tuna actually swim faster at depth to minimize contact with the cooler temperatures.