Hammerhead sharks were the first fish to be found to “hold their breath”

Off the sea near Hawaii, a juvenile hammerhead shark with an open mouth and gills.

Image credit: Biosphoto/Alamy

Many fish and marine mammals are known to dive deeper into deeper waters from warmer waters for hunting. The challenge, however, is how to maintain their body temperature when the surrounding water temperature is only a few degrees above freezing, so that the metabolism is active enough to hunt.

In the depths of the cold ocean, if you want to be a “warm hunter”, it is a good way to stop oxygen and save heat. Hammerhead sharks have evolved a unique ability to “close their gills” to avoid a drop in body temperature when they dive into cold, deep water to hunt.

“For any fish, the fastest place to lose heat is always at the gills.” Mark Royer, a postdoctoral researcher in shark physiology and behavior at the University of Hawaii’s Institute for Marine Biobiology, said that because of the large amount of warm blood flowing through the gills, the gills are “essentially like giant radiators tied to the head.”

Some fish, such as whale sharks, are large enough to preserve their body temperature while diving. Other fish, such as tuna, marlin and shark families including great white and mako sharks, have evolved specialized heat exchange systems at the gills to avoid excessive heat loss from the body.

Hammerhead sharks have neither these physical advantages nor these adaptations, but they have been found to dive quickly and repeatedly at depths of about 800 meters.

To understand how sharks respond to temperature changes, Royer and colleagues developed a device that consists of instruments that measure depth, water temperature, position and movement, as well as probes embedded in muscles near the dorsal fin to record the shark’s core temperature. The device is designed to break off after a few weeks, float to the surface and send a signal.

Three hammerhead sharks caught off the coast of Hawaii were tagged with the device.

In a paper published in the journal Science, the team reports that sharks dive several times in deep water with temperatures between 5 and 11 degrees Celsius, with one shark dives 6 times a night in deep water about 20 degrees Celsius below the surface, staying for 5-7 minutes each time before surfacing. Body temperature remains constant for most of the shark’s diving.

Royer believes that sharks effectively “hold their breath” without opening their gills or mouths during dives in order to keep their core temperature stable. “If there is no water on the gills, it will not discharge heat from the body.” “When they get close to the surface, they can slow down, open their gills and start breathing again because it’s not as cold as the bottom of the sea,” Royer said. ”

Mark Meekan, a fish ecologist at the University of Western Australia’s Oceanographic Institute, said stopping oxygen intake in this way showed that hammerhead sharks must be able to cope with a sharp drop in blood oxygen levels when diving, although the mechanism has yet to be discovered.

“What they do is probably slow down the movement of the heart muscle and thus the flow of blood throughout the body.” Meekan said. Shark tissue and blood may have evolved to hold more oxygen per unit volume, similar to the adaptive capacity of people living at high altitudes.

So far, hammerhead sharks have been the first fish to be found to have the ability to “hold their breath,” but Simpfendorfer, a marine biologist at James Cook University in Townsville, Australia, says other sharks and fish may have the same ability to adapt. (Source: Li Huiyu, China Science News)

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