To Keep Warm In The Deep While Hunting, Hammerhead Sharks Hold Their Breath: Study

The researchers found that warm muscle temperatures were maintained throughout the deepest portion of each dive made by hammerhead sharks. 

Scalloped hammerhead sharks (Sphryna lewini) keep their bodies warm in cold, deep water by holding their breaths, a new study has found. They foray into deep water to hunt prey such as deep sea squids, stingrays, and bottom dwelling crustaceans. The study, led by University of Hawai'i at Mānoa, was recently published in the journal Science. 

The ecology (relationship between living beings and their surrounding environment) of a species acts as an important link between deep and shallow water habitats, and the findings provide key insights to this. 

Most fish are completely ectothermic, which means that their body temperatures are heavily regulated by their environment. Since large, predatory fish must maintain a certain body temperature to function properly, and often need to venture into different thermal environments to hunt prey, being ectothermic can be challenging, the study said. 

When fish move through different thermal environments, they experience heat exchange via conduction through the body wall, and convection from blood flow across the gills, the authors noted in the study.

Why do sharks hold their breaths when they dive deep?

Adult scalloped hammerhead sharks use a strategy to prevent convective heat loss at the gills during excursions into deep, cold water. They dive rapidly and repeatedly from warm surface waters, which have temperatures of about 26 degrees Celsius, to depths exceeding 800 metres, which have temperatures as low as five degrees Celsius. They hold their breath to prevent convective heat loss at the gills.

If the sharks did not close their gill slits during deep dives into cold water, their gills, which are natural radiators, would rapidly cool the blood, muscles and organs. 

Biologgers, or instruments used to track wild animals, were attached to adult sharks, as part of the study. The devices simultaneously measured the sharks' muscle temperature, depth, body orientation and activity levels, a statement released by the University of Hawai'i at Mānoa said.  

The researchers found that warm muscle temperatures were maintained throughout the deepest portion of each dive made by hammerhead sharks. 

During the latter stages of the ascent phase, substantive cooling occurred. Once cooling started, it was rapid. 

According to the study, the sharks suppressed gill function by holding their breaths during deep dives to suspend convective heat transfer. This strategy had not been observed earlier. It has broad similarities to marine mammal "breath hold" diving, the authors noted. 

Royer, the lead author on the paper, was quoted as saying in the statement that it was unexpected for sharks to hold their breath to hunt like a diving marine mammal, and that it is an "extraordinary behaviour" from an incredible animal. 

Royer said although it is obvious that air-breathing marine mammals hold their breath while diving, the researchers did not expect to see sharks exhibiting similar behaviour. He explained that this behaviour reveals that scalloped hammerhead sharks have feeding strategies that are broadly similar to those of some marine mammals, like pilot whales. 

The statement said that a video of a scalloped hammerhead shark swimming along the seabed at a depth of 1,044 metres showed its gill slits tightly closed, while similar images from surface waters show the sharks swimming with their slits wide open. 

In the statement, Royer said that holding their breath keeps scalloped hammerhead sharks warm, but also shuts off their oxygen supply, so, although the animals hold their breath for an average of 17 minutes, they only spend an average of four minutes at the bottom of their dives at extreme depths before quickly returning to warmer water, which is well-oxygenated. 

More about the “breath-holding” mechanism

The authors noted that instead of body cooling routinely beginning and persisting during the deeper stages of the dive, the sharks maintained an elevated body temperature, up to 20 degrees Celsius above ambient temperature, during most of each dive, and only rapidly lost heat near the inflection point in the ascent.

Sharks have an active mechanism to prevent heat loss from their gills during excursions into cold water, the apparent lack of heat transfer suggests. 

The sharks probably shunt blood away from their gills, or reduce the flow of water across the gills by closing their mouths, gill slits, or both, the authors noted. This inhibits the sharks' ability to absorb oxygen from the environment. 

By maintaining a warm body temperature deep in the ocean, scalloped hammerhead sharks are able to exploit resources in the cold environment. 

Since the feeding events are very brief, the sharks fleetingly opening their gills while feeding at depth would be unlikely to be reflected in the core muscle temperature. 

The authors concluded that scalloped hammerhead sharks preserve cardiac function during deep dives by reducing convective heat loss at the gills, and maintaining both muscle and heart temperature.