However, hammerhead sharks will brave the chill for a tasty meal. Each night, the flat-headed predators make multiple dives over 2,600 feet deep into the frigid depths of the ocean in search of fish and squid, braving a temperature drop of 68 degrees Fahrenheit.
How can chondrichthyans, which are cold-blooded, stay alive at temperatures like this? Sphyrna lewini, also known as scalloped hammerhead sharks, are shown in a study published on Thursday in the journal Science to close their gills during their nightly dives in order to conserve body heat.
Cold-blooded fish have never been seen using this method of controlling their body temperature before, and it sets them apart from high-performance fish (yes, that’s the scientific term) like great white sharks and tuna, which use very strategies to endure extreme cold.
Mark Royer, a shark researcher at the University of Hawaii in Manoa, was conducting research on a different topic when he became curious about the scalloped hammerhead’s secret heating process. He rigged up a set of sensors on the backs of six hammerheads off the coast of Hawaii. After a certain period of time, the packages were programmed to disconnect from the sharks and send out a satellite signal, signalling that they were ready to be retrieved.
Dr. Royer explained that the tags worked like shark Fitbits, recording information such as the shark’s depth and temperature. They were so sensitive that they picked up on each and every twitch of the fish’s tail. Researchers led by Dr. Royer found that hammerheads lose some heat as they dive, but quickly regain it as they swim below the water’s surface. The sharks’ average body temperature during hour-long dives was 75 degrees Fahrenheit, even though the surrounding water was just 39 degrees.
The temperature of the water in which a shark swims greatly influences its body temperature because sharks are ectotherms. Using a mathematical model, Dr. Royer and his team demonstrated that the temperature data they collected made no sense unless the sharks were actively conserving body heat. Dead scalloped hammerheads that had been washed up on the beach were also tested, and similar heat exchange rates were found between the sharks and a water bath as between living deep-diving sharks and ocean water.
Dr. Royer concluded that the fish were “holding their breath,” somehow preventing the flow of water over their gills and the ability to take in oxygen, because they had conserved body heat and had no other physical adaptations that could prevent heat loss. Based on an observation of a scalloped hammerhead doing this at depths greater than 3,000 feet below the surface in 2015, researchers believe the hammerheads do this by physically closing the gill slits. The next step in Dr. Royer’s plan to prove this notion involves attaching video cameras to diving hammerheads.
In agreement with the team’s reasoning, University of Miami marine biologist Catherine Macdonald, who was not involved in the study, said that she couldn’t “see a way” that the sharks could be inhaling while maintaining the body temperatures seen in the data.
Dr. Royer plans to investigate the hammerheads’ metabolism next to learn more about their rest and recuperation after their nightly athletic achievement. It’s like expecting a world-class sprinter to complete a marathon, and he thinks that’s why hammerheads have such a hard time surviving when caught on fishing lines for many hours.