How deep-diving sharks keep warm will take your breath away
Hammerhead sharks like it warm, but for a good meal they are willing to go cold. The flat-headed predators dive more than 2,600 feet from tropical surface waters into the frigid depths of the ocean several times a night to hunt for fish and squid, tolerating a drop in temperature of 68 degrees Fahrenheit for dining.
How do these cold-blooded chondrichthyans tolerate these temperatures without turning into frozen fish? A study published Thursday in the journal Science shows how one species, Sphyrna lewini, or scalloped hammerhead sharks, stay warm during their night dives: they skip the fringes and close their gills, essentially holding their breath.
This strategy for regulating a cold-blooded fish’s temperature has never been observed before and sets them apart from powerful fish (yes, that’s the scientific term) like great white sharks or Atlantic bluefin tuna that use vastly different strategies to tolerate extreme cold.
Mark Royer, a shark biologist at the University of Hawai’i at Mānoa, was inspired to investigate the scalloped hammerhead’s secret heating technique after noticing how deep they dived during another research project. He attached a package of sensors near the dorsal fins of six hammerhead sharks near Hawaii. The packs were designed to detach from the sharks after several weeks and broadcast a satellite signal when they were ready to be scooped out of the sea.
The tags were like shark Fitbits, said Dr. Royer, and collected data such as depth and body temperature. They were even sensitive enough to detect every single movement of the fish tail. Dr. Royer and colleagues found that the hammerheads lose some body heat as they begin their descent, but then quickly return to the same surface temperature as they swim deeper. Even when the surrounding water was as cold as 39 degrees Fahrenheit, the sharks had a body temperature of around 75 degrees during hours of dives.
Sharks are ectotherms, meaning their body temperature is largely determined by the surrounding water temperature. Dr. Royer and his team used a mathematical model to show that the temperature data they collected made no sense unless the sharks were actively trapping body heat in some way. They also measured heat exchange between dead scalloped hammerheads (which had washed up on the beach) and a water bath, finding rates comparable to those between live deep-diving sharks and ocean water. The main similarity between the two? “No conductive heat loss across the gills,” said Dr. Royer. And the gills are the number 1 source of heat loss in a fish’s body.
“Gills are essentially giant radiators attached to the head,” he said.
The retained body heat and the lack of other physical adaptations that could prevent heat loss convinced Dr. Royer believed that the fish were “holding their breath,” somehow stopping the flow of water over their gills — and their ability to take in oxygen. The researchers suspect that the hammerhead sharks do this by physically closing the gill slits an observation from 2015 of a scalloped hammerhead doing this more than 3,000 feet below the surface. Dr. Royer next wants to attach video cameras to hammerhead sharks to confirm this hypothesis.
Catherine Macdonald, a marine biologist at the University of Miami who was not involved in the study, agreed with the team’s reasoning, saying she saw no “way” for the sharks to breathe normally while maintaining body temperature as in the data.
Dr. Royer next plans to study the hammerheads’ metabolism to better understand the recovery period that follows the extreme athletic performance they put in each night. He suspects that the hammerheads’ propensity for relatively short periods of high activity may explain why they die so easily when attached to fishing lines for hours; it’s like asking an elite sprinter to run a marathon.
“This study invites many additional studies,” said Dr. McDonald. “I’m always thrilled with sharks’ ability to surprise me.”