Hammerhead sharks love warmth, but in order to have a good meal, they are willing to get cold. The flat-headed predators dive more than 2,600 feet from tropical surface waters into the frigid ocean depths several times each night to hunt for fish and squid, enduring 68-degree Fahrenheit dips in temperature to eat.
How do these cold-blooded cartilage survive such temperatures without turning into frozen fish? A study published Thursday in the journal Science shows how one species, Sphyrna lewini or scalloped hammerhead sharks,And Staying warm during night dives: They skip the frills and close their nostrils, essentially holding their breath.
This strategy of temperature regulation in cold-blooded fish has not been observed before and is distinguished from high-functioning fish (yes, that is the scientific term) such as great white sharks or Atlantic bluefin tuna that use vastly different strategies to withstand extreme cold.
Mark Royer, a shark biologist at the University of Hawaii at Manoa, was inspired to investigate the scalloped hammerhead’s secret heating technique after noticing how deep it dived during a different research project. He attached a bundle of sensors near the dorsal fins of a six hammerhead near Hawaii. The packages were designed to separate from the sharks after several weeks and sent a satellite signal when they were ready to be released from the sea.
The tags were like Fitbits sharks, Dr. Royer said, collecting data such as depth and body temperature. It was even sensitive enough to detect every flick of the fish’s tail. Dr. Royer and his colleagues found that hammerheads lose a little body heat as they begin their descent, but soon return to the same surface temperatures as they swim deeper. Even when the surrounding water was as cold as 39 degrees Fahrenheit, the sharks’ temperature was around 75 degrees during the hour-long dive.
Sharks are ectotherms, which means that their body temperature is largely determined by the temperature of the surrounding water. Dr. Royer and his team used a mathematical model to show that the temperature data they collected didn’t make sense unless the sharks were somehow actively conserving body heat. They also measured rates of heat exchange between dead scalloped hammerheads (which washed up on the beach) and a water bath and found rates similar to those between live sharks in deep, ocean water. The main similarity between the two? “There is no heat loss conductive across the gills,” said Dr. Royer. The gills are the first source of heat loss in the body of the fish.
“The nostrils are basically giant radiators attached to the head,” he said.
The preserved body heat and lack of other physical adaptations that could prevent heat loss convinced Dr. Royer that the fish were “holding their breath” in some way that stopped the flow of water over the gills – and their ability to absorb oxygen. The researchers suspect that the hammerhead does this by actually closing the gill slits, based on this Note 2015 The scalloped hammerhead that’s doing the work more than 3,000 feet below the surface. wants dr. Royer attached video cameras to the diving heads of the hammer to confirm this hypothesis.
Katherine McDonald, a University of Miami marine biologist who was not involved in the study, agreed with the team’s reasoning, saying she could “see no way” that sharks could breathe normally while maintaining body temperatures seen in the data.
Dr. Royer next plans to study the metabolism of the hammerheads to better understand the recovery period that follows the intense athletic achievement they do each night. He suspects that hammerheads’ propensity for relatively short periods of high activity might explain why they die so easily when caught in fishing lines for many hours; It’s like asking an elite runner to run a marathon.
“This study calls for a lot of additional studies,” said Dr. McDonald. “It always makes me happy that sharks can surprise me.”