New research suggests that scalloped hammerheads halt their breathing when making dives into deeper and colder water.
Hammerhead sharks were observed on frequent dives off the coast of Hawaii where they descended to depths of up to 2,600 feet. Water temperatures would quickly go from 75 degrees to 40 degrees and below.
According to new research from Mark Royer, a marine biologist at the University of Hawaii Manoa which was presented at the Ocean Sciences Meeting 2020, since the cold-blooded sharks are not known to have physiological means to retain or generate heat internally, the hammerheads are modulating their behavior to keep their muscles warm enough for active hunting at depth.
“Hammerheads are a tropical and warm temperate species. When their body temperature gets too low, they lose muscle function, visual acuity, and their metabolism slows down. If a shark gets too cold, it can’t keep itself moving and breathe,” Royer said.
The sharks replenish oxygen in their blood from ocean water as it rushes into their mouths and out through their gill slits. This can also rapidly cool their blood to the surrounding water.
In order to keep this from happening, it is theorized that
scalloped hammerhead sharks may be simply shutting their mouths, or clamping their gills shut, on dives to keep out the water and avoid getting chilled.
“The laws of thermodynamics hold,” Royer said. “We know the water is super cold and the sharks are staying warm. So the sharks can’t be dumping body heat out through their gills.”
To maintain their core body temperature, the sharks essentially hold their breath.
Royer tagged hammerheads with a package of sensors that recorded the sharks’ depth, tailbeats, and the pitch and roll of their bodies, as well as the temperature of their muscle and that of the surrounding water, following them from Kāneʻohe Bay, in the island of Oahu, as they made repeated dives of off the coast to depths over 800 meters (2,600 feet). He followed nine sharks for 7 to 23 days, gathering over 1,750 hours of data.
Body temperatures held constant past 150 meters (500 feet) depth, where researchers would expect to observe the sharks beginning to cool down, and as the sharks swam vigorously at depth. After about five minutes, the sharks dashed back up to about 300 meters (1,000 feet), where they slowed down, and their body temperatures abruptly dropped.
The sharks spent roughly 45 minutes at the surface, as their bodies slowly warmed, before repeating the dive cycle.
Royer plans to include cameras in future work, to learn more about what the sharks are doing on their dives and observe their mouths and gills.