University of Hawai’i at Mānoa researchers have made a surprising discovery about scalloped hammerhead sharks: they hold their breath during deep dives into cold water to keep their bodies warm while hunting prey like deep sea squids. This behavior provides new insights into the physiology and ecology of the species, which serves as a crucial link between deep and shallow water habitats.
Lead author and researcher Mark Royer of the Shark Research Group at the Hawai’i Institute of Marine Biology (HIMB) in the UH Mānoa School of Ocean and Earth Science and Technology described the finding as unexpected, noting that it’s “an extraordinary behavior from an incredible animal.” While shark gills act as natural radiators that would quickly cool the blood, muscles, and organs during deep dives into cold water, the sharks are warm-water animals that feed at depths where seawater temperatures are similar to those found in Kodiak Alaska (around 5ºC/40ºF). As a result, they need to maintain their body temperature to effectively hunt.
Although air-breathing marine mammals are known to hold their breath while diving, the discovery of sharks exhibiting similar behavior was previously unobserved. Royer explained that the findings suggest scalloped hammerhead sharks and some marine mammals like pilot whales have similar feeding strategies, both having evolved to exploit deep-dwelling prey and holding their breath to access physically challenging environments for short periods.
The research team discovered this behavior by attaching devices to deep-diving scalloped hammerhead sharks that simultaneously measured their muscle temperature, depth, body orientation, and activity levels. They observed that the sharks’ muscles stayed warm throughout the dive into deep cold water but cooled suddenly as they approached the surface at the end of each dive. Computer modeling suggested that the sharks are preventing heat loss from their gills to keep their bodies warm during deep dives into cold water.
In addition to holding their breath during deep dives into cold water, researchers have discovered that scalloped hammerhead sharks close their gill slits tightly while swimming along the seabed at depths of over 1,000 meters. When they approach the surface at the end of each dive, their muscle temperature suddenly cools, indicating that they reopen their gill slits to resume breathing while still in cooler water.
The discovery of these behaviors sheds light on the physiological and ecological adaptations of scalloped hammerhead sharks, which are important links between deep and shallow water habitats. Although they are not currently threatened in Hawaii, they are endangered in other parts of the world due to overfishing, bycatch, and habitat loss.
The researchers note that understanding the sharks’ physiology and ecology can help with conservation efforts, particularly in light of potential threats like deep-sea mining or large-scale fishing in the mesopelagic “twilight zone.” These activities could make it harder or more dangerous for the sharks to hunt their natural prey and could impact their ability to forage successfully in the deep sea. The researchers caution that this extraordinary physiological feat that allows scalloped hammerhead sharks to expand their ecological niche into the deep sea could make them vulnerable to additional human impacts.
Source: University of Hawaii at Manoa