Scalloped hammerhead sharks hold their breath to stay warm in the deep

Plunging into the frigid depths of the ocean, scalloped hammerhead sharks exhibit a fascinating behavior that enables them to thrive in chilly waters. Despite being cold-blooded creatures typically found in tropical regions, these predators effortlessly navigate the depths of 800 meters or more to feed on deep-sea squid. How do they manage this feat? According to a recent report published in Science, researchers propose that scalloped hammerhead sharks close their gills and mouths as they dive, effectively holding their breath. This maneuver not only prevents them from taking in oxygen but also retains their body heat, preventing it from escaping into the cold ocean.

If this discovery proves accurate, it would introduce a new technique for maintaining body temperature that was previously unknown. Julia Spaet, an ecologist at the University of Cambridge, who was not involved in the study, expresses her interest, highlighting the novelty of the finding.

While certain deep-sea fish like tuna and lamnid sharks can regulate heat flow to specific organs even in freezing temperatures, scalloped hammerhead sharks lack this ability. With each deep dive, the environment around them can drop by over 20°C, akin to a human immersing themselves in icy waters. Such a drastic temperature change should impair the shark’s vision and brain function and may even prove fatal by immobilizing the muscles and preventing the intake of oxygenated water through the gills.

To unravel the mystery of how these sharks stay warm, a team led by marine biologist Mark Royer from the University of Hawaii at Manoa attached a bundle of sensors to the animals off the coast of Oahu. The sensors included an accelerometer that tracked the sharks’ movements, tail beats, body orientation, as well as depth, water temperature, and internal body temperature. After 23 days, the sensor pack detached and floated to the ocean’s surface, where it sent its location to the scientists. They then set out on a boat to retrieve the data, scanning the waves for the small bundle.

The data they collected yielded surprising results. The sharks were able to maintain their body temperature within a mere 0.1°C throughout their dives and most of their ascent to the surface. Only during the final 300 meters of their ascent did their body temperature drop by around 2°C. The most plausible explanation is that the sharks closed their gills and mouths, engaging in freediving as they descended to the ocean floor. This strategy would allow them to preserve their body heat while foraging for food. The slight temperature drop during the final part of the ascent likely occurred as the sharks opened their gills to breathe again when nearing the more comfortable surface waters. On average, the sharks held their breath for approximately 17 minutes per dive.

Royer describes the scalloped hammerhead shark as the first deep-diving fish known to hold its breath, describing the finding as astonishing and completely unexpected. He notes that the rewards of the meal at the bottom must be truly delectable, as the energy expenditure involved would otherwise not be justified.

While Spaet finds the results intriguing, she suggests that visual evidence of the sharks closing their gills and mouths during dives would further support the theory of breath-holding.

Moving forward, Royer and his team aim to investigate the sharks’ metabolism to gain a deeper understanding of how they can perform their deepwater dives. He emphasizes that these findings highlight the existence of many extraordinary phenomena that remain hidden from us.

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