Hammerhead sharks are first fish found to ‘hold their breath’ when deep diving

Nature News

11 May 2023

By Bianca Nogrady

It pays to be a warm hunter in the cold ocean depths, so this animals shuts down its oxygen intake to conserve heat.

A juvenile scalloped hammerhead shark (Sphyrna lewini), with its mouth and gills open, near the ocean surface off Hawaii.Credit: Biosphoto/Alamy

Because it makes them better hunters, scalloped hammerhead sharks (Sphyrna lewini) have evolved a unique method to avoid losing body heat when they dive for prey in deep, cold waters: they close their gills.

Numerous fish and marine-mammal species are known to dive from the warm surface to deeper waters to hunt. However, ectothermic, or ‘cold-blooded’, animals face the challenge of how to conserve their body temperature to keep their metabolism active enough for hunting when the surrounding water can be just a few degrees above freezing.

“The most rapid point of heat loss for any fish, even a high-performance fish, is always at the gills,” says Mark Royer, a postdoctoral researcher in shark physiology and behaviour at the Hawaiʻi Institute of Marine Biology in Kaneohe, part of the University of Hawaiʻi. Because of the high volume of warm blood flowing through the gills, they are “essentially just giant radiators strapped to your head”, he says.

Some fish, such as the whale shark (Rhincodon typus), are able to conserve their body heat when diving through sheer size. Others, such as tuna, marlin and the family that includes great white (Carcharodon carcharias) and mako sharks (Isurus oxyrinchus), have evolved specialized heat-exchange systems at the gills that avoid too much body heat being lost.

The scalloped hammerhead has neither of these advantages or adaptations, yet has been tracked doing rapid, repeated dives to depths of around 800 metres.

To understand how sharks were coping with the temperature changes, Royer and his colleagues developed a device consisting of instruments that measured depth, water temperature, location and movement, as well as a probe embedded into muscles near the dorsal fin that recorded the shark’s core temperature. The device was designed to break off after several weeks, float to the surface and send out a signal to enable its recovery.

Three scalloped hammerhead sharks captured off the Hawaiian coast were tagged with the device.

In a paper published in Science1, the team reported that the sharks would dive several times — six in an evening, for one shark — into deep water at temperatures of 5–11 °C, around 20 °C colder than at the surface, and remain there for 5–7 minutes at a time before surfacing.

Body temperature remained constant for most of the dive until the final stage of their ascent back to warmer waters, when it would decline rapidly.

Keeping warm

Royer suggests that the sharks are keeping their core temperature stable by simply not opening their gills or mouth during the dive; effectively ‘holding their breath’. “If you don’t have water going over your gills, then you won’t be dumping your body heat into the environment,” he says.

The drop in body temperature near the surface coincides with the sharks’ steep ascent flattening out slightly, which Royer suggests is the moment they start to allow water to flow over the gills. “They can slow themselves down, open their gills and start breathing again [because] the water that they’re in, it’s not as cold as it is at the bottom,” he says.

To shut down oxygen intake in this way suggests that the scalloped hammerhead must be able to deal with plummeting blood-oxygen levels during dives, says Mark Meekan, a fish ecologist at the University of Western Australia Oceans Institute in Perth, although the mechanism is yet to be discovered. “What they could be doing is slowing the heart muscle, slowing the pumping of blood around the body,” Meekan says. The shark’s tissues and blood could have evolved to hold more oxygen per unit of volume — akin to the adaptation seen in people who live at altitude — or might be able to deal with the molecular by-products of anaerobic respiration, which can be toxic at high levels.

Marine biologist Colin Simpfendorfer at James Cook University in Townsville, Australia, says the study shows how sharks are well adapted to the limits of their environment. “Diving to over 1,000 metres from tropical temperatures at the surface down to just a couple of degrees centigrade to feed is a fairly extreme movement to do on a regular basis,” Simpfendorfer says.

So far, scalloped hammerheads are the first fish found to do this, but Simpfendorfer says other sharks and fish might have the same adaptation. “There is a big advantage in being able to hunt when you’re warm and everything else is cold.”

doi: https://doi.org/10.1038/d41586-023-01569-x


  1. Royer, M. et al. Science https://doi.org/10.1126/science.add4445 (2023).
See: https://www.nature.com/articles/d41...ail&utm_term=0_c9dfd39373-22debc20c3-46554234

Swimming in cold, deep waters, scalloped hammerhead sharks have found an unusual way of keeping warm: They close their gills and hold their breath. This unusual behavior, documented Thursday in Science, has never been recorded in any other fish. Hammerheads are ectotherms—or cold-blooded creatures—which means their body temperatures are regulated by their environment. Their gills, which let them breathe by picking up dissolved oxygen in the water, are also the primary points of body heat loss. If a shark stops moving, then water can’t flow through its gills, which can lead to suffocation and eventual death. Specialized sensors were attached to the creatures’ backs in a shallow bay off Oahu. The sensors tracked each shark’s muscle temperature and body orientation, the surrounding water temperature and how deep the animal was diving to and swimming in. It is amazing that the hammerhead shark, pecialized sensors to the creatures’ backs in a shallow bay off Oahu. The sensors tracked each shark’s muscle temperature and body orientation, the surrounding water temperature and how deep the animal was swimming.
he Early Jurassic Period (195 million years ago) the oldest-known group of modern sharks, the Hexanchiformes or sixgill sharks, had evolved. They were followed during the rest of the Jurassic by most modern shark groups. They evolved flexible, protruding jaws, allowing the animals to eat prey bigger than themselves, while also evolving the ability to swim faster to catch this prey. The hammerhead sharks were thought to be the youngest living group of sharks. There are at least eight different species of hammerhead shark, and while fossil teeth evidence suggests that their ancestors may possibly have existed 45 million years ago, molecular data points to a much more recent appearance during the Neogene period* (which began 23 million years ago). The strange shape of their head is thought to help in electroreception (the detection of naturally occurring electric fields or currents) as they hunt for prey. It may also improve their vision, enabling better prey differentiation at greater distances, enhance their swimming and refine their ability to smell.

* The Neogene period was the second of three divisions of the Cenozoic Era. The Neogene Period encompasses the interval between 23 million and 2.6 million years ago and includes the Miocene (23 million to 5.3 million years ago) and the Pliocene (5.3 million to 2.6 million years ago) epochs. The Neogene, which means “new born,” was designated as such to emphasize that the marine and terrestrial fossils found in the strata of this time were more closely related to each other than to those of the preceding period, called the Paleogene (66 million to 23 million years ago). The term Neogene is widely used in Europe as a geologic division, and it is increasingly employed in North America, where the Cenozoic Era has traditionally been divided into the Tertiary Period (66 million to 2.6 million years ago) and the Quaternary Period (2.6 million years ago to the present).

See: https://www.britannica.com/science/Neogene-Period