In a surprising revelation that adds a new dimension to our understanding of marine biology, a recent study published in the journal Science suggests that hammerhead sharks have a unique way of maintaining their body temperature while hunting in deep, chilly waters – they “hold their breath.”
This unexpected finding, reminiscent of thermoregulation strategies employed by marine mammals, raises intriguing possibilities about similar tactics being prevalent among other deep-diving fish species.
As a scientific journalist, it’s my pleasure to dive into the details of this fascinating discovery and shed light on the complexities of life beneath the surface of our oceans.
Most fish, including sharks, are ectothermic, which means that their body temperatures are largely determined by the environment around them. This poses a physiological conundrum for large predators, who need to maintain a specific body temperature to operate efficiently, yet venture into cold, deep waters in search of food.
Take, for instance, the scalloped hammerhead sharks (Sphyrna lewini). These formidable predators, which dwell in warm surface waters of temperate and tropical coastal regions, are known to undertake frequent deep dives, sometimes exceeding 800 meters. In such depths, water temperatures can plummet to a frigid four degrees Celsius.
The question that puzzled scientists was how these sharks managed to maintain their body temperature in such icy waters, given their lack of specific morphological or vascular adaptations to conserve body heat actively.
To unravel this enigma, Mark Royer and his team developed cutting-edge remote biologgers. These devices, implanted internally in adult sharks, recorded an array of data including depth, ambient water temperature, activity rates, body movements, and, crucially, internal body temperature.
The team discovered that the hammerhead sharks maintained a body temperature significantly elevated – up to 20 degrees Celsius above the surrounding water temperatures – throughout the deepest part of each dive. Intriguingly, the sharks only began to lose body heat rapidly when they started their return journey to the surface.
The researchers proposed a compelling explanation for this phenomenon. They suggest that the sharks effectively “hold their breath” while diving by closing their mouths and/or gill slits. This action reduces the flow of cold water across their gills, thereby minimizing heat loss that would typically occur due to the respiration process in cold water.
The swift body heat loss during the sharks’ ascent to warmer waters is likely a result of the gill slits reopening, leading to convective heat transfer. While this finding is a significant leap in our understanding, the authors caution that more research is needed to confirm this thermoregulation strategy.
Mark Meekan and Adrian Gleiss, in a related perspective, write, “Given the effectiveness of breath holding and the strong selective forces that shape behavioral and physiological thermoregulation, this strategy could be widespread among other epipelagic and teleost fishes.”
This innovative study offers a fascinating glimpse into the adaptive strategies of one of nature’s most intriguing predators, potentially opening doors to better comprehend the survival mechanisms of other marine species.
Hammerhead sharks are a group of sharks that belong to the family Sphyrnidae. As their name suggests, these sharks are best known for their distinctive head shape, which is flattened and laterally extended into a “hammer” shape called a cephalofoil. There are nine recognized species of hammerhead sharks. Here’s what we know about them:
Apart from their unique hammer-shaped heads, these sharks are typically gray-brown to olive-green on their dorsal (upper) side and lighter on their ventral (lower) side. Their size can range from 0.9 to 6 meters (3 to 20 feet), depending on the species. The great hammerhead is the largest species, while the bonnethead and the scoophead are among the smallest.
The hammer-shaped head, or cephalofoil, provides several advantages. It increases the shark’s sensory perception by spreading the sensory organs over a larger area, enhancing its ability to find prey. The wide head also allows for better maneuverability and the ability to scan larger areas of the ocean floor for food.
Hammerhead sharks are carnivorous and feed on a variety of prey, including fish, squid, octopus, crustaceans, and other sharks. The great hammerhead has a particular taste for stingrays.
Like all sharks, hammerheads have electroreceptive sensory organs known as ampullae of Lorenzini. These allow them to detect the electric fields created by other animals, which is particularly useful when hunting for prey. Their wide-set eyes give them a better visual range than most other shark species.
Hammerhead sharks are viviparous, meaning they give birth to live young. The embryos are initially sustained by a yolk sac, and once this is depleted, the yolk sac transforms into a structure similar to a mammalian placenta through which the mother provides nutrition.
Hammerhead sharks are known for their schooling behavior. Large schools, primarily composed of females, can be seen in the open ocean during the day. At night, the sharks disperse to hunt.
Many species of hammerhead sharks, including the scalloped hammerhead (Sphyrna lewini), are considered endangered by the International Union for Conservation of Nature (IUCN) due to overfishing and demand for their fins, which are highly prized in the shark fin trade.
Hammerhead sharks are found in warm waters along coastlines and continental shelves. They’re present in various oceans worldwide, including the Atlantic, Pacific, and Indian Oceans.
Some species, like the scalloped hammerhead, are known to make frequent deep dives, sometimes exceeding 800 meters. They’re known to maintain an elevated body temperature in these frigid waters, potentially by “holding their breath” to reduce the flow of cold water across their gills – a phenomenon that’s currently being studied.
Please note that research is ongoing and our understanding of these remarkable creatures is continually evolving.