Surprisingly, similar to marine mammals holding their breath, hammerhead sharks sometimes stop breathing in the water, researchers report: In order not to cool down during their hunting trips into the cold deep water, the fish close their gills. This is evident from data logger records and observations of scalloped hammerhead sharks. The discovery suggests that this strategy may also be common in other deep-diving shark and fish species.
In the course of evolution, successful hunters of the seas emerged from two very different groups of animals: in addition to fish, mammals later developed into skilled predators of the underwater world. However, the physical characteristics and strategies of both animal groups differ significantly. Seals, whales and the like breathe through lungs and therefore have to come up again and again to get air. What is also special is that they can always keep their body temperature at a high level, even in cold water, because like all mammals they are warm-blooded. This is completely different with sharks and co. They absorb oxygen directly from the water through their gills and are cold-blooded - their body temperature adapts to the environment.
Both systems have advantages and disadvantages. Although fish can save energy through their temperature concept, they lose a great deal of their performance at low values. This becomes a particular challenge for species that move between warm and very cold sea layers. This is particularly pronounced in the case of scalloped hammerhead sharks (Sphyrna lewini): They live in tropical waters with surface temperatures of around 26 degrees Celsius. To hunt squid, however, they regularly dive to depths of more than 800 meters, where the temperature is only around five degrees Celsius.
Spied dives
So far it has been unclear how the scalloped hammerhead sharks cope with the low temperatures during their hunting trips. Therefore, the researchers led by Mark Royer from the University of Hawai'i in Mānoa decided to investigate possible strategies of thermoregulation. They equipped some scalloped hammerhead sharks with specially developed biologgers. In addition to the water temperature, the depth and the physical activity, these devices could also record the temperature in the muscles of the test animals via a probe.
The scientists were amazed to find that during the approximately 17-minute dive into the cold depths, the muscle temperature of the sharks remained at a level well above 20 degrees Celsius. The predators were still warm when they probably showed strong movement activity at the maximum diving depth during the hunt. According to computer models, the strong heat loss through the large surface structures in the sharks' gills should actually cause a very rapid temperature drop in the cold water, say the researchers. However, the data showed that the body temperature only dropped significantly during the course of surfacing.
"Freediving" keeps hammerhead sharks warm
As the scientists explain, this pattern can only be explained by the fact that the hammerhead sharks close their gills when they dive into the cold depths and then hunt there with bated breath, as it were. Only when they surface do they open their gills in cold water, which leads to a significant drop in temperature at the end of the trip. As the researchers report, observations by diving robots also fit this explanation: They show scalloped hammerhead sharks that are in the deep sea with tightly closed gill slits. "The study results thus expand our understanding of how hammerhead sharks are able to dive to great depths to hunt and withstand the low temperatures there," summarizes Royer.
The hammerheads' "breath-holding" behavior is also an interesting parallel to the behavior of marine mammals, the scientists point out. "The fact that lung-breathing marine mammals hold their breath while diving is well known and obvious, but we didn't expect something similar to happen in sharks," says Royer. "This previously unknown behavior shows that hunting strategies have evolved in both groups of animals, giving them short-term access to the physically demanding environments of the deep," says the marine biologist.
It might now also be worth exploring the interesting thermoregulation strategy further. As Mark Meekan of the University of Western Australia at Crawley and Adrian Gleiss of Murdoch University point out in an article accompanying the study, the concept could be widespread in other deep-diving shark and fish species.
Source: University of Hawaii at Manoa, Science, doi: 10.1126/science.add4445