Hammerhead sharks can stay warm even when diving to extreme depths – by performing the equivalent of marine mammals’ breath-hold dives. In previously unobserved behaviour, scientists now believe the sharks “hold their breath” simply by closing their gill-slits at critical moments.
Sharks are among the many big fish known to dive regularly to “mesopelagic” depths, between 200 and 1,000m in open ocean. “Because these vertical movements typically begin and end in shallow waters, they are commonly referred to as dives, even though these fishes are not compelled to return to the ocean’s surface to breathe, unlike marine mammals, reptiles and birds,” says a scientific team led by Mark Royer of the Hawaii Institute of Marine Biology, University of Hawaii.
But the parallels between animals that breathe using lungs and gills are no co-incidence, they say, after concluding that tropical scalloped hammerhead sharks (Sphryna lewini) effectively freedive.
The body temperature of most fish is heavily regulated by their environment, and this presents a challenge for those large predators that need to maintain a certain temperature to fully function when they venture into different thermal environments in search of prey.
“Fish moving between different thermal environments experience heat exchange via conduction through the body wall and convection from blood flow across the gills,” says Royer’s team. Using state-of-the-art remote biologgers, they found that the hammerheads were able to maintain warmer body temperatures during dives beyond 800m seemingly by closing their gill slits.
Video evidence beyond 1,000m
The scientists believe that, by suppressing gill function, convective heat transfer is suspended. “Functionally, these sharks hold their breath during dives to facilitate access to prey in deep, cold waters,” they say, suggesting that it is oxygen storage capacity, rather than simply temperature, that determines dive durations.
The adult sharks dived rapidly and repeatedly from surface waters at 26°C and above to beyond 800m, where they encountered temperatures as low as 5°C, maintaining warm muscle and heart temperatures throughout. Significant and rapid cooling occurred only during the latter stages of the ascent phase.
ROV video footage of one of the hammerheads swimming 1,043m deep off Tanzania showed that it appeared to have its gill-slits closed, whereas videos in surface waters showed the slits open, but further research is needed to put the theory beyond all doubt. The scientists would like to attach cameras to the sharks’ pectoral fins to see what exactly happens to the gill-slits and when.
Maintaining a warm body temperature at depth would enable the sharks to swim faster, as required when feeding. Should they need to open their gills at that time it would probably not be for long enough to affect their muscle temperature. The findings have been published in Science.