Hammerhead sharks like it warm, but for a good meal they’re
willing to get cold. The flat-headed predators dive more than 792m from
tropical surface waters into the ocean’s frigid depths multiple times every
night to hunt for fish and squid, tolerating a 20°C plunge in temperature to
dine.
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How do these coldblooded chondrichthyans tolerate these
temperatures without turning into frozen fish? A study published Thursday in
the journal Science shows how one species, Sphyrna lewini or scalloped
hammerhead sharks, stay warm during their nightly dives: They skip the
frills and close their gills, essentially holding their breath.
This strategy for regulating a coldblooded fish’s
temperature has never been observed before and distinguishes them from
high-performance fish (yes, that is the scientific term) such as great white
sharks or Atlantic bluefin tuna that use vastly different strategies to
tolerate extreme cold.
Mark Royer, a shark biologist at the University of Hawaii at
Mānoa, was inspired to investigate the scalloped hammerhead’s secret heating
technique after noticing how deep they were diving during a different research
project. He attached a package of sensors near the dorsal fins of six
hammerheads near Hawaii. The packages were designed to detach from the sharks
after several weeks and emitted a satellite signal when they were ready to be
scooped out of the sea.
The tags were like shark Fitbits, Royer said, collecting
data such as depth and body temperature. They were even sensitive enough to
detect each individual flick of the fish’s tail. Royer and colleagues found
that the hammerheads lose a little body heat when they start their descent, but
then quickly return to the same temperatures they were at the surface as they
swim deeper. Even when the surrounding water was as cold as 3.8°C, the sharks
had body temperatures around 23°C during hourlong dives.
Sharks are ectotherms, which means their body temperature is
largely determined by the surrounding water temperature. Royer and his team
used a mathematical model to show that temperature data they collected didn’t
make sense unless the sharks were somehow actively conserving body heat. They
also measured rates of heat exchange between dead scalloped hammerheads (that
had washed up on the beach) and a water bath and found rates similar to those
between live deep-diving sharks and ocean water. The key similarity between the
two? “No conductive heat loss across the gills,” Royer said. And the gills are
the No. 1 source of heat loss in a fish’s body.
“Gills are essentially giant radiators strapped to the
head,” he said.
The conserved body heat and the lack of other physical
adaptations that could prevent heat loss convinced Royer that the fish were
“holding their breath,” somehow stopping the flow of water over their gills —
and their ability to take in oxygen. Researchers suspect the hammerheads do
this by physically closing the gill slits, based on a 2015 observation of a
scalloped hammerhead doing so more than 914m below the surface. Royer wants to
attach video cameras to diving hammerheads next to confirm this hypothesis.
Catherine Macdonald, a marine biologist at the University of
Miami who was not involved with the study, agreed with the team’s reasoning,
saying that she could not “see a way” the sharks could be breathing normally
while maintaining the body temperatures seen in the data.
Royer is next planning to study the hammerheads’ metabolism
to better understand the recovery period that follows the extreme athletic feat
they perform each night. He suspects that the hammerheads’ propensity for
relatively short periods of high activity may explain why they die so easily
when trapped on fishing lines for many hours; it’s like asking an elite
sprinter to run a marathon.
“This study invites a lot of additional studies,” Macdonald
said. “I am always delighted by sharks’ capacity to surprise me.”
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