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 Researchers said in the journal Science on Thursday that this
deepwater denizen is the first fish known to be fully warm-blooded,
circulating heated blood throughout its body, enabling it to be a
vigorous predator in frigid ocean depths.
Tuna and certain sharks can warm specific regions of their body such
as swimming muscles, brain and eyes in order to forage in chilly
depths but must return to the surface to protect vital organs such
as the heart from the effects of the cold.
The opah, also called the moonfish, internally generates heat
through constant flapping of wing-like pectoral fins, with an
average muscle temperature about 7 degrees to 9 degrees Fahrenheit
(4-5 degrees Celsius) above the surrounding water temperature at the
time.
The opah boasts a unique structure that prevents this heat from
being lost to the environment.
Warm-blooded animals, such as birds and mammals, and known as
endotherms, generate their own heat and maintain a body temperature
independent of the environment. Cold-blooded animals, known as
ectotherms, include amphibians, reptiles, invertebrates and most
fish.
 "With a more whole-body form of endothermy, opah don't need to
return to surface waters to warm and can thus stay deep near their
food source continually," said fisheries biologist Nicholas Wegner
of the U.S. National Oceanic and Atmospheric Administration's
National Marine Fisheries Service.
The opah is a rusty reddish color, has white spots and bright red
fins. It weighs up to 200 pounds (90 kg) and is about the size of a
car tire, with an oval body shape. Found in oceans worldwide, it
spends most of its time at depths of 165-1,300 feet (50-400 meters),
hunting fish and squid.
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A unique structure within its gills lets warm blood that leaves the
body core help heat up cold blood returning from the gills'
respiratory surface, said fisheries biologist Owyn Snodgrass of NOAA
and Ocean Associates Inc.
Being warm-blooded gives it distinct advantages over its cold-bodied
prey and competitors including faster swimming speeds and reaction
times, better eye and brain function and the ability to withstand
the effects of cold on vital organs.
Fish dwelling at such depths typically are slow and sluggish,
ambushing rather than pursuing prey.
The researchers documented that opah are warm-blooded by tagging and
tracking them off California's coast, measuring their body
temperature, water temperature and the depths at which they swam.
(Reporting by Will Dunham; Editing by Grant McCool)
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