Claire Corlett

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Opah or Moonfish: A deepwater fish joins mammals, birds in the warm-blooded club

Opah or Moonfish: A deepwater fish joins mammals, birds in the warm-blooded club


Move over, mammals and birds, and make room
for a fish called the opah in the warm-blooded club. 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 to 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 to 1,300 feet (50 to 400 meters), hunting fish and squid. 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.

1 comment on “Opah or Moonfish: A deepwater fish joins mammals, birds in the warm-blooded club

  1. How so very interesting documentary!
    Thanks for sharing with us my dear friend!
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