The discovery of a warmblooded
fish, announced in
May in the journal Science,
was no fisherman’s tale.
The opah, or moonfish (Lampris
guttatus), with small fins relative to
its big, disk-shaped body, seems an
odd candidate for warmbloodedness:
The deep-sea fish has lots of surface
area for heat loss and definitely isn’t shaped
for speed. But the opah generates
heat by continually flapping those
fins, says the study’s lead author,
Nick Wegner, a fisheries biologist
with the National Oceanic and
Atmospheric Administration in
La Jolla, Calif.
Fatty connective tissues around the
opah’s muscles and fat layers around
its organs — particularly the heart
— help retain heat. Veins carrying
warm blood from the heart to the
gills interweave with arteries bringing
colder, oxygen-rich blood from
respiration. Heat exchange occurs
between blood vessels in the gills,
which are exposed to cold water.
The opah’s ability to maintain
its body slightly above ambient
temperatures gives it advantages
over the slower, colder prey it hunts.
“Their warmer heart provides
muscles with oxygen and nutrients.
They swim faster and with more
power,” says Wegner.
Tuna, marlin and great white
sharks heat up certain areas —
swimming muscles, parts of their
viscera and the eye and brain — but
these regional endotherms can stay
at lower depths only for short periods
and must rise to warmer waters,
unlike the deep-dwelling opah.
No other known fish has such an
adaptation, though Wegner notes
the opah has a cousin, the southern
opah (Lampris immaculatus), in the
Southern Hemisphere. “We would
expect it’s using the same strategy,”