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.