New research by NOAA Fisheries shows that the silvery fish, roughly the size of a large automobile tire, can swim faster and see better, thanks to the heated blood that circulates throughout its body much like mammals and birds, giving it a competitive advantage in the cold ocean depths.
It dwells hundreds of feet beneath the surface in chilly, dimly lit waters, and swims by rapidly flapping its large, red pectoral fins like wings through the water. Lead author, Nicholas Wegner of NOAA Fisheries, said that because it can warm its body, it turns out to be a very active predator that chases down agile prey like squid and can migrate long distances.
Wegner realized the opah had an unusual design: Blood vessels that carry warm blood into the fish’s gills wind around those carrying cold blood back to the body core after absorbing oxygen from water. The design is known in engineering as “counter-current heat exchange.” In opah it means that warm blood leaving the body core helps heat up cold blood returning from the respiratory surface of the gills where it absorbs oxygen.
Wegner said that there had never been anything like this seen in a fish’s gills before. This was a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before we thought of it.
Other fish have developed limited warm-bloodedness (known as regional endothermy) to help expand their reach from shallower waters into the colder depths. But the opah’s evolutionary lineage suggests that it evolved its warming mechanisms in the cold depths, where the fish can remain with a consistent edge over other competitors and prey.
The research is reported in the journal Science.
