The opah, Lampris guttatus, is rewriting the rules of fish biology with a recently discovered ability to keep most of its body warm. Fish biologists have long since known that certain fish species such as tuna can keep certain parts of their bodies warm, such as their eyes and their core but the opah is the first fish discovered to have special adaptations that allow it to keep its body warmer than the ambient temperature of surrounding water.
Well known as a large species of sportfish, the opah has long been suspected to be different from other deep water living predators. Instead of being a sluggish ambush predator, the opah is built for speed with a muscular body, large powerful fins, and large eyes that all come together to make it a swift and efficient hunter.
The ability to be warm blooded in deep water gives the Opah and advantage over all other cold blooded animals in the ocean depths who are restricted by their cold blooded nature. Being warm blooded allows the opah to act and react quickly, as well as to move much faster than other fish.
What makes the opah so unique is how it generates heat, and how it retains it to keep as much of that warmth inside its body as possible. For starters, you may have noticed the cartoonishly round shape of the opah, which strikes a balance between being streamlined to move through the water while also reducing the surface of its body which is exposed to the cold water around it.
Furthermore, the opah moves by flapping its pectoral fins really fast, instead of undulating its body. The large pectoral fins and deep pectoral muscles reminds us of a much smaller freshwater fish, the hatchetfish, which can flap its pectoral fins so fast as to fly out of the water over a distance to escape predators.
The smoking gun of warm-blooded-ness for the opah is the unique shape of its gills which uses counter current flow to keep its blood from losing too much heat to the open water from which it must breathe. The opah’s combination of special adaptations to warm blooded life allows it to keep most of its body up to 5C above the temperature of the surrounding water.
Like so many things we were taught in school to be black and white, the truth about Nature is so much more interesting than what we’ve been led to believe. Fish may be more primitive than us but that doesn’t mean they have stopped evolving, and we gotta give props to the opah fish for pulling off a very important feature of mammals and birds that not even reptiles or amphibians have been able to reproduce – endothermy. [Wash. Post]