The Opah /Moonfish... the world’s first warm-blooded fish? / by Francisco Blaha

Anyone that spent time in a longliner in the pacific, knows Opah (moonfish) Lampris guttatus. As it normally comes as by catch. Is a quite a good looking fish, but not particularly fast or smart... but it looks like it keep a good secret up its fin!

 NOAA Fisheries West Coast

NOAA Fisheries West Coast

One of the most basic biology facts we’re taught in school growing up: Birds and mammals are warm-blooded, while reptiles, amphibians and fish are cold-blooded.

In a recent paper published published in Science, Wegner et all, explain that

Endothermy (the metabolic production and retention of heat to warm body temperature above ambient) enhances physiological function, and whole-body endothermy generally sets mammals and birds apart from other animals. Here, we describe a whole-body form of endothermy in a fish, the opah (Lampris guttatus), that produces heat through the constant “flapping” of wing-like pectoral fins and minimizes heat loss through a series of counter-current heat exchangers within its gills. Unlike other fish, opah distribute warmed blood throughout the body, including to the heart, enhancing physiological performance and buffering internal organ function while foraging in the cold, nutrient-rich waters below the ocean thermocline.

The secret lies in a specially designed set of blood vessels in the fish’s gills, which allows the fish to circulate warm blood throughout its entire body.

Scientists already suspected the opah was special. Most fish who live where the opah does — that is, hundreds of feet deep, in some of the ocean’s darkest and coldest places — are sluggish, thanks to the low temperatures. At these depths, even predatory fish tend to be slow-moving, waiting patiently for prey to come by rather than actively chasing it down. But the opah, which spends all its time in these deep places, has many features usually associated with a quick-moving, active predator, such as a large heart, lots of muscle and big eyes. These characteristics made the opah “a curiosity”.

The opah’s secret first started to come out when NOAA researcher and lead author of the paper looked at a gill sample and noticed something intriguing.

All fish have two kinds of blood vessels in their gills: vessels carrying blood in from the body to pick up oxygen, and other vessels carrying oxygenated blood back out again. In the opah, the incoming blood is warm after circulating through the fish’s body. This is because the opah swims by quickly flapping its pectoral fins, rather than undulating its body like many other fish do, to propel itself through the water — a process that generates high heat. But outgoing blood, which has just been in contact with water in the gills, is cold. Wegner noticed that in the opah’s gills, the two sets of vessels are tightly bundled against each other, so that the incoming blood vessels can warm up the outgoing blood before it goes anywhere else. This set-up, known as “counter-current heat exchange,” allows warm blood to be delivered throughout the body.

Some other types of fish, such as tuna, have similarly designed blood vessels in certain parts of their bodies, allowing for “regional endothermy” — warm-bloodedness that’s limited to certain organs or muscles, such as the eyes, liver or swimming muscles. But the opah is the only fish scientists know of that has this design in its gills, where most fish lose the majority of their body heat to the surrounding cold water. By warming up the blood in the gills before it goes anywhere else, the opah achieves not just regional endothermy, but whole-body endothermy, according to the paper’s authors. Testing showed that the opah is able to maintain a core body temperature about 5 degrees Celsius warmer than the surrounding water.

While only one species of opah is currently recognized — Lampris guttatus — scientists are starting to believe that they should actually divide the opah into several different species based on genetic variations in different populations around the world. The opah in this study were found off the West Coast of North America, so the next step will be to start sampling opah in other parts of the world to see if they all have the same specialized gills.

Down the road, future studies could also examine other related types of fish to try and figure out how and when those special gills evolved. And since a variety of different fish already exhibit regional endothermy, including tuna and certain types of sharks, it may be possible that this kind of physiological adaptation, has evolved numerous times in different lineages of fishes.

It’s possible that other deep-water species have similar adaptations as the Opah, although it’s unlikely scientists will ever discover a fish that’s truly warm-blooded, in the way whales or other marine mammals are warm-blooded.

In the meantime, Opah is an unlikely star!