It’s a big, round fish called the opah, found in deep waters around the earth and looking a bit like someone’s goldfish that seriously outgrew its bowl. Some fish, like tuna and sharks, can control temperature in parts of the body. This one can keep its whole body warm, giving it improved performance in the coldness of the depths. It’s the first example of whole-body endothermy in a fish, raising new questions about the evolution of a complex trait.

The story caught the attention of many news sites, such as the BBC News and the Washington Post. The Weather Channel featured a short news video and report, saying that warm-bloodedness gives this fish a competitive advantage when it hunts slower, cold-blooded prey. Nate Scott at USA Today got a little crazy with his coverage, saying “Scientists have found a warm-blooded fish and we’re probably all dead… They’re evolving.”

This is a sign. This is a message from the deep. Fish are changing. They’re getting smarter. It won’t be long before the warm blood of this fish starts burning hot, hot with a desire to eat us one by one.

Or perhaps not. Let’s take a look at this nice harmless fish scientifically. Calling it “warm-blooded” is a bit of a misnomer, since it lacks the tightly regulated endothermic homeostasis seen in higher vertebrates like mammals and birds. Instead, as Sacha Vignieri explains in a short statement in This Week in Science, the opah distributes heat around its body that’s generated by its swimming muscles.

Mammals and birds warm their entire bodies above the ambient temperature. Generally, this ability is lacking in other vertebrates, although some highly active fish can temporarily warm their swim muscles. Wegner et al. show that the opah, a large deepwater fish, can generate heat with its swim muscles and use this heat to warm both its heart and brain. This ability increases its metabolic function in cold deep waters, which will help the fish compete with other, colder-blooded species. [Emphasis added.]

Conservation of muscle heat is not that unusual. As noted, tuna and some sharks can warm parts of their body with it. The source paper in Science identifies the key to the mechanism in the opah: counter-current heat exchangers in the gills.

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.

Counter-current heat exchangers (CCHE) are common in vertebrates. We have them in our kidneys. Similar mechanisms are found in sea turtles, foxes, and dolphins. Illustra Media‘s new film Living Waters (to be completed this month) will illustrate a particularly amazing example of a CCHE in humpback whales that creates a severe challenge for Darwinian evolution.

Other fish have CCHE’s, too, so it’s not overly surprising that the opah can use the mechanism to distribute heat generated by its fin muscles to warm itself. This does not, however, minimize the wonder of a CCHE. It is achieved by a “wonderful net” of blood vessels (rete mirabile, literally “miraculous web” in Latin) that provides an ingenious method to regulate core body temperature.

In a rete (plural: retia), arteries and veins mesh into networks of fine vessels that flow in opposite directions, as the term counter-current implies. This allows heat to diffuse from warm arteries to cold veins. In dolphins, excess heat from swimming is shed to the environment through retia in the dorsal fin and tail, where the blubber layer is absent.

The opah has retia in its gills. The authors explain how this provides warm-bloodedness in an unusual way for a fish:

What is exceptional about the opah is its arrangement of counter-current retia mirabilia located inside each thick, fat-insulated gill arch (Fig. 2), which thermally isolate the respiratory exchange surfaces from the rest of the body. Vascular casts of the gills (Fig. 2, A, C, and E) reveal that unlike other fishes, extensions of the afferent and efferent filament arteries (which deliver and collect blood immediately pre- and post-gas exchange at the gill lamellae) are embedded within each gill arch in a tightly bundled and contorted manner to form an arterio-arterial rete. Specifically, the afferent and efferent arteries of each individual filament are closely coupled (Fig. 2E) and stacked in an alternating pattern within the arch (Fig. 2, C and D) so that the cold oxygenated blood of each efferent vessel (returning from the respiratory exchange surfaces) should be warmed by the conduction of heat from the warm deoxygenated blood in the afferent filament arteries on either side (which are carrying blood to the gas exchange surfaces). As a result, oxygenated blood leaving the respiratory exchange surfaces should be warmed before entering into efferent branchial arteries for distribution to the rest of the body.

These retia, in other words, are unusual in that they are all composed of arteries, not arteries and veins. Heat from the warmer deoxygenated blood is transferred to oxygenated blood in the gills, so that it is not lost to the ocean water. But even that is not enough to keep the fish’s brain warm. The authors found an additional rete in the muscles that move the eyes that adds a little more heat to the cranium. This way, the fish can avoid the mental sluggishness of a cold brain.

“Of particular importance is the capacity of opah to increase the temperature of the heart,” they continue, by conserving body heat with specialized fat layers. The combined systems work well for a fish that spends most of its time below the thermocline in the frigid waters of the deep. “With a warm body core and heart, and even warmer cranial region, opah have the capacity for enhanced physiological function in their deep, cold habitat.”

In short, the opah employs specialized adaptations of traits that are present in other fish. They are arranged in ways to conserve and channel heat for this species’ deep-water needs. Consequently, the fish can call the world’s deep oceans home.

And Now, Evolution

What do the authors say about how these adaptations evolved? Not much.

This study presents morphological, temperature, and behavioral data that demonstrate an independent evolution of a more whole-body form of endothermy present in the opah, Lampris gutattus — a poorly studied, large, mesopelagic fish with a circumglobal distribution….

In many respects, the opah has converged with regionally endothermic fishes such as tunas and lamnid sharks for increased aerobic capacity. However, unlike these active, more surface-oriented predators that are thought to be derived from tropical ancestors and to use regional endothermy to expand their thermal tolerance or habitat utilization into deep and colder waters, the opah’s evolutionary history is likely tied to greater oceanic depths, with all but the most basal lineage of the Lampridiformes inhabiting the mesopelagic zone (200 to 1000 m depth). Therefore, rather than using regional endothermy to dive below the thermocline during temporary forages, the opah (with its more whole-body form of endothermy) is distinctively specialized to exploit cold, deeper waters while maintaining elevated levels of physiological performance. The discovery of this form of endothermy, coupled with the recent finding of several distinct opah species inhabiting different regions of the world’s oceans (including the subpolar southern opah, L. immaculatus), sets the stage for future comparative studies to further explore this key evolutionary innovation.

The explanation? Convergent evolution to the rescue! This fish’s evolutionary history is tied to the depths. Give us more funding, and we can “further explore this key evolutionary innovation.”

If that leaves you feeling unsatisfied, consider intelligence as a cause. Intelligence can take a solution that works in one environment and apply it in different animals in different environments. A rete mirabile is a complex system that cannot arise in a gradual, stepwise manner, because all the parts have to function together before any part has survival value. “Convergence” and “innovation” are magic words that provide no understanding. But since we know of a cause — intelligence — that can adapt a similar solution in multiple ways, that is a cause that a rational scientist should pursue.

Image credit: NOAA Fisheries/Southwest Fisheries Science Center.