This week scientists discovered the first known warm-blooded fish; except that this fish was already well-known to humans. The comically appointed opah, a large silver and red fish that is large, circular, and flat, has been making appearances in fishing nets off the coast of west Africa and Hawaii for years.
Now Nicholas Wegner from the National Oceanic and Atmospheric Administration (NOAA) and his team have discovered that not only is the opah warm-blooded, it is also an active predator. The reasons these facts about the opah have only recently been discovered may provide a hint about its successful survival over time.
"Nature has a way of surprising us with clever strategies where you least expect them," Wegner says. "It's hard to stay warm when you're surrounded by cold water but the opah has figured it out."
Most fish are cold-blooded, maintaining body temperatures similar to their surroundings. This means that they can hunt in cold, deep waters but must warm themselves near the surface periodically. Certain species have "warming" skills that give them an advantage. For example, certain sharks can use their swimming muscles to generate more heat for a short time. Other species including the marlin and the swordfish can heat their brains and eyes for short bursts of time to provide keener eyesight as they hunt.
The opah has the distinct advantage of being able to maintain a temperature that is approximately 5 degrees Celsius higher than its environment. It therefore has less need to warm at the surface and make long trips back and forth in between the deep and the surface while hunting. In fact, Wegner's team confirms that opah do not regularly surface and spend most of their time at depths of 150 to 1,300 feet."
The opah is not warm-blooded in the same way that mammals or birds are; its system is not that efficient. However, its ability to maintain a higher temperature throughout its body is unique among fish-and a special advantage for a predator.
"Endothermy is particularly advantageous in cold or thermally-variable habitats as it allows for increased physiological function at cold temperatures" Wegner says. "For opah, having a warm body should give it advantages, like faster swimming and reaction times, over its cold-bodied prey and competitors. However, many species are highly successful at being cold, slow moving ambush predators. The cost of being warm is having higher energetic demands."
This monumental finding, however, was an accidental discovery by Wegner and his team. They had been catching more opah than usual during a research trip and took that chance to study them more closely. In dissecting the opah the team noted that their gills were full of red and blue blood vessels.
"That was when we realized what it was capable of," Wegner says. These retia mirabilia-Latin for "wonderful nets"-are the secret weapons that sharks have to warm portions of their bodies. For opah, though, they are enough to maintain the temperature throughout the body.
The placement of these retia mirabilia is significant. Fish typically lose heat most through their gills, so the opah, with retia mirabilia in its gills, have the best chance of retaining heat. This is enhanced by the thick layers of fat that insulate the opah. Tuna and some sharks, in contrast, can retain more heat because they have retia mirabilia, but they still lose heat from their gills.
The opah's muscles, brain, and heart are all warm, and this is a first. "That's why opah can stay at depth," says Wegner. "These guys are specialized for living deeper than those other predators."
How then did this stay a mystery for so long?
For one thing, it seems intuitive that predators have more need for warmth. Scientists already knew that fish like sharks had the ability to retain some heat in parts of their bodies. The unexpected look of the opah may have blinded scientists to the fact that these fish are in fact quick, active predators.
"That's what's really blew my mind about this discovery," says Wegner. "Just from looking at it, I really thought it was a slow, sluggish, deep-water fish that doesn't do very much. But all indications are that this is a very fast fish and an active predator. We've put some tags on them to show that they migrate thousands of kilometers."
"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments. But 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."
Opah can reach nearly 7 feet in length. Opah belong within the order lampriformes, most of whom are slow-moving ambush predators. This is because in deep, cold water high levels of activity are too taxing on a fish's energy reserves. However, opah are able to move quickly thanks to their horizontally positioned and powerful pectoral fins.
Why was Wegner the first to note the unique structure of opah gills?
Perhaps no one was looking for speed and warmth in these fish. Because Wegner is an expert in respiratory adaptations in fast-swimming fish is somewhat of a "gill expert." This led the study's co-author, Owyn Snodgrass, to send him samples from the opah caught on recent research trips.
"The tissue he collected for me sat around in a jar for several months before I actually looked at it," Wegner admits. "When I did, I saw many structural similarities to the gills of other fast swimming fish, which provided the first clue that this wasn't a typical sluggish, deep-dwelling fish. When I cut open one of the gill arches, I noticed an incredible network of blood vessels, something I had never seen before in the gills. Something no one had ever seen before."
This adaptation toward endothermy appears to be unique to the opah-for now. But the discovery prompts important questions. Might this finding soften the distinction between fish and the "warm-blooded" classes? And if endothermy isn't unique to birds and mammals, isn't it true that the opah might not be the only example of this phenomenon-not the sole fish to show this adaptation?
"The fact that we didn't know that the opah could warm its entire body until now goes to show how little we really know," Wegner says. "There are currently two described opah species: the opah, Lampris guttatus, and the southern opah, Lampris immaculatus. We hope to be able to examine some southern opah in the future, as this species is known from the Southern Ocean where the water temperatures are even colder. That means endothermy would probably be even more advantageous, but also harder to maintain."
For now, scientists will have many ongoing inquiries to make when it comes to marine life; this discovery points to how much we still don't know about fish.
"Discoveries like this help us understand the role species play in the marine ecosystem, and why we find them where we do," said Francisco Werner, director of the Southwest Fisheries Science Center. "It really demonstrates how much we learn from basic research out on the water, thanks to curious scientists asking good questions about why this fish appeared to be different."
The study is now available in Science.
