We met 30 years ago, Barbara Block and I, over the body of a huge Pacific blue marlin. We were standing on the dock at Kailua-Kona on the Big Island of Hawaii, on opposite sides of the table supporting the fish. Professor Block, then a young post-doc, was doing research on specimens brought in by the anglers competing in the Pacific Gamefish Tournament. I was covering that contest as a journalist.

The 950-pound (431-kilogram) marlin between us was stone cold, five hours out of water, its iridescent colors long since faded to gray. Block suggested that I poke my finger in behind the marlin's large eye. I had no idea who this young woman was, but I did as she suggested.

The eye tilted forward as my finger worked its way into the socket behind. Two sport fishermen standing nearby looked away, squeamish and unhappy. My finger worked deeper. I was startled. Heat. This cold slab of marlin was warm, almost hot, behind the eye. "Heater organ," Block explained.

In marlin and other billfish, she said, some of the muscle used for movement of the eyeball has undergone evolutionary modification, enabling mitochondria there to produce heat. The phenomenon is called NST, non-shivering thermogenesis. The heater organ allows the marlin to move freely from the sunny surface to the cold depths with its eyes and brain warmed and working efficiently.

Removing my finger from the eye of the marlin, I muttered my astonishment. Block, smiling slightly, nodded farther down the dock, to a table where several big ahi, yellowfin tuna, awaited transport to the icehouse. In the tunas, she said, the design was even better: a countercurrent heat-exchange system that warmed the whole body.

This is not what I had learned in school. All fish are not ectotherms—cold-blooded—as many of us were taught. The tunas are endotherms of a sort, warm-blooded, as are mackerel sharks like the great white, mako, and salmon shark. In these creatures a rete mirabile, a "wonderful net" of intertwined arteries and veins, lie so close together that the heat in the former is captured by the latter and recirculated. The marlin is partway there: ectothermic aft, but endothermic forward in its eye and brain.

Barbara Block's marlin lesson that day was the best kind of hands-on education. Thirty years after the fact my fingertip still remembers the surprise of that heat behind the eye.

View Images These 20-year-old bluefin in the Gulf of St. Lawrence are about 9 feet (2.7 meters) long and 800 pounds (363 kilograms) each. Photograph by Brian Skerry, National Geographic

Tuna Research on Cannery Row

"We're doing some of the same stuff as when I last saw you," Block told me recently, when we met again. In the interim she had won the MacArthur "Genius" Award, a professorship at Stanford, and renown as a leading researcher into the biology of the bluefin tuna. I dropped by her lab at Hopkins Marine Station, on Cannery Row in Monterey, California, to interview her about the bluefin, greatest of tunas, a fish I was researching myself for a story. (Read "Quicksilver" in National Geographic magazine). We sat in her office and she picked up where we had left off three decades before.

"When I started my career, I was studying how animals stay warm, how animals make heat. I'm still doing that. You could ask the birds and mammals, Why do you want to be warm? It's because if you're warm, you can go wherever you want. Just like us. We're not limited by temperature. You also increase metabolic performance, because you warm up your insides. You get increased muscular performance, because you're running at a higher temperature. It doesn't matter whether you're a bird, a mammal, or a tuna. Not all tunas are as warm as bluefin, though. If there's one thing we've learned, it's that bluefin are the warmest of the animals called thunnids."

The walls of Block's laboratory are papered with charts, graphs, posters, and enlarged reproductions of pages from scientific journals. They make a kind of gallery of the professor's work. After our interview, I slowly read my way down the walls, marveling at all that Block had accomplished since that day on the Hawaiian dock.

The Secret Life of Fish

Several posters and maps told the story of the wave glider, an oceangoing robot that Block and her people, in collaboration with the maker, Liquid Robotics, have been deploying in various experiments. To any passing ship or albatross, the wave glider appears as a slow-moving surfboard, with a mast or pole at center jauntily flying a little flag. To any tuna or shark encountering it underwater, the glider appears as a small, multi-winged submarine connected by a tether to the shadowy underside of the surfboard 20 feet above. The tether is designed to capture the energy in the motion of the waves and drive the glider forward. Solar panels power the onboard computers and sensors. The glider can run autonomously, or it can be remotely controlled through the Internet by technicians ashore.

Block's team had steered one wave glider in circles around the sea stacks of the Farallon Islands, just north of San Francisco, to monitor the gathering of great white sharks there. They had deployed another glider off North Carolina, in the rough winter seas along Cape Hatteras, to gather data on sturgeon, sandbar sharks, and bluefin tuna.

The wave glider, then, is a sea drone that blows up nothing and invades no privacy, unless you count the secret lives of the fishes. Its only job is to soak up information.

Warm Brains, Cool Hearts

Most of the scientific art on the walls of Block's lab celebrates the bluefin tuna. One graph charts the periodic rise in visceral temperature of a tagged bluefin after its feeding sessions. The heat of digestion raises a line of spikes across the graph, the sort of sawtooth pattern traced by an earthquake across seismograph paper.

Graphs like this had not existed 30 years ago, when Block was a newly minted Ph.D. In the intervening years, after much trial and error, Block's team and their consulting engineers have succeeded in producing what they call the "archival tag," which is surgically implanted in the fish. The internal end measures both the temperature inside the peritoneal cavity and the depth at which the fish is swimming. The tag's stalk, which protrudes outside the body, has sensors for measuring external water temperature and ambient light.

The tag's mini-computer takes snapshot readings of this information at two-minute intervals for as long as several years. If a fisherman catches the fish and sends in the tag, he collects a $1,000 reward. Back at the lab, the tag pours out its cornucopia of data on each fish's movements—daily, seasonal, horizontal, vertical—as well as its hunting habits and physiology.

One of the revelations of this internal tag is that the fish captures not only the heat generated by its swimming muscles, but also those spikes of visceral heat on the graph, recycling the postprandial glow of digestion.

Some months after my first visit, I was back at the Monterey lab. Block was showing me a series of graphs depicting the dive profiles of Atlantic bluefin tuna. "Here we see a lazy tuna at the surface relaxing," she said, with a nod toward a graph depicting shallow dives. "It's a very warm water column. The fish is out in the Gulf Stream."

She pointed to a graph depicting much deeper dives. "Then the tuna comes into the Gulf of Maine, and what you see is the tuna's going up and down, up and down, up and down. When I first saw this, I didn't think about it. I knew it was odd. And then at a lecture one of my colleagues said, 'Why do they act like mammals that have to come up to get air?' And I looked at the graph, and I thought, Yeah, it is kinda weird, isn't it?"

The tags were trying to tell Block something, but she and her colleagues were slow to grasp it. The tags showed bluefin diving to 250 meters, spending maybe 20 minutes down there in ice-cold water, and then going back up for a while before diving again.

"What we think is that in very cold water the tuna is getting bradycardia, a slowing of the heart. They have to come up and rewarm it. They're not like us completely, in that our heart is warm all the time. The tuna heart, close to the gills, gets cold. What we discovered in our lab is that the capacity of the bluefin's heart to function in the cold is extraordinary."

Block explained that cardiac contraction is dependent on intracellular stores of calcium in the sarcoplasmic reticulum. Of all tunas tested by Block and various co-authors, at all temperatures tested, the bluefin displays the highest rate of calcium uptake in the tuna family, twice that of albacore and three times that of yellowfin tuna and bigeye tuna.

While the bluefin's countercurrent heat-exchange system is key to its unmatched athleticism and endurance, it turns out that its great range owes as much to the opposite capacity: a cool heart that functions well in cold. This permits the western Atlantic bluefin to travel, in just two weeks, from its Gulf of Mexico spawning grounds, at 30°C (86°F), to its Canadian foraging grounds at 10°C (50°F). And it allows the fish to commute comfortably from the warm surface to cold depths.

No Time for Applause

In 2012 Block was awarded the Rolex Prize for the applied technology of her tagging projects. The prize came sweetened by 100,000 Swiss francs (about U.S. $113,500) and a Rolex watch. As it happened, I was in the professor's office when UPS delivered the plain brown cardboard box containing the watch. Block grinned on reading the label and tore into the box like a five-year-old at Christmas. Extricating the watch from the smaller box within, she slipped her wrist through the white gold band and held it aloft to the applause of two staff members who leaned in to see.

The Rolex gave her about 16 seconds of pleasure, by my count, and then she was back on the phone, working out the logistics of a shipment of tuna tags. The watch was forgotten, just something snug on her wrist. For the rest of my time in the office that day, she never looked at it again.