Extracts

Discover how the Greenland Shark can live to over 300 years old

How The Greenland Shark Lives to Over 300 Years Old

The Nature of the Sea.

The chill beneath the tropics.

Just beyond the northern limit of the Atlantic Ocean, the Arctic Circle crosses Greenland and swoops eastwards across a 2,000- kilometre stretch of open ocean, barely skimming Iceland as it crosses grey choppy water towards northern Norway. At noon on the winter solstice each year the sun scrapes the horizon only briefly, and then buries itself behind the curve of the Earth, leaving the Arctic to face the unforgiving darkness of the universe for another day. Even six months later, when sunlight slants across the ocean surface for 24 hours continuously, the light rays are weedy, tightly rationed by the tilt of the Earth and weakened by long transit through the atmosphere. The consequence is a very different face of the global ocean: it’s cold here, and relatively dark. There’s still plenty of life, but the details have been tweaked by evolution to match the local challenges. Every creature carries the imprint of its environment.

Halfway between Iceland and Greenland, 400 metres below the surface, a patch of the gloom is moving. It glides slowly, taking four seconds to cover 1 metre, and its grey mottled skin hangs loosely on its long body. The sharp corners and sleek silhouette of closely related species are replaced by a softer look, reminiscent of a baggy old jumper which won’t ever be thrown out because it’s too comfortable. This lumpy- looking creature is 4.5 metres long and weighs a gigantic 400 kilograms. Speed is not its game. The Greenland shark never goes anywhere in a hurry. But then it has no need to. An individual of this size was probably born around the start of the industrial revolution, and has been gliding around these waters for 240 years. It’s thought that this species can live to be at least 300 years old (possibly more), doesn’t reach sexual maturity until it’s about 150 and keeps on growing throughout its life at about one centimetre each year. As far as we know, it’s the world’s longest- living vertebrate. This exceptional lifespan seems to be directly connected to the cold, which slows down the processes of life1, stretching out the shark’s existence by a factor of ten. This slow giant will spend its leisurely life in water that’s around 0°, mostly hidden several hundred metres below the surface. One of the greatest mysteries carried by that baggy body is how it catches prey – adult Greenland sharks are found with stomachs full of fish like flounder and skate, and sometimes even with freshly killed seal.

Any of those species could easily outswim this creeping predator. Adding to the mystery, almost all Greenland sharks are at least partially blinded by a parasite that sits in one or both of their eyes. Such a hindrance won’t matter in the inky blackness of the ocean depths, but it will limit their hunting nearer the surface. We often assume that the predatory thrones at the top of the food chain must be occupied by the fastest, most observant and most aggressive animals, endowed with nature’s most dangerous weapons in the form of teeth, claws and heft. But in the deep dark cold, the Greenland shark shows us that there is another way to live, and another way to hunt.

The contrast between a tropical coral reef and the normal Arctic habitat of the sluggish Greenland shark shows us the broad pattern of ocean temperature in near- surface waters. It’s set by exposure to sunlight, which is tightly tied to latitude and season. The sunny equatorial seas delight visitors with temperatures of around 30°, but in the central Arctic, water temperature can drop as low as −1.8°. The global ocean energy reservoir is bursting at the equator, but offers only slim pickings nearer the poles. And yet every detailed map of sea surface temperature is a beautiful study of minor exceptions to this rule, each one opening a new door on the complexity of the ocean beneath. There are cool patches and warm patches parked incongruously next to coastlines, elegant swirls of balmy water far from the shore, and striking, snaking intrusions of cold water into friendlier surroundings. We’ll explore some of those later in this book, but the foundation that sits beneath all beautiful complexity of these patterns is that ocean temperature varies between −2° and 30° and is broadly correlated with how close to the equator you are. But this pattern of energy storage isn’t just about the surface. The depths have a tale to tell too.

In August 2013, the research vessel R/V Apalachee was working in the Gulf of Mexico, close to the site of the blowout that had killed 11 crew on the Deepwater Horizon oil rig three years earlier. The latitude of this site is close to 29° , far closer to the equator than to the poles, and the typical surface water temperature at that time of year is around 30° . The researchers on board, led by a team from Florida State University, were sampling deep- sea fish communities to assess the impact of the oil spill. One of the fish they hooked was brownish- grey, 3.7 metres long and quite clearly a juvenile Greenland shark. It was brought out into the hot summer sunshine in a green net stretcher and parked on deck, far warmer in death than it had ever been in life. It was the first of its kind to be captured in the Gulf of Mexico— noteworthy enough to earn a write- up on a few online news sites, but the researchers weren’t that surprised. So what was a polar shark doing in the Gulf of Mexico in the height of summer?

The hook that snagged the shark had been hanging 1,749 metres below the sea surface. And down there, the water temperature was 4° well within the comfort zone of a Greenland shark. The upper ocean might have been warm enough to take a bath in, but the depths were not, and this is normal. The shark would have been at home at that depth anywhere between the North Pole and where it met the hook, because the deep ocean is almost all that cold. That bright colourful map of ocean surface temperatures, with the nice broad red stripe at the equator, is just that: a map of surface temperatures. The temperature difference between the surface water, which basks in bright sunlight, and the deeper waters that haven’t been touched by sunlight for decades can be dramatic. In the Gulf of Mexico, that warm surface layer is only 100– 200 metres thick, but a huge section of the ocean basin inside the Gulf is almost 4,000 metres deep. Below 1,000 metres, this basin is filled with cold salty water very similar to the cold salty water that fills much of the depths of the North Atlantic basin. It flows in from the Caribbean, continually refreshing the pool. There are also other, more subtle, layers above that.

The ocean that we humans see – the sunlit water that you can comfortably wade into, full of life and character and food – forms only a small fraction of the whole ocean. Tropical coral reefs are an exception, albeit one that is vital for life in the ocean, and one that is an enormous joy to share a planet with. But it is essential that you don’t give up on the deep ocean at this point, on the basis that it sounds best left alone. That cold dark water beneath is just as interesting and necessary and important as anything that happens at the surface. Looking down at a flat map of ocean temperature seen from above isn’t enough. We also have to pay attention to how things change with depth.


1The name of a recent international collaboration to study this species was brilliantly to the point: ‘Old and Cold – the Biology of the Greenland Shark’.

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