Thanks to Stephanie Pain, Knowable Magazine and Smithsonian for this:

What Are Scientists Learning About the Deepest Diving Creatures in the Ocean?

Animals-turned-oceanographers are helping biologists find out what they do when they get to the cold, dark depths

Chilean devil rays swim in the Atlantic Ocean near the Azores. Reinhard Dirscherl / ullstein bild via Getty Images

There’s only one word for it: indescribable. “It’s one of those awesome experiences you can’t put into words,” says fish ecologist Simon Thorrold. Thorrold is trying to explain how it feels to dive into the ocean and attach a tag to a whale shark — the most stupendous fish in the sea. “Every single time I do it, I get this huge adrenaline rush,” he says. “That’s partly about the science and the mad race to get the tags fixed. But part of it is just being human and amazed by nature and huge animals.”

Whale sharks are one of a select group of large marine animals that scientists like Thorrold, of the Woods Hole Oceanographic Institution in Massachusetts, have signed up as ocean-going research assistants. Fitted with electronic tags incorporating a suite of sensors, tracking devices and occasionally tiny cameras, they gather information where human researchers can’t. They have revealed remarkable journeys across entire oceans, and they have shown that diving deep is pretty much ubiquitous among large marine predators of all kinds.

Many regularly plunge hundreds and sometimes thousands of meters — to depths where the water can be dangerously cold and short of oxygen, there’s little or no light except for the flickers and flashes of bioluminescent organisms, and the pressure is immense, putting some animals at risk of fatal decompression sickness.

To function at such depths, deep-diving species have evolved an array of anatomical and physiological adaptations — thick, insulating blubber, for instance, or blood vessels transformed into heat-exchange systems, collapsible lungs and oxygen-storing muscles, and ultra-sensitive eyes, to name but a few. But what drove these great predators to acquire their remarkable diving skills?

For most biologists, the answer is a no-brainer: Food. Yet that’s been remarkably hard to prove. After decades of tagging studies, there’s enough circumstantial evidence to be confident that many top predators do dive deep in search of prey. But even now, only one species has been seen in action. The northern elephant seal (Mirounga angustirostris) is now something of a superstar, thanks to a pioneering series of mini-movies featuring its own snout and whiskers and a supporting cast of deep-sea fish and squid.

Food, though, might not be the deep sea’s only attraction, says Thorrold, coauthor of an article that examines the motivation of diving predators in the 2022 Annual Review of Marine Science. Dives and diving behavior vary: Some animals dive many times an hour, others sporadically. Most stick to depths between 200 and 1,000 meters, a region officially named the mesopelagic but better known as the twilight zone; others plunge far deeper. The shapes of dives hint at more than one function, too. A quick downward plunge and equally steep ascent, for instance, suggests a different purpose from a long, slow, flat-bottomed dive. “If the same individual does different types of dive at different times,” says Thorrold, “then that’s good evidence they are for different purposes.”

There is no shortage of suggestions for what those purposes might be. Deep, dimly lit waters could provide refuge from other predators; somewhere to cool an overheating body; navigational cues for those able to detect them; even a long-distance communication channel. “All these ideas are in play,” says Thorrold. “The fact we can’t rule out any of them reflects how mysterious a lot of these large pelagic animals are to us.”…

Read the whole article here.