There's a reason the giant oceanic manta ray has been compared to a B-2 stealth bomber. These graceful fish "fly" through the water with wings that stretch seven metres (23ft) from tip to tip. But with a broad range that spans most of the world's tropical waters, they're also hard to find. Between graceful backflips at the surface, oceanic mantas are known to disappear some 1,000 metres beneath the waves – and until now, we didn't really understand why.

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When a surface-cruising marine animal hits the deep ocean, it's usually for one reason: there's food down there. Sperm whales dive for squid, tuna dive for smaller fish and even sharks sample the deep-water bounty. But oceanic mantas (Manta birostrishave long been considered planktivores – animals that feed exclusively on tiny creatures like copepods, mysid shrimp and acorn worms. These drifting "zooplankton" typically feed on even smaller organisms like algae, which in turn depend on light to survive. That means the known prey species of oceanic mantas live near the light-filled top of the water column – so why do these giants head fathoms below?

Puzzled by that question, researchers from the Marine Megafauna Foundation (MMF) and Queensland University set off for Isla de la Plata, a small island off the coast of Ecuador that hosts the largest aggregation of oceanic manta rays in the world. What they discovered is that we may have been wrong about manta diets all along. At least locally, surface-dwelling plankton makes up just 23 percent of what these animals eat – which makes it more of an occasional snack than a staple.

"Usually researchers examine stomach contents to determine the diet of a fish," explains MMF. "However, this is a distressing or even lethal procedure, which would not be appropriate considering manta rays are listed as vulnerable to extinction."

Katherine Burgess, taking a tissue sample of a giant manta ray in Ecuador- copyright Andrea Marshall.jpg
Katherine Burgess takes a tissue sample. Image: Andrea Marshall, MMF/used with permission

Instead, the team used a small tissue sampler, which pricks the mantas' skin – much like a human blood sugar test – in order to compare each ray's chemical makeup to that of various surface plankton. "We studied the giant manta rays' diet using biochemical tests, which work on the 'you are what you eat' paradigm," says Queensland University biologist Katherine Burgess.

The vast majority of the mantas' diet, it turned out, comes from animals that live between 200 and 1,000 metres deep. This was a big surprise – though it's not unheard of in other species. Another marine behemoth, the ocean sunfish or Mola mola, is typically encountered by divers near the surface, but recent studies have shown that they too descend to hunt. 

Sunfish dive up to 792 metres (2,600 ft) in search of prey, and it's thought that their massive size helps them do it. This could also be true for mantas. Having a large body surface area means you lose heat more slowly, and in cold, deep water, the ability to thermoregulate means an animal can stay down for longer intervals. After a dive, sunfish have to spend time at the surface to warm back up, and the team suspects this could be why mantas still feed in the shallows. 

The waters surrounding Isla de la Plata are shallow, warm and teeming with life, but the island is also situated near a 3,000-metre drop-off. For an animal that needs to make numerous trips down into the depths and back up again, this is the perfect home. When they get too cold, mantas can return to the shallows to recuperate, whilst enjoying a bit of TLC from the various cleaner fish that reside there.

"The closely related mobula ray (or devil ray) relies heavily on krill species usually found at depths, but we have yet to find out what kind of species the manta rays prey on," says the team.

Besides providing intel about manta ecology, this research has big implications for how we approach the future conservation of the species. Like sharks and other rays, giant mantas reproduce very slowly (just one pup every two to five years!), and they're also extremely sensitive to fishing pressure. If these animals are utilising deep-sea habitat, current trawling practices could have unforeseen impacts on their survival.

This is particularly concerning because rays share something else with their shark kin: trade in their body parts has decimated manta populations in recent years. "It's a tale played out in our oceans and throughout our planet on a regular basis. Mantas are paying the price of becoming the latest commodity in the Chinese medicinal trade," says the Manta Trust

Mantas use gill rakers (or gill plates) to sieve plankton and other food from the surrounding water – and these bristled, feathery structures are prized in traditional medicine. Known locally as peng yu sai, the gill rakers are promoted as an alleged cure for a number of ailments, including poor fertility, cancer and poor blood circulation, to name a few. The rakers are cooked into soups and tonics, but in reality they offer no proven health benefits.

The gill raker trade has become so pervasive, in fact, that all manta species gained protections under appendix II of the Convention on International Trade in Endangered Species (CITES) back in 2013, a listing that requires all trade in their parts to be sustainable. Illegal activity has continued to undermine these efforts.

Some progress is being made, however. India, Sri Lanka and Indonesia, which maintain the largest manta fisheries, have begun to see stricter enforcement against poaching. And earlier this year, Peru announced its commitment to protecting one of the world's largest populations of giant oceanic mantas, in part by tightening regulations on gillnet fishing. The move was applauded by conservationists and scientists alike, as it came eight months after a mind-bogglingly large manta was caught as bycatch off the Peruvian coast.

Burgess and her colleagues plan to expand their study in the hope of solidifying our knowledge of how large rays interact with the open-ocean food web. Not only will this will help us to pinpoint critical habitats for the ocean giants, but it might also better equip responsible fisheries to protect them in future.

"There are still many questions to be answered, but this study brings us a step closer," says the team.

Top header image: Clifton Beard, Flickr