Giant manta rays are usually portrayed gliding elegantly through sunbeams filtering through shimmering surface waters. But new research shows they have a deeper, darker side, too. Very little is known about the diet of these huge fish, which can reach 7m across. To find out, biologists have taken tissue samples from free-swimming mantas off the coast of Ecuador, using a biopsy punch mounted on a hand-spear.

An animal’s tissues contain chemical signatures derived from its prey, allowing the biologists to estimate that surface plankton comprised only 27 per cent of their diet. The rest was from the mesopelagic zone at depths of 2001000m. “The mesopelagic zone is the next frontier for open ocean fisheries,” write the scientists. “It is concerning that we still do not fully understand the reliance on this zone by marine megafauna that already face threats in well characterised surface habitats.”