Her bulging belly and swollen abdomen told researcher Niv Froman all he needed to know: This reef manta ray hovering near the surface in the Maldives was unmistakably pregnant.

"She basically looks like a pregnant woman. We call it the pregnancy bulge," says Froman, an ecologist at The Manta Trust, a U.K.-based nonprofit that works to protect rays across the world.

At full term, the fetus folded up inside her womb had a wingspan of almost five feet, as wide as a park bench. But pregnancy only becomes visible after around six months, meaning scientists might not know a manta is pregnant until halfway through her 12.5-month gestation.

That’s why Froman and the team have been using new