“There are a lot of things that we don’t understand about why animals are the way they are and how they have evolved,” said Stephanie Venables, a graduate student at the University of Western Australia and lead author of the study.

To study the manta-ray enigma, the researchers looked at the frequency of melanism in thousands of photos uploaded by divers, dive operators and underwater photographers between 2003 and 2018 in different locations in and around the Indian and Pacific oceans.

They found that melanism varied from place to place. In Indonesia’s Raja Ampat islands, for example, 40 percent of the reef manta population was melanistic, but in others, it was hard to find even two melanistic mantas. This suggested that some evolutionary advantage could be selecting for this trait in these populations, but what?

Mantas have few predators, in part because of their size. They also tend to have white bellies, which enable them to blend in with contrasting light from the surface and avoid being seen from below. Presumably a black belly would make them more conspicuous, and more prone to predation. The researchers tested that idea by following a few individual animals through years of photos, then determining whether coloration influenced the animals’ survival.

The results, described in the new paper, showed no difference in the survivorship of white and melanistic rays. Whatever the role of those dark blotches, they weren’t making the rays more visible to predators.