Have you ever seen a black manta ray and wondered what makes it that color? Or whether the darker coloration has any effect on the way the animal lives and behaves?

In a new study – the first to investigate melanism in a marine species – scientists from the Marine Megafauna Foundation (MMF), University of Western Australia, University of Papua, Udayana University, Murdoch University, and Macquarie University set out to learn more about this fascinating color variation. Their research, which is published today, found that the frequency of melanism varied across the studied locations, and that this variation was more likely a result of random evolutionary processes such as genetic drift, as opposed to selection by predation.

The secret of the black manta rays in the Indo-Pacific. Video credit: Janneman Conradie and Lukas Müller

Melanism occurs when an increase in darker pigments in an animal’s skin or fur results in darker-colored or entirely black individuals. Although they are often mistaken as a different species, these melanistic individuals are simply a different color variation (or ‘color morph’) within a species. While melanism is relatively common in terrestrial animals, like black panthers (which are in fact leopards or jaguars), it is rare in marine species.

Melanistic morph alfredi – Steph Venables

The most prolific example of melanism in the ocean occurs in manta rays. Photo credit: lead author of the study and MMF researcher, Stephanie Venables

Interestingly, the most prolific example in the ocean occurs in manta rays. This is particularly unusual as manta rays are the only shark or ray known to exhibit melanism, especially in such high proportions. While typical manta rays are dark grey on their backs and mostly white on their undersides, melanistic (or black) mantas are completely black on their backs and almost entirely black on their underside with a central white blaze that varies in size and shape, and can be used to identify individuals.

Melanistic and typical morphs LEI StephVenables

Melanism was most prominent in the Raja Ampat (West Papua, Indonesia) population of reef manta rays. Photo credit: lead author of the study and MMF researcher, Stephanie Venables

The study, published today in the journal Proceedings of the Royal Society B, used long-term manta ray photo identification catalogs to investigate melanism in giant and reef manta rays across a number of locations in the Indian and Pacific Ocean. Researchers found that black mantas were common in some locations, making up 40% of some populations. Strangely, they were completely absent in others.

“Melanism was most prominent in the Raja Ampat (West Papua, Indonesia) population of reef manta rays (40%) and in the Ecuador population of giant manta rays (16%), which is interesting considering the distance between these two regions,” said lead author Stephanie Venables, a researcher for the Marine Megafauna Foundation and PhD candidate at the University of Western Australia. “The variation in melanism frequency across locations raises the questions of why melanism has persisted in manta rays, and why it is more common in some populations than others,” she added.

Steph with melanistic birostris – Doug Keim

Stephanie Venables, lead author of the study, with a melanistic giant manta ray (Mobula birostris). Photo credit: Doug Keim

In some terrestrial species, melanism has been linked to improved camouflage or increased thermoregulation. But scientists questioned whether it could have any benefits or disadvantages to manta rays. While black manta rays may look stealthy to us, their darker coloration could, in fact, make them more visible to predators. To begin to answer this question, the study also looked at whether survival rates were different for melanistic mantas when compared to typical-colored manta rays.

“Because of their size, manta rays don’t have many predators, mainly larger sharks and orcas. Despite being less camouflaged, we found that predators don't appear to target black mantas more than normal colored mantas, as survival rates were equal,” said Dr Andrea Marshall, co-founder and principal scientist of MMF’s global manta ray program.

The findings suggest that predation does not influence melanism in these populations. It is likely that the variation in melanism percentages across locations may be a result of more random evolutionary processes such as genetic drift, where traits occur in a population by pure chance. The study also found evidence that melanism may spread between neighbouring populations through gene flow in giant manta rays, which occurs when individuals travel between locations.

Melanistic morph alfredi 3 – Steph Venables

While black manta rays may look stealthy to us, their darker coloration could, in fact, make them more visible to predators. Photo credit: Stephanie Venables

Researchers made use of MantaMatcher – a global database for manta rays where scientists and members of the public record encounters – to access identification photos of individuals from a range of locations.

“This study also highlights the benefits of long-term photo-ID catalogs and collaborative citizen science platforms like MantaMatcher, which enables research of this kind to take place,” added Dr. Marshall.

Melanism is typically a heritable trait that is passed down from generation to generation, yet scientists are still trying to identify the genes that are responsible for melanism in manta rays.

“Understanding melanism can give us insight into how global manta ray populations are connected, and how certain traits spread across different locations. We are only just beginning to understand melanism in manta rays and we hope that future research will reveal more about this fascinating trait,” concluded Venables.

The study by Venables et al. titled “It’s not all black and white: investigating colour polymorphism in manta rays across Indo-Pacific populations” was published in the journal Proceedings of the Royal Society B on 9th October 2019 and is available here.