Scientists from the Marine Megafauna Foundation (MMF) have released their findings from a new study investigating the distinctive colouration of black manta rays. The study, carried out by MMF and scientists from the University of Western Australia, University of Papua, Udayana University, Murdoch University, and Macquarie University is the first of its kind to research melanism in marine species.

Melanism is caused by an increase of the dark pigment melanin in the skin or fur of an animal causing it to appear almost, or completely, black in colour. Melanism occurs in a number of terrestrial animals, often leading to their mistaken identification as different species. One such example is the black panther, actually a melanistic leopard (in Asia and Africa) or the closely related jaguar (in America).

Melanism in the aquatic realm is much rarer. It has been witnessed in a number of species such as seals, whales and dolphins, but there has been little research into its frequency and distribution.

By far the most prolific example of marine melanism occurs in manta rays, which are the only known species of elasmobranch – the sub-class of cartilaginous fish which includes sharks and rays – known to exhibit melanism. While manta rays typically have a dark grey dorsal surface and a mostly white underside, melanistic mantas are almost entirely black with only a central white ‘blaze’ of variable size on the underside which, like the spot patterns of regular mantas, can be used to identify individuals. 

The study, published in the journal Proceedings of the Royal Society B, used long-term manta ray photo identification catalogues such as MantaMatcher – a global database for manta rays where scientists and members of the public record encounters – to access identification photos of both giant (oceanic) and reef mantas across a number of locations in the Indian and Pacific Oceans. Researchers found that black mantas were common in some locations, making up 40 per cent of the population, but were completely absent in others.

Melanistic and typically coloured manta circle a cleaning station (Photo: Steph Venables/Marine Megafauna Foundation, University of Western Australia)

‘Melanism was most prominent in the Raja Ampat (West Papua, Indonesia) population of reef manta rays (40 per cent) and in the Ecuador population of giant manta rays (16 per cent), 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.

The study also investigated whether the darker colouration of melanistic mantas affected their survival rates when compared to typically-coloured mantas. In some terrestrial species, melanism has been linked to improved camouflage or increased thermoregulation, but scientists questioned whether the same benefits would apply to manta rays in the ocean. While black manta are sometimes jokingly referred to as ‘stealth mantas’, their darker colouration could, in fact, make them more visible to predators.

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

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. Melanism is known to be a heritable trait, however, scientists have yet 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,’ said Stephanie Venables. ‘We are only just beginning to understand melanism in manta rays and we hope that future research will reveal more about this fascinating trait.’ 

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

This article was edited from the original blog post on the Marine Megafauna Foundation website. 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 9 October 2019 and is available here.