Melanism in Manta Rays

manta ray

After reading a New York Times article on melanism in manta rays, our group felt inspired to learn more about the species and its interesting condition. For more information, the article is linked here, as well as in the References section!

Background on Manta Rays

There are many organisms of astonishing size in the ocean, but one of the biggest and most magnificent is the manta ray. They are about 25 feet long wing to wing, and due to their extreme size, they have very few predators. There are two different species: the Mobula birostris (the pelagic type) and the Mobula alfredi (the reef type), both of which generally reside in the Indo-Pacific waters. Both species are listed as vulnerable on the IUCN Red List of Threatened Species. When viewed from below, one can observe that most manta rays have a solid white stomach; however, some have unique black spots. These black spots result from melanism—a common condition among land animals but very rare in aquatic animals.

What is Melanism?

Melanism is defined as an increased production of melanin pigments, resulting in darker colored individuals. This is very common on land and has been linked to evolutionary advantages across land animals such as pocket mice, snakes and insects. Underwater, however, melanism is highly uncommon, and out of hundreds of species of cartilaginous fish only two species—both the only known mantas—exhibit melanism. Additionally, melanism exhibited in these mantas only occur in some populations. The variation in melanism frequency in mantas from different locations has caused researchers to wonder if this phenotype provides any evolutionary advantage. 

Melanism and Natural Selection

Melanism in terrestrial animals has proven to be an advantageous result of natural selection. For example, pocket mice in contrastingly light and dark terrain have adapted to match their coat to their surroundings as a means to avoid predators. Beyond camouflage, melanism aids some species in regulating body temperature and resisting disease. Given mantas’ unique development of melanism, researchers aimed to identify which, if any, aspect(s) of fitness it could facilitate. Most manta rays have white bellies, enabling them to blend in with contrasting light from the surface and avoid being seen from below. Though manta rays have few predators, a black belly, or even a spotted one, would presumably make them more prone to predation. Given this hypothesis, marine researchers from the United States, Australia, and Indonesia recently conducted a study to determine whether melanism affects the manta ray’s survival.

Conclusion and Future Work

There is no one definitive answer as to why melanism started to show up in certain manta ray populations around the world. Current studies concluded that melanism does not give mantas survival advantage over their predators; rather, it may just be a product of genetic drift. However, it would be interesting to see a future study on their hunting habits (e.g. time of day), which can help researchers understand if melanism provides an advantage related to when mantas look for their prey. However, future research is increasingly becoming difficult with manta populations dwindling. Nevertheless, it is important to continue to better understand mantas and its important ecological role in the environment.

Ecotourism: The Value of Protecting the Manta Rays

The dwindling populations of Mobula birostris and Mobula alfredi mean their protection and conservation is more important than ever. Off the North Sudanese coast of the Red Sea, high concentrations of reef type mantas frequently congregate in and around Sanganeb Marine National Park and Dungonab Bay – Mukkawar Island Marine National Park. These two sites comprise a marine protected area (MPA) and UNESCO World Heritage Site. In fact, the only documented sighting of a Mobula birostris X Mobula alfredi hybrid ray occurred in this MPA. However, two conflicting proposals may determine the future of the area and the mantas. The Heart of the World, a Dubai-esque proposal including an airport and a huge skyscraper, would involve heavy coastal dredging, increasing turbidity and decreasing plankton abundance—two challenges for the mantas. On the other hand, there’s potential for small-scale ecotourism development based in Mohamed Qol and Dungonab, where manta-watching would promote local economic development while ensuring the MPA stays well protected. Ecotourism is rising across the world and has already shown promise in Sudan. Promoting such practices will provide the best protection for the mantas, facilitating further research into melanism and many other phenomena we’ve yet to discover.   

 

References

Augliere, B. (2020 Jan 8). For Manta Rays, Survival Isn’t Black and White. Hakai Magazine.

Kessel, S. T., Elamin, N. A., Yurkowski, D. J., Chekchak, T., Walter, R. P., Klaus, R., Hill, G., & Hussey, N. E. (2017). Conservation of reef manta rays (Manta alfredi) in a UNESCO World Heritage Site: Large-scale island development or sustainable tourism? PLOS ONE, 12(10), e0185419. doi: 10.1371/journal.pone.0185419

Klein, J. (2019 Oct 14). The Mystery of Melanistic Manta Rays. New York Times Science. 

Márquez, M. C. (2019 Dec 26). It’s Not All Black and White: Melanism in Manta Rays. Forbes

Murray, A. (2019 Sept 15). Protecting the Million-Dollar Mantas. Save our Seas.Venables, S., Marshall, A., Germanov, E., Perryman, R. & Tapilatu, R. (2019 Oct 9). It’s not all black and white: investigating color polymorphism in manta rays across Indo-Pacific populations. The Royal Society Publishing. 100(9). 5268-73. doi: 10.1073/pnas.0431157100

6 thoughts on “Melanism in Manta Rays

  1. Matias Junghahn

    I found the topic of melanism in manta rays to be fascinating, particularly because you mention that it does not occur in any other species of cartilaginous fish. The only logical reason I could come up with for this is that melanism in other species of fish could cause an increase in visibility and thus predation, meaning that individuals that experience genetic variations leading to melanism usually do not reproduce and pass on the trait for melanism. Since manta rays do not experience predation pressures as intense as most other fish, perhaps the individuals that developed melanism were able to effectively reproduce and eventually the trait became common. I also found a Chinese study focused on cod that mentions that the fish may become melanized as a response to parasitism, which could perhaps be the case in manta rays as well.

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  2. Paige Bernstein

    I find this article to be particularly interesting as researchers still have yet to find a definite conclusion on the exact purpose, if there is one, of the melanism found in some manta rays. I also find it interesting that melanism is a trait found in other terrestrial species, yet it functions to aid in either camouflage or thermoregulation, which is not the case for manta rays. This makes me curious as to whether different species can make use of the same trait in differing ways depending on their environments. In a different article on this topic, it describes that the percentage of rays with melanism also differs greatly between different populations, with it occurring in over 40% of rays in some populations and only 16% in others. Because of the lack of knowledge on this topic, as well as the declining population of manta rays, I think it is even more crucial for these animals to be protected and maintained in order to continue studying them.

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  3. Mason Berger

    Conservation is a constant race against time. With manta ray populations dwindling, we need every piece of information possible about them in order to devise the most effective plan to save them. And if there is one thing this study reveals, it’s that there is more going on with manta rays than we may have previously thought. Perhaps it is genetic drift, as the post mentions, or perhaps there is a deeper reason for the various patterns of melanism we see. Either way, there are questions still to be answered about manta rays and thus there is still the potential for more effective conservation methods to be discovered. I also like that this group talked about ecotourism and its potential to promote conservation. In creating an economic incentive to preserve the manta ray, the argument becomes much easier to make with governments, corporations, etc. Thank you for this great presentation and I am excited to see further research on this topic!

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  4. Lydia Sellers

    I really was intrigued by this article due to previous studies I have looked at about the coloration in manta rays. After listening to the presentation, I was still wondering if the melanism in rays is permanent or whether the coloration can change. I found in research by Dr. Csilla Ari that manta rays can undergo pigment changes through maturation. The changes also resemble two separate species of manta rays based on coloring at the beginning and the end of the observation. The males coloration change was also more prominent than the females. I wonder if this could make identifying specific species difficult or even if this could enhance their fitness like suggested in the group’s article. It would also be interesting to know if this coloration change can be seen in the terrestrial species they mentioned that exhibit melanism.

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  5. James Zhou

    I have a few (probably bad) ideas for why the melanism might persist, partly drawing from what my classmates already commented.

    I read from SeaWorld (sorry couldn’t find a better source) that for orcas’ black and white coloring, “The dark side blends in with the murky ocean depths when viewed from above. The light ventral side blends in with lighter surface of the sea when seen from below. The result is that prey have a difficult time seeing a contrast between the counter-shaded animal and the environment.” It’s possible mantas evolved that as well, but with a much higher degree of variation over how the melanism is distributed.

    Also, since orcas and sharks seem to be their main predators outside of humans, maybe we need to question the assumption that the melanism makes them easier to spot. Since orcas and sharks rely heavily on sound and smell, maybe sight is less important in their predation of mantas, and once they get close enough, both the melanated and non-melanated mantas are about equally easy to spot and catch.

    It’s also possible that since, as Lydia pointed out, the coloring changes over time, maybe oftentimes mantas that will develop melanism later in life are able to reproduce before that happens, thus ensuring that their genes are still passed on before their fitness decreases.

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  6. Ansley Arnow

    I found this blog post fascinating, and I am very interested in seeing what scientists will be able to learn in the future through more studies on manta rays. I think your group did a great job at really explaining the concepts brought up by the article (ex: melanism) in a very clear way, and relating them to things we see on a daily basis. Great job!

    When I was going through these blog posts again more recently after our lecture on plastic and pollution, I began to wonder whether this melanism had anything to do with industrial melanism. I had previously thought that industrial melanism could only be seen on species on land, for example with the peppered moth during the Industrial Revolution, but I recently came across an article that talked about how the turtle-headed sea snake has evolved to a darker color and has lost its stripes (https://www.nationalgeographic.com/news/2017/08/sea-snakes-color-pollution-environment-oceans/).
    A study believes that the pigment may actually help these snakes rid themselves of industrial pollutants when they shed their skin due to higher concentrations of trace elements such as arsenic and zinc. They also tended to shed their skin more than snakes in areas further away from industrial pollution. Although manta rays obviously do not shed their skin, I was curious whether the melanism spots had a correlation with increased pollution in the area. When doing a little more research on the North Sudanese coast, I found out that they experience oil pollution and contaminants due to the expansion of international oil trade (https://www.nationalgeographic.com/news/2017/08/sea-snakes-color-pollution-environment-oceans/). This could be interesting for a future study.

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