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.
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
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