Scientists Are Learning Fascinating Things About The Manta Rays In Mexico

Mantas are famous for their coloration, typically dark grey on top with a mostly white underbelly. Like a fingerprint, the spots splayed across this milky underside allows researchers to identify individuals within a population, especially if they are constantly coming to the same spots. Every now and then, though, you might see one or two that look completely different: almost entirely black. Melanism is generally defined as an increase of dark pigment in the skin, fur, or plumage of animals, resulting in a blackish appearance. While relatively common in terrestrial animals, it’s quite rare in marine animals... except in mantas. Melanistic (or black) mantas aren’t as uncommon as you would think! While previous research completed by the Marine Megafauna Foundation has found melanism 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%), it wasn’t quite known how prevalent it was in the Mexican manta population.

Some 250 miles (400 kilometers) south of Baja California lies the Revillagigedo archipelago, a cluster of volcanic islands that are home to one of the largest aggregations of sharks and manta rays in the world. Known as the “Galápagos of Mexico,” the government of Mexico created the Revillagigedo National Park (RNP) in 2017, expanding a previously small marine reserve around the archipelago to become, at the time, the largest fully protected marine protected area in North America. Since then, RNP has become a well-known aggregation site for oceanic manta rays (Mobula birostris) and other tropical marine megafauna in North America. However, authors of a new paper point out that published baseline data on population structure and dynamics, and the influence of oceanographic variables on the abundance of oceanic manta rays in the region are lacking.

All photo‐ID images were opportunistically collected by professional and recreational divers (1–3 citizen scientists per trip) from liveaboard dive boats and were compiled into a photo‐ID database, and used them to describe baseline demographic characteristics of this population and to also evaluate the influence of environmental factors on population dynamics. By analyzing the unique spot pattern each oceanic manta ray has on their belly, the scientists were able to identify and catalogue individuals with information on sex and color morph (whether they were melanistic or had the black-and-white pattern mantas are so famous for). A total of 493 individual oceanic manta rays were identified from a total of 778 photo‐ID sightings. A total of 399 mantas (81.1%) were seen in only one year, 71 mantas (14.4%) were observed in 2 years, 20 were observed in 3 years and only 2 mantas were sighted in all four years. The team was also able to determine the sex in 79.2% of these individuals, with over half (58.6%; n = 218) being female. Most of the photo-ID sightings that the researchers were unable to determine the sex were of the black morphology type (78.9%) as it’s difficult to visually identify the sex of black mantas as everything sort of blends in. Because of this, authors recommend that sex might be routinely collected for all individuals for future studies. While 369 (75%) of the mantas had a ‘chevron’ color morph, 123 (25%) were black mantas (that had white markings on their bellies to help ID them). This is higher than other populations around the world! 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.

The team also used a mark-recapture technique to estimate monthly abundance of the mantas in this area. A method commonly used to estimate the size of a population where it is impractical to count every single individual, the basic idea is that you capture a small number of individuals, put some sort of identifier on them (like a tag or mark of), and then release them. Later on, you’ll catch another small group and record how many you recapture (how many you catch that have a mark or identifier on them). In a small population, you are more likely to recapture those marked individuals, while in a large population, you less likely. The team did something similar, but with used an entirely non-invasive methodology and used the photos as ID reference that works as a capture. Across the full four-year study period, they calculated a superpopulation size for the RNP population, of 1172 (± 90) individuals, an important baseline record for this population as well as one of the few estimates of what the size of a typical oceanic manta population might be. “These superpopulation sizes demonstrate how oceanic manta ray populations are extremely vulnerable to fisheries pressure,” the authors state in their paper. “This also suggests that population sizes of oceanic manta rays may vary substantially by region, as the annual cap-tures in major fisheries nations such as Sri Lanka and Indonesia would rapidly extirpate a population of similar size to the one estimated here.”

As filter feeders, oceanic manta rays are closely linked to local primary and secondary productivity, and by measuring sea surface temperature (SST), chlorophyll-a (Chl-a), and tracking the El Niño–Southern Oscillation (ENSO) the team also found that the RNP may have more favorable prey conditions than other parts of this population’s geographic range, therefore serving as a refuge for the oceanic manta rays here. “The use of other approaches such as satellite telemetry will be necessary to understand the expansion and contraction of the population distribution range, the fine scale vertical use of the water column, and to evaluate movement patterns offshore and away from the specific coastal and insular portions of their range that are accessible to field researchers,” the authors conclude.