In a First, Scientists Discover Oceanic Manta Ray Nursery
Oceanic manta rays seem to glide forth from the depths as goliaths, fully formed. Marine biologists have struggled for years to identify where and when juveniles come of age. A new study published today by marine biologists from the Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration (NOAA) has, for the first time, identified a nursery for these 20-foot enigmas. This finding will buoy federal efforts to protect the species, which was recently listed as threatened under the Endangered Species Act.
Mysteries abound about the movements and reproduction of oceanic mantas—researchers even recently suggested that what had long been considered a single species may in fact be two.
But the new study manages to chip away at one of manta rays’ riddles. The discovery of the nursery seems to resolve why juvenile oceanic manta rays are rarely seen mixed in with adults, which is a common occurrence for their coastal cousins, the reef manta. It turns out that oceanic manta young have been squirreling themselves away in secret kindergartens. Fittingly, the one researchers have identified was hiding in plain sight.
The nursery unmasked by the team lies in Flower Garden Banks Marine Sanctuary off the coast of the Texas-Louisiana border. Finding it was doubly serendipitous.
The first bit of luck was that the site has long been frequented by NOAA biologists conducting a variety of biological surveys. Their meticulous notes and opportunistic photographs chronicled 357 different manta sightings dating back to 1993, forming a long-term dataset needed to conclusively prove the site is indeed a nursery. None of the researchers, however, typically worked on manta rays, and did not realize the importance of what they were seeing.
Cue Joshua Stewart, the lead author of the paper and a manta specialist at Scripps. “I started working with the sanctuary on some other projects related to mantas. […] I was there to collect some tissue samples, and the very first manta I dived with at the banks looked a little small.” Upon further investigation he realized it was a male with “teenie-weenie” reproductive organs indicative of a juvenile. “I said ‘Woah’ because I had only seen two or three juveniles anywhere else in the world.”
They came up from the dive and Stewart turned to the NOAA researchers to express his excitement, only to find their response blasé. According to Stewart, “They said ‘that’s pretty common for around here.’”
The team then combed through the manta sighting records, an important step because the high abundance of juveniles only satisfies one criterion for qualifying as a salt-water nursery. The NOAA dataset, however, ticked the other two boxes; the area had been utilized in the same way across many years and individual juveniles did stick around for more than transitory period.
They were able to establish the last point by pouring over their photo archive, which reveal the unique spotting pattern on each manta’s belly. Despite lacking hands, it turns out mantas have a fingerprint of sorts.
While the nursery may be the only one that has been conclusively identified, Dr. Andrea Marshall, co-founder of the Marine Megafauna Foundation and a scientist unaffiliated with the study, was quick to contextualize the finding. “It’s certainly not the only one known,” she said. Marshall has quite the credentials to back up the assertion. Her personal website points out she was the first person in the world to finish a doctorate on mantas.
Marshall continued, “I know of about four other places where we are seeing small-sized oceanic mantas on a regular basis.” She also pointed out that “at some of the other places they I mentioned they are newborn-sized consistently.”
Watch a video about the newly-discovered nursery.
Stewart readily conceded that Flower Garden Banks is not entirely unique, but pushed back at the suggestion that newborns were not well-represented there. “The spread of sizes we see suggests to me that there is a range of juvenile ages represented at the banks.” He added more than a quarter of the sightings were animals with a wingspan less than 1.6 meters, which despite being the height of an adult human, falls well under the size that manta babies hit by their first birthday.
Stewart and Marshall both agreed that this type of investigation fills dangerous gaps in our knowledge of the now imperiled species. As Marshall put it, “Understanding more about a very critical habitat for mantas is hugely important.”
Internationally, oceanic manta rays are under threat from targeted fishing for the lucrative market for Chinese traditional medicines, the efficacy of which is not underpinned by medical evidence. In U.S. waters though, the biggest challenge is when the rays are caught as by-catch when fisherman use nets to target other species or when some types fishing gear disturb the sea floor.
The paper stops short of explaining why Flower Garden Banks acts as a magnet for young mantas, but Stewart speculates that its location is critical. The Banks lay in close proximity to deep water feeding grounds but are also shallow protected reefs. That means that baby mantas can safely warm themselves in the sun after a feeding foray in the adjacent cold dark depths. Sunning directly above their deepwater snack bar could leave them exposed. Bite marks on the wings of some juvenile mantas at the site are a testament to the dangers of tiger sharks and other predators for the rays lacking their immense adult stature.
The study will no doubt be a boon for NOAA. The agency is required to identify critical habitat and measures to protect the species now that oceanic mantas have been listed under the U.S. Endangered Species Act. As the authors of the paper posit, Flower Garden Banks may be a particularly important site to protect, because young mantas do not come around often. Manta moms carry their developing young for more than a year before giving live birth to just a single pup. They then go two to seven years before giving birth again.
A manta ray giving birth
The mysteries of the manta may never be fully unravelled, but Stewart’s study suggests another reason why Flower Garden Banks Marine Sanctuary may be a great place to start looking. It turns out the nursery is a multicultural manta kindergarten. Through DNA samples and photos, the authors are confident that both the original oceanic manta and the putative second species frequent the site.
Images courtesy FGBNMS/G.P. Schmahl, Joshua Stewart