Her bulging belly and swollen abdomen told researcher Niv Froman all he needed to know: This reef manta ray hovering near the surface in the Maldives was unmistakably pregnant.
"She basically looks like a pregnant woman. We call it the pregnancy bulge," says Froman, an ecologist at The Manta Trust, a U.K.-based nonprofit that works to protect rays across the world.
At full term, the fetus folded up inside her womb had a wingspan of almost five feet, as wide as a park bench. But pregnancy only becomes visible after around six months, meaning scientists might not know a manta is pregnant until halfway through her 12.5-month gestation.
That’s why Froman and the team have been using new contactless ultrasound technology to identify early-stage pregnancy in reef mantas, a key piece of information for conserving the species, which the International Union for Conservation of Nature lists as vulnerable to extinction. This technology allows researchers to examine an animal’s internal anatomy while minimizing stress.
This is the first time scientists have used contactless scanners to obtain images of a wild pregnant manta ray. (Read how manta rays form friendships.)
In all, Froman describes 81 scans of 55 individual free-swimming reef mantas in his new study in the Journal of Fish Biology.
But the results were "very, very alarming," said Froman. Out of the 21 scans that captured images of the uterus, seven females were pregnant, with only four of those showing visual signs of pregnancy. A further five individuals—which had no visual signs of maturity, such as mating scars, pregnancy, or taking part in courtship behaviors—had large, developed uteri. These findings confirm that scientists have been underestimating the number of mature females in the reef manta ray population in the Maldives.
Knowing there are more mature females in the Maldives’ population of around 4,000 reef mantas—while the number of newborns remains the same—suggests the species' fertility rate is lower than previous estimates.
This brings the population “to an even more vulnerable level than we previously thought," Froman says.
A beneficial technology
Froman, who has been scuba diving with mantas for over a decade, knows how to predict manta movements. After taking an ID photo from below, he repositions himself ready for the scan and waits for the manta to pass him again.
When taking the ultrasound images, he approaches the ray from above and waves a probe of the small, portable waterproof scanner about an inch from the animal’s skin, ensuring the imaging can reach the uterus. In most cases, the ray doesn’t react, and in 20 seconds, the scan is complete.
Andrea Marshall, a National Geographic Explorer and co-founder of the Mozambique-based Marine Megafauna Foundation, is most excited that the technology has been validated for the first time.
She wasn’t surprised to hear researchers have been underestimating reef manta ray fertility, since their usual approach—looking for mating scars on the females’ pectoral fins—isn’t always reliable. “Scars don’t always occur from mating; they heal and they can be difficult to see,” she says.
She also adds the cost of the contactless ultrasound makes the technology prohibitive for most scientists studying manta rays. (Go inside the underwater world of manta rays with a National Geographic photographer.)
But for those who can afford it, she adds, the potential to gather more reliable data could fine-tune our knowledge of mantas’ reproductive ecology.
A boost for conservation?
The new technology has major conservation implications for Maldivian reef mantas, the largest known population of the species, which lives across the tropical and sub-tropical Indo-Pacific.
Now that scientists know fertility rates in Maldivian rays may be lower than previous data suggested, the race is on to find out how reef manta rays are faring, Froman says.
To that end, the Manta Trust is analyzing over 10 years of photo data to find out the population's survival rate, trends over time and region, and which pressures—such as tourism, fishing, and climate change—could be affecting the fish’s fertility. (Read about the discovery of a rare manta ray nursery.)
Mysteriously, manta fertility rates in the Maldives seem “wildly different” compared with other parts of the world, Marshall says, with that population producing offspring less often than those in other regions.
Maldivian females are thought to produce an average of one pup about every seven years, while births have been reported around every four years in Japan, and every two years in Hawaii and Mozambique.
But if researchers can confirm many pregnant females are gathering in a particular area, she says, it could help conservationists “designate that area as a critical habitat.”
