How to ultrasound a 15-foot-wide giant mother manta

More than 300 miles off Mexico’s Pacific coast, a giant oceanic manta ray—its wingspan stretching around 15 feet across—glides past Madalena Cabral. She stays calm, and waits for the animal to see her.

Cabral, a marine biologist with the organization Mares De Mexico, has spent the past six years studying the manta rays at Revillagigedo National Park, the largest fully protected marine reserve in North America. And while oceanic manta rays are among the largest fishes in the ocean and are listed as endangered, many aspects of their basic biology remain poorly understood.

When the giant manta has accepted her presence and seems comfortable, Cabral swims above the manta with a handheld device and presses gently against the animals’ back. As she scans the manta’s back, an image flickers onto her phone. Inside the animal, a pup, a baby manta, is moving—its wings flexing, its mouth opening and closing as it pumps nutrient-rich uterine fluid.

“It’s like a superpower,” Cabral says, “having vision to see inside an animal.” The first time she recognized what she was seeing in real time she burst into tears. “I cried underwater,” says Cabral.

Cabral uses underwater ultrasound technology to confirm pregnancies in wild oceanic manta rays (Mobula birostris). So far, Cabral has confirmed three pregnancies on oceanic manta rays. This paper is the first scientific record of ultrasounds confirming pregnancy in wild oceanic mantas.    

Ultrasound on wild, free-swimming mantas has been done before, but on reef mantas (Mobula alfredi), a closely related species that lives in more coastal environments. Scientists know considerably more about reef mantas, which are more plentiful and stay closer to shore than oceanic manta rays. Additionally, marine biologist Deni Ramirez Macias scanned oceanic mantas with ultrasound in both Ecuador and the Revillagigedo Islands, though this work remains unpublished.

Confirming the pregnancy is only the first step, says Cabral. The next question is figuring out where those pregnant females go. Using multiple research techniques, such as satellite tagging, combined with the underwater touchless ultrasound imaging and long-term photo identification, she hopes to uncover how these rays move, reproduce, and use protected areas throughout their life cycle.

During the two expeditions, one each in January and March of 2026, Cabral tagged three manta rays with satellite devices designed to track their movements. The tags remain attached for about six months before automatically detaching, floating to the surface, and transmitting their data via satellite.

Once the data is retrieved, Cabral can begin to piece together where pregnant mantas spend their time—whether they stay within protected waters or move into the open ocean, and how their depth and behavior may change over the course of pregnancy.

“I am very comfortable saying that we do not know, it's a mystery, " she says. “Nobody knows.”

She hopes this data informs decision and policy making to effectively protect the species, including if the protected area needs to be expanded. We don’t want to “just create a rectangle in the middle of the ocean — animals don’t know borders,” Cabral says.

Beyond ocean borders

In the waters surrounding the reserve, the primary threat is bycatch from industrial tuna fisheries. Massive nets can entangle manta rays, preventing them from swimming. They need to swim to breathe, so “if they’re entangled, they are not going to make it,” Cabral says.

Even individuals that are released often suffer severe injuries. With such slow reproduction, populations may struggle to recover from sustained losses, she says. In some parts of the world, oceanic manta ray numbers have declined by 90%, she says.

Oceanic manta rays reproduce very slowly, the slowest in fact of any shark or ray. Females don’t start having babies until they’re about 8 to 10 years old, and even then, they usually have just one pup every four to five years, with a 12-month pregnancy. Because they have so few offspring, their populations are especially vulnerable to threats like fishing and human activity, says Cabral. That’s why identifying where pregnant females are going and where they are pupping is essential for their long-term survival.

This work will pave the way for Cabral to describe and establish the technique to ultrasound oceanic giant mantas, because they behave quite differently from the reef mantas. For the smaller reef mantas, “they are more skittish and you have to approach from their blindspot,” Cabral says. 

For conservation, it’s really important to know how much a population reproduces, but with wild animals, scientists often have to estimate this instead of measuring it directly, says Asia Armstrong, a manta ray biologist in Australia with the University of Queensland who was not involved in the project. “To be able to use this technology with underwater observations is a total game changer. It's really quite amazing,” says Armstrong. Scientists need to know how often manta rays reproduce and how many are pregnant to understand how healthy their population is and how much pressure they can handle, she says.

“But you do need that expertise to know what you're looking at,” says Armstrong.

And for that, Cabral turned to the publications from a group of researchers with Okinawa Churaumi Aquarium in Japan who pioneered underwater ultrasounds in sharks and rays. “They have the whole progression of gestation,” says Cabral, “so the images they provide in that paper were a good baseline for me to compare what I have to their images.”

“I think it is very exciting news,” says Kiyomi Murakumo, a biologist at the Okinawa Churaumi Aquarium. “Confirming pregnancy shows that this technique can be applied not only in managed environments but also in the field.    

Early detection

Using the ultrasound machine also helps scientists spot pregnancy in manta rays much earlier, instead of only when they look obviously pregnant and confirm that pregnant mantas live in Revillagigedo National Park.      

Normally, manta rays look like a flat pancake, says Armstrong. When they are late in pregnancy, they “start to look more like a ravioli,” she says, as the animal's belly area swells, creating a noticeable bulge, and then a bulge becomes visible on the upper surface as well.

But, Cabral didn’t see these big fat mantas at the park. With ultrasound, Cabral can now detect pregnancies much earlier, even before mantas look visibly pregnant. By combining this with tracking tags, she can see where pregnant mantas spend their time and where they go later in pregnancy. This helps identify important habitats and makes it easier to protect those areas.

Manta ray expert Guy Stevens, founder of Manta Trust, agrees that this is a great first step for oceanic manta conservation. “What we really need is repeat scans of the same individuals over time and a lot more scans of the adult females in the population,” he says. This will help understand how many pregnant females exist in the population, if there is seasonality to their reproduction, and where and if they leave to give birth elsewhere, since few pups are seen at this location.

Cabral is planning to scan and tag more rays in 2027. For her, studying the rays at Revillagigedo is a window into their lives that keeps unlocking more mysteries. For instance, she recently documented Galapagos sharks rubbing against manta rays for the first time, a behavior that may help the sharks remove parasites. “Every time we learn something new about manta rays, we’re reminded how much of their lives still happens beyond our sight. Protecting places like Revillagigedo gives us the chance to keep discovering those secrets.”