The Insane Biology of: The Giant Manta Ray

The world of mobula rays transcends the barriers between oceans, the borders between shallow seas and cold ocean depths, and even the ocean’s ultimate barrier, the one between sea and sky. Found in tropical and temperate seas around the world, the genus Mobula contains some of the most incredible organisms that have ever graced this planet. There are over 600 species of rays in the ocean, but only 11 different species of mobula rays, all of which have a distinct diamond-shaped appearance with wings that allow them to glide through the water and put on stunning displays of elegance and agility.

Some are small and congregate in incredible numbers, occasionally flying out of the water in acrobatic leaps. Others are absolutely enormous, so large that their elegance appears to defy gravity. The giant oceanic manta ray is the largest species of ray in the world, with wingspans that can reach over 8 meters. Their sheer size makes them a striking presence in the ocean, captivating researchers and divers who are lucky enough to encounter them. More than just being huge, they are also gregarious and often curious about human swimmers, just as the swimmers are about them. This curiosity is due to the giant oceanic manta ray’s incredible intelligence; they have the largest brains of all fish, with a brain-to-body ratio that exceeds that of the enormous whale shark.

Until very recently, scientists struggled to categorize them. In 2018, scientists listed giant mantas and reef mantas under the genus Manta, but further analysis of their morphology and genetics resulted in placing them in the Mobula genus along with smaller devil rays. Now, scientists are learning that the giant oceanic manta ray isn’t simply a shallow water plankton eater but a deep-sea renegade with movement so efficient that teams of roboticists around the world are working to emulate it. Not only do they dive deeper than previously thought, but they have also been observed to rapidly change the color and pattern of their skin, something that is basically unheard of in the world of elasmobranchs.

As the giant oceanic manta ray glides through the water, it flaps its massive pectoral fins, giving the appearance that it’s flying. When rays diverged from sharks around 200 million years ago, the pectoral fins expanded into what we now call the ray disc. For the giant oceanic manta ray, this expansion was extreme; their wings can reach over 8 meters across, and their pectoral fins make up around 85% of their body length. In contrast, for sharks of a similar size, their pectoral fins make up just 10 to 15% of their body length.

The benefit of such massive wings compared to the more typical shark body plan becomes clear when observing the behavior of the giant manta ray compared to the similarly sized great white shark. The great white shark is a powerful, fast swimmer, but its bursts of speed usually only happen in one direction, making it hard to turn quickly. In contrast, the giant manta ray is often seen swimming in tight, acrobatic circles, circling back to eat clusters of plankton or evade predators. Its maneuverability and agility are almost unheard of in such large animals, thanks to its large and flexible wings.

While the overall shape of a manta ray’s wing is similar to that of birds, these fins can undulate and bend in many more ways than bird wings can. The manta ray has numerous support structures inside the pectoral fin, which can all be controlled separately. However, as useful as maneuverability is for evading predators, it comes at a cost; maneuverability is simply controlled instability, and without stability, an animal can’t move steadily and efficiently along a predictable path. Fortunately, the giant oceanic manta ray strikes a balance between these opposing qualities, being maneuverable while also maintaining stability and efficiency.

One way the giant manta ray maintains stability is by having wings with a fairly high aspect ratio. During flight, whether in the sea or the sky, longer, narrower wings give a plane or manta ray more stability. While manta wings aren’t exactly long and skinny like glider wings, their length does help balance the animal while gliding. High aspect ratio wings also help generate lift efficiently. The way the giant manta moves its pectoral fins allows its swimming to be highly efficient and sometimes surprisingly fast. On average, giant manta rays swim about 9 mph (14.5 km/h), but to evade danger, they can sprint as fast as 22 mph (35.5 km/h).

To reach these speeds, they combine fin oscillations with undulations. Oscillations are the flapping of the wing up and down, generating large propulsive and lift forces. At the same time, the wing is undulating, sending a traveling wave outward from its body towards its flexible wing tips. As giant rays flap in this manner, their fins disturb the water and create vortices that push against the surrounding water, propelling the animal forward. These two types of motion make the giant manta ray an extremely efficient swimmer, with a propulsive efficiency measured at 89%.

To understand more about the biology of manta rays, I spoke with Jessica Pate, founder of the Florida Manta Project and research scientist at the Marine Megafauna Foundation. She explained that manta rays have to keep swimming in order to breathe, so they are designed to swim. You’ll never see one resting on the bottom. Their hydrodynamic efficiency is remarkable, with a more pointed diamond-shaped body structure compared to stingrays, which are rounder and live on the bottom. This design allows them to generate a lot more power out of their wing tips.

Researchers are even designing underwater autonomous vehicles modeled after giant mantas, hoping these bio-inspired machines will help map the seafloor, perform search and rescue missions, and inspect underwater technology far more nimbly and efficiently than current systems.

While all mobulas belong to the subclass of cartilaginous fish known as elasmobranchs, there are significant differences between mobulas and stingrays. Mobulas have mouths on the front of their bodies rather than underneath, and they possess calicles—two fin-like appendages around their mouths. Unlike the anterior part of most stingrays, these calicles do not have electrosensory abilities; rather, they help funnel water and plankton straight into their mouths. This makes mobula rays the only vertebrates with six paired appendages instead of four: two by their mouth, two pectoral fins acting as wings, and two pelvic fins at the back.

While stingrays have long stingers, most mobula species, including the giant manta ray, have lost their ability to sting. One of their other differences is something you can’t see unless you get up close and personal with either of these animals: their teeth. Stingrays have crushing teeth plates that help them feed on crustaceans and fish, while mobula rays only have small, peg-shaped, uneven teeth on their lower jaw, which are pretty useless for chewing. These teeth are primarily used for mating when males latch onto females.

Manta rays are filter feeders. When plankton is abundant, they open their mouths and swim through it. There’s no sucking; everything in their path simply travels into the large oral cavity and passes through their gill rakers. Manta rays are like giant flying colanders, filtering out the tasty bits. This transition from grinding teeth to filter feeding dates back to sometime before 50 million years ago in the fossil record, possibly after the extinction of the dinosaurs 66 million years ago when other ocean-dwelling planktivores disappeared, opening up a large environmental niche.

There was also a geological event called the Paleocene-Eocene Thermal Maximum about 55 million years ago, during which global temperatures increased from 5 to 8°C, leading to a boom in plankton growth. Whatever the reason, mantas became voracious plankton eaters, and as a result, something remarkable occurred: manta rays became giants.

But how is it that a diet of tiny plankton pushed these creatures to become so big? Researchers have found that gigantism isn’t limited by the trophic level (which type of food an animal is eating) but rather by the quality and abundance of that food and the animal’s ability to exploit it. There’s a biological theory called Cope’s Rule, which hypothesizes that evolutionary lineages tend to increase in size over time because being larger improves survival for apex carnivores. This is only true up to a point; there’s not much point in being larger than a size where you have no enemies and can prey on anything you want.

For plankton eaters, being bigger is almost always better. Being massive gives them an increased foraging range since greater swimming efficiency enables traveling farther in search of plankton. A larger mouth and increased surface area also allow them to process much more water and gather much more plankton. Perhaps the biggest advantage of being so large is that you’re less likely to be eaten. When manta rays are full-grown adults, they have very few predators; really, it’s only orcas.

There has been a recent orca predation in Hawaii on a manta, and large sharks in some study sites have been known to take bites out of mantas. However, mantas are surviving because they are only taking small chunks out of the back. History has shown that large animals are more at risk of extinction because they are often more vulnerable to environmental crises due to their need for more resources like food, as well as their long reproductive phases. Organisms that reproduce in a span of a few weeks are much more likely to undergo rapid evolution than something like a manta ray, which has a pregnancy lasting a full year and females that only give birth every few years.

Their overall life history strategy is more similar to that of an elephant than to another kind of fish because they grow really big but reproduce very slowly. Generally, when you have that kind of strategy, you have higher survivorship because you’re putting all your eggs in one reproductive basket. But for the giant manta ray, being gigantic and eating lots of plankton has so far been worth the risk.

For a long time, we thought shallow water plankton was all they were eating. We would see them swirling around shallow coastal waters, mouths open, catching plankton, and we thought that was the end of the story. However, it turns out manta rays aren’t just inhabitants of the sunny shallows; they dive deep—sometimes really deep—all the way down to the ocean’s midnight zone.

For decades, scientists have struggled to fully understand the lives of giant manta rays because they are simply hard to track down. They are known to travel huge distances, and when scientists are lucky enough to spot one, it’s usually from a boat in shallow water. Thus, for a long time, all the data surrounding giant mantas came from observations in this environment.

As time went on, some researchers had a hunch that there was more to manta ray behavior than they were seeing. To gain more insight, they traveled to Ecuador, where the largest number of giant manta rays are known to congregate. Here, they collected small muscle samples from the rays as well as samples of plankton. They wanted to find evidence in the rays’ bodies that their diet was made up entirely of shallow water plankton. To do this, they compared the nitrogen and carbon isotopes in both the plankton and the manta ray muscle tissue.

Nitrogen isotopes are particularly useful because they can tell us about what marine organisms have been eating; the ratio of nitrogen increases with each step up the food web. In other words, top predators like sharks and tuna will have a much higher nitrogen ratio than something like plankton, which eat algae. Carbon isotopes are useful in telling us the origin of food sources; for example, surface plankton and mesopelagic plankton have different levels of carbon-13.

What the researchers found in the case of the giant manta rays was that their chemical signature wasn’t correlated to surface plankton; it looked much more like they were eating food from the mesopelagic, a much deeper layer of the ocean. Later, scientists put satellite tags on the manta rays and found that they were diving to depths of 1,400 meters. Another group of researchers tagged mantas off the coast of Peru to see what their movements would be like throughout the day and found that three of the rays dove much deeper at night than during the day.

It’s possible that they stayed closer to the surface during the daytime as a way of warming up since they’re ectothermic, meaning they can’t produce much of their own body heat and rely on the surrounding temperature to keep them warm. But the deep dives aren’t the only unexpected movements these oceanic mantas are making. It also seems like they’re moving around their habitats in different ways than we once thought.

For a long time, giant oceanic manta rays were believed to be intrepid travelers, migrating huge distances to get from one feeding ground to another. While that is probably the case for some mantas, one survey that stretched over four years and relied on citizen scientists along the east coast of the U.S. collected more than 5,000 sightings of giant manta rays and saw them moving hundreds of miles up and down the coast depending on water temperature.

They can make long-distance migrations; our team just published a paper where they tracked a manta going a thousand kilometers, which is a pretty long-distance migration, but maybe not like the white shark that swam from South Africa to Australia. It’s not something as extreme as that. A couple of years ago, a manta ray showed up at Coco Island, which is about in the middle of the ocean off the coast of Costa Rica, but it was a reef manta ray, which reef manta rays are not known to inhabit the eastern tropical Pacific. The closest place it could have come from is Hawaii, which is thousands of miles away.

Other researchers looking at giant manta rays around Mexico and Indonesia found that the rays didn’t swim more than 150 miles from where they were first tagged; they stuck much more closely to their own neighborhood than researchers expected. They also ate a diet that was very specific to their region. So, their non-migration could be due to the abundance of food in those particular locations. Finding and studying these more sedentary populations could open up a new chapter in manta ray research, and we clearly still have a lot to learn about them, including something that seems fundamental: can these mysterious creatures change the color of their skin, and if so, how did we only just find out about this spectacular ability?

Mantas are typically described as having one main type of coloration called the chevron, which is a dark navy or black shade over most of their dorsal side, with two white triangular patches across their head. Their ventral side, or belly, is largely white in color, with different patterns and patches of black. However, when observing captive individuals, researchers were surprised to see the coloration of the giant manta rays rapidly change. The skin around their eyes, mouths, and the inside of their calicles, as well as the chevron shapes on their backs, all rapidly lightened or darkened during intense social interactions or just before they were fed.

This is something our team is actually looking into. We were always a little skeptical about it because we had never seen it. The way we do our research is quite different from everyone else; we usually look from a drone in the sky. While you can’t necessarily tell who is who based on their dorsal coloration, I can usually identify individuals based on slight differences. One day, I noticed a new manta, but when we got in closer, it was the same manta from two hours before, just looking dramatically different. The color-changing was reversed fairly quickly, but researchers really don’t know how or why the giant manta rays put on such a show.

We do know that they have good visual abilities and large brains, so maybe the color changes have to do with social behaviors. This ability isn’t just limited to the giant oceanic manta ray; the smaller reef manta has been observed to change colors too. It’s a tantalizing hint that these mantas are more complex than we might give them credit for.

Another piece of evidence suggesting the extent of their intelligence is the mirror self-recognition test. This experiment is generally thought to be one of the most reliable ways to gauge an animal’s self-awareness or self-recognition. For land animals like chimpanzees, it’s easy to put some kind of marking on the animal that they can’t see without the aid of a mirror. If they notice the mark on themselves in the mirror and respond to it, that suggests they’re aware that the image in the mirror is their own body.

While researchers couldn’t easily place a marking on manta rays, the mantas did significantly more repetitive movements in front of a mirror than in control conditions. Their calicle fin movements also increased, almost as if they were making faces at themselves. It’s not quite possible to say whether they passed the mirror test, but the results are intriguing nonetheless.

While it is difficult to quantify the intelligence of any animal, especially the giant manta ray, we do know that they have the biggest brain of any fish. In the Mobula genus, the telencephalon is the largest part of the brain, accounting for 61% of the total mass. This is also the area of the brain associated with memory, learning, complex social behavior, and higher sensory processing. They don’t just have a big brain because they’re a giant animal; they have a big brain for their body size.

We think that manta rays are likely pretty intelligent, probably because they’re very social creatures. Many divers or people who’ve been in the water with them report feeling as though the mantas are looking at them. There are also experiences when they become entangled in fishing line; they just stop swimming and let you remove it before swimming off. They have complex lives and are really intelligent, more similar to dolphins than to typical fish.

There are so many characteristics of the giant manta ray that make them unique: their brains, their behavior, their bodies—the list goes on. However, many aspects of their lifestyle also make them increasingly vulnerable to extinction. In 2018, the giant manta ray was listed as threatened under the U.S. Endangered Species Act. Because they feed on plankton, they are also more likely to ingest large quantities of microplastics, and we just don’t know what that might do to the health of animals that can live for more than 40 years.

They are exceptionally vulnerable to almost every kind of fishing, and in shallow water habitats, they are often hit by boats speeding above them. Additionally, people falsely believe that manta gill plates provide health and vitality to those who consume them, leading to their killing in huge numbers specifically for that one body part.

Researchers are doing their best to understand manta ray behaviors in order to protect them, whether that’s creating locations with special designations as nurseries or monitoring their movement from the mesopelagic back up to shallow waters. The more we learn, the better we can ensure our activities don’t interfere with theirs, increasing our chances of seeing them gliding by in the water.

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