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An Interview With a Marine Biologist About ‘47 Meters Down: Uncaged’

Surprise: The science of this one isn’t exactly solid

Getty Images/Ringer illustration
Spoiler warning

Throughout the summer of 2019, audiences have been deprived of what’s become an annual tradition at the box office: a silly shark movie. But while Crawl—featuring alligators attacking Kaya Scodelario during a Category 5 hurricane in Florida—was a worthy salve in July, we finally got a proper shark movie over the weekend with the release of 47 Meters Down: Uncaged. Shark-movie enthusiasts will already know that Uncaged is the sequel to 2017’s 47 Meters Down, a surprise hit featuring two tourists in Mexico trapped at the bottom of the ocean—at, you guessed it, precisely 47 meters—after their shark cage detaches from the boat, desperately trying to get back to the surface before they run out of air while also not getting torn to shreds by great white sharks. (Spoiler: It proves rather difficult.)

But as a sequel, Uncaged is different in everything but name; in fact, if you’re a stickler for diving minutiae, I regret to inform you that it’s never quite clear how many meters down its characters are submerged. The new setup, while still based in Mexico—sidenote: Why does this franchise hate Mexico so much?—sees four high schoolers scuba diving through underwater Mayan ruins, when debris eventually traps them inside a labyrinthine underwater cave system. Unfortunately, the caves are also the home of great white sharks that have evolved from their time living in them for what could be centuries. Hopefully, you’re already aware this premise is completely fabricated, and that anyone going on an underwater cave jaunt will not encounter mutated great white sharks … right?

Well, just to be safe, I hopped on the phone with Dr. Stephen Kajiura, a professor at Florida Atlantic University’s department of biological sciences, to talk about Uncaged’s cave sharks, what one can expect to find in underwater caves, and the ways sharks are depicted in pop culture. Dr. Kajiura and I have previously discussed the ocean as presented by Aquaman, which featured secret Atlantean societies, sharks being ridden like horses into battle, and an ancient leviathan voiced by Julie Andrews, and turned out to be somewhat unrealistic. Light spoilers for Uncaged ahead.

Stephen, the last time we spoke I was explaining Aquaman’s radical interpretation of our oceans. Have you had a chance to watch the movie?

I did see the movie. I saw it right after I talked to you, in fact. And it was nothing but entertaining.

I’m not sure if you heard about this, but in addition to making an Aquaman sequel, Warner Bros. is planning to make a horror spinoff about the Trench and all the creatures that inhabit it.

Oh, yeah? Good. Oh, man. Yeah, those were pretty funny. They reminded me of isopods—far too active to actually be deep-water creatures, but entertaining nonetheless.

I would love to see whatever else they come up with. Any of these marine programs, I like to tune in, just to see what sort of silliness the writers come up with. I always think there’s a real potential here—if they would actually consult with scientists, they could make these things even better. But I don’t think they’re particularly interested in consulting with the real scientists.

Anyway, let me briefly explain what’s going on in 47 Meters Down: Uncaged. Four high school girls go scuba diving in Mexico through these underwater caves with Mayan ruins and get trapped after some debris seals off their exit to the surface. It also unleashes a bunch of great white sharks, who have evolved from their time living exclusively in underwater cave passages—the biggest difference is they’re pale and completely blind, and, obviously, they start attacking the girls who try to escape. I know that’s a lot to take in, but what’s your initial reaction to this?

Oh, I have to commend the writers on their imagination. If nothing else, that’s unique. There was a movie years ago, a horror movie, and I don’t know what it was called, about women who went spelunking in some cave—

Oh, The Descent!

The Descent. And yeah, there were these, whatever they were, monsters in the caves. They’re rehashing it, and making an underwater version of the same thing.

That was my reaction when I saw [Uncaged]. I was like, “It’s kind of like The Descent, except unfortunately not as compelling.”

That was one I was actually scared by.

So in this movie, it’s implied these great whites had evolved over centuries surviving in the caves. Would this type of evolution be possible for great whites if, hypothetically, a group of them got trapped in underwater caves? What kind of stuff would happen to their physiology?

There’s a couple of factors that come into play straight away. Number one is that caves are a very low-energy environment. There’s very little food there. So, certainly nothing that’s going to support a large predator like a white shark. Straight away, trapped in the cave, any sharks would basically die because there wouldn’t be enough for them to eat.

Putting that aside, secondly, there is no possible way that they could have lived long enough to reproduce and had populations continue to reproduce over multiple generations in order to get these adaptations appearing like the loss of vision, et cetera. That’s something that does not happen right away. That’s something where you need some mutation, like a loss of eyes, which is advantageous and which then spreads throughout the whole population. But that’s, again, a longer-term change rather than something that would happen right away.

I think they’re probably basing this on actual blind cave fish who are related to fish on the surface who have eyes, but blind cave fish just never developed them. And those blind cave fish are a different species. They’re related to the ones on the surface, but they’re not exactly the same thing. And they’ve lost their eyes and their pigmentation over a much longer time span. I mean, they’re like the size of your finger. They can live in caves where there is allegedly small amounts of food.

Well, it’s interesting you mentioned that because they do briefly show a cave fish with completely clear skin and no eyes, although it’s closer to the size of a tuna and it scares the girls. So, it’s a bit extrapolated.

[Laughs] Right. This is quite the interesting cave they’ve found themselves in.

It seems like the sharks would need a steady supply of high schoolers getting lost in those caves to survive for that long.

Also if these fish are that big, this cave system must be extremely extensive. If you consider just one white shark, these sharks can range over vast distances. They tagged a shark off Australia that ended up off South Africa and went back again. There’s ranges of movements for these white sharks, and to say that they’re doing fine in a cave system in Mexico, it doesn’t really strike me as something that’s going to work for a big predatory fish like that.

Maybe the movie adheres to Hollow Earth theory or something like that.

Of course [laughs]. I forgot that altogether.

So are there any shark species that exclusively live in underwater caves or areas similar to that, and if so, what are they like?

The problem is that underwater caves are typically open to the outside. Whereas you might get sharks that go into cracks and crevices and little caves, it’s always with an exit. They went in, they can get back out again. And so in that sense, there are none that are living in caves. They may use them occasionally for shelter, but certainly not constrained to the cave environment.

In that case, what are some cool creatures that do live in underwater caves?

The only real underwater caves, the only extensive underwater caves are freshwater ones. In freshwater, you’ve got the little blind cave fish. There’s a few arthropods that live in caves, because you’ve got spiders and you’ve got centipedes and insects and stuff.

In the water there’s really not much in a large space, because there’s so little energy there, and so few nutrients available that it just can’t support a whole lot.

If I’m not mistaken, there are species of sharks that live very deep in the ocean and get no natural light. How do they differ from, say, a great white shark? Are they slower? Are they scavengers instead of hunters?

You can have some sharks and their relatives, like skates, that are found in great deep water, where, you’re right, there’s very little light. And what you typically see are very large eyes because they’re trying to gather any light that they can. The bigger the eye, even a few photons is going to help.

So you have a lot of these species that have big eyes, and then as you get even deeper, they still retain their eyes. But what’s interesting is that, with the skates, they actually have more electroreceptors than some other shallow-water species. And it’s been suggested that what they’re doing is they’re compensating for the fact that they can’t see anything; they’re relying on their electroreceptors to help them find their prey items. That’s a nonvisual sensory modality, which is effective in the dark. And so that might be one adaptation that you might expect is to see [with the cave sharks] is this increase in electrosensory distribution to help them compensate for the loss of vision at that depth or in those caves.

Well, it’s interesting, since Uncaged kind of plays like a haunted house movie, where the girls have to evade the cave sharks and avoid any sudden movements that give away their location. Do great whites normally use their vision to hunt, or do they have other methods to detect prey?

I think what they’re trying to get at is your hypersensitive lateral line system. What they’re proposing is that since the cave waters are probably mostly pretty still, any sort of turbulence would indicate that something swimming around would be prey and that’s what the sharks are tuning in on. In the real world, we see this in cave fish where they have well-developed systems to help them detect water movement. So, that I’ll buy.

White sharks are primarily visual hunters, and think of it this way, if you’re a big white shark—a big adult white shark—you’re feeding a lot on mammals, like a seal or a sea lion or something that’s moving on the surface. A lot of these white sharks come up from below and rocket up to the surface toward their prey and nail them that way. So, the prey really can’t see them coming because they’re coming up from below them. The sharks are reliant mostly on vision.

The 47 Meters Down cave sharks aren’t the first, and definitely won’t be the last, sharks featured in pop culture. How do you feel about the way sharks have been depicted on screen?

Have you ever seen a movie with a friendly shark?

Yeah, you never really see that, unless it’s animated.

Everyone loves dolphins because, you know, Flipper. Dolphins are your friends. They’re out there to rescue you if you fall over your boat or whatever scenario that’s been established. That’s why people love dolphins—and who’s always the bad guy? It’s always the sharks chasing Flipper, or whatever. Sharks have always had this negative rap.

But for the most part, sharks leave people alone. Sharks stay out of the way. They’re really skittish. They’re not particularly interested in getting close to you unless you’re baiting them in with dead fish on the line or something. For the most part, we have this negative perception of them that’s been propagated over the years with all these other movies. And I think what it boils down to is our innate fear of something out there that’s outside of our control that could do damage to us, right?

Humans like to think of themselves as the top, the masters of their domain and everything. But when you’ve been in the water, you’re not. You get in the water, you’re in the shark’s domain and suddenly the humans are the odd man out. It’s a little disconcerting to not be in control and to have something else that is better suited to that environment and that could do you physical damage that’s as big as you or bigger than you. And I think that’s the basis behind this fear of sharks. I think we’ve got this fear of sharks that’s already there, and it’s been cultivated by the way the sharks are depicted in the media.

In terms of pop culture sharks, on a scale of 1 to 10—let’s say Sharknado is a 10—how absurd would you say the blind great white cave sharks of 47 Meters Down: Uncaged are?

Yeah, that would also be a 10 [laughs]. What happens, I think, quite often is that people get hold of a couple of disparate facts and then stick them together to create a story, when in fact those two things do not go well together at all.

For example, yes, there are fish that live in caves. It’s a fact, and the fish that live in caves are blind. Now you can say maybe—maybe, it’s a stretch—maybe there are sharks that live in caves. Not really, but let’s say that’s fine. Then they can say, “Well then, these sharks would be blind as well.” Well, no, that’s really not the case at all. Just because one thing happens to one group of animals doesn’t mean that another group of animals is going to have that same outcome. Piecing things together like, “Well, fish live in caves and fish are blind and maybe we can make a story about blind cave sharks,” yeah, there’s no such thing. It’s entertaining. It’s a fun idea for a movie, but it’s really not gonna work.

This interview has been edited and condensed for clarity.