Thanks to Nature on PBS for supporting this episode.

Check out Spy in the Wild 2 on PBS, the PBS Video App, and pbs.org/spyinthewild to follow animatronic spy creatures as they go undercover in the animal world. [♪ INTRO]. Birds fly.

Fish swim. We learn this when we're two. But of course, things in nature are never quite so simple.

One creature that muddies the waters, so to speak, is Limacina helicina, an arctic-dwelling species of a sea butterfly -- a type of mollusk. Rather than swim like most other aquatic life, this creature seems to fly through the water with tiny wings. And scientists studying these wing movements noticed something interesting: they looked remarkably similar to the wing movements of hummingbirds and fruit flies.

But why would you need to fly in water? Isn't flying different than swimming for a reason? Well the answer lies in a concept from the field of fluid dynamics called the Reynolds number.

The Reynolds number is used to predict how a fluid behaves, based on the speed of a movement and the physical properties of the fluid. A high Reynolds number describes fast, turbulent movements, such as humans swimming through water, while a low Reynolds number describes slow, smooth -- or laminar -- movements like that of this sea butterfly. But what's really interesting is that the Reynolds number for these sea butterflies “flying” in water closely matches the Reynolds number of fruit flies flying in air.

Because while water is denser and more viscous than air, sea butterflies are bigger and slower than fruit flies -- so from a fluid-dynamics point of view, the numbers work out about the same. This means that the water and the air behave similarly when these two creatures generate lift through their flying movements. But why fly at all?

Why not just swim like most of the other self-respecting ocean dwellers? Well, like many things in nature, this unusual behavior may be motivated by the need to feed. Sea butterflies feed by deploying a large spherical mucus web that captures plankton.

As you do. While feeding, this sea butterfly is neutrally buoyant. It will continue to drift in the ocean currents, neither floating nor sinking.

However, if disturbed, it will retract into its shell and sink. It's no longer neutrally buoyant -- so it can't just swim back up to where it needs to be the way many fish would. To make its way back to where the food is, it flaps its wings to generate lift until it can redeploy its web.

This is a great example of convergent evolution, when two different creatures with only distant relation to one another develop similar behaviors to adapt to similar environments. In this case, sea butterflies and fruit flies both evolved to use flight mechanisms that manipulate the fluid around them in similar ways -- even if the fluids themselves are completely different. Maybe we should start calling sea butterflies “sea fruit flies”!

Then again… maybe not. If you enjoyed learning about how sea butterflies move, you almost definitely will like a new mini-series from PBS that features some actual butterflies. It's called

Nature: Spy in the Wild 2, and you're looking at footage from it right now, of a huge flock of monarch butterflies filmed by -- believe it or not -- a robot hummingbird. Over four episodes, more than 50 robots like this go undercover to film animals in the wild. The robots can get close to animals in a way that humans just can't, so the series is full of cool insights into animal behavior.

Nature: Spy in the Wild 2 currently has three episodes streaming on the PBS video app and pbs.org/spyinthewild. And tune in or stream the final episode tomorrow night — Wednesday, May 20th — at 8/7 central on PBS, pbs.org/spyinthewild and the PBS Video app. [♪ OUTRO].