The Snail That Breathes Through Its Foot

Breathing Without Gills

When we think of sea creatures, we often imagine gills—thin, feathery organs that extract oxygen from water. Fish have them. So do many crustaceans and mollusks.

But in the strange world of marine gastropods, not all animals follow the rules.

Meet Melibe viridis, a sea slug so unusual that it breathes through its foot.

Yes, its foot.

Not lungs. Not gills. Just a wide, flat, translucent foot pressed against the sea floor—acting as a giant respiratory surface.

This is one of the weirdest ways any creature has evolved to survive underwater.

What Is Melibe viridis?

Melibe viridis is a type of nudibranch, a soft-bodied sea slug known for bizarre shapes and striking colors. It's often called the hooded nudibranch because of its enormous, mouth-like head flap, which it uses to trap tiny prey.

It glides slowly along the ocean floor using its muscular foot, which extends across the entire underside of its body.

But this foot is more than just a way to move.

It’s a multifunctional organ—helping the animal not just crawl, but also breathe.

The Science Behind Foot-Breathing

Here’s how it works:

Unlike gilled animals, Melibe viridis has a thin, highly vascularized foot. This means it’s packed with tiny blood vessels just below the skin.

As the slug moves across the ocean floor, oxygen from the surrounding water diffuses directly into its bloodstream through this skin.

It’s like breathing through your skin—but over a surface area much wider than your body.

Because the foot stays in constant contact with water, it acts like a giant open lung. No filters. No pumps. Just pure surface exchange.

No Shell, No Problem

You might wonder: why doesn't this slug have gills or a shell like a "normal" snail?

Well, Melibe viridis is a nudibranch, and nudibranchs lost their shells millions of years ago through evolution. That loss forced them to adapt in other creative ways—especially in how they breathe and defend themselves.

In the case of Melibe viridis, the answer was to maximize surface area on the body and use diffusion for respiration.

This solution works best in shallow, oxygen-rich waters—where movement and contact with flowing currents help deliver oxygen to the skin.

Movement Matters

Breathing through a foot isn’t passive. It requires motion.

That’s why Melibe viridis is almost always moving—gliding, bending, or flexing its foot—ensuring that new, oxygen-rich water passes beneath its body.

If the water becomes stagnant or low in oxygen, the slug’s breathing slows down, and its survival becomes harder. It relies entirely on its environment to breathe.

That makes it incredibly sensitive to pollution, rising sea temperatures, and oxygen-depleted zones—conditions that are becoming more common in modern oceans.

A Hunter with a Net

This slug might breathe through its foot, but it feeds with its head—which looks like a transparent hood or open bag.

Melibe viridis swings this hood side to side like a fishing net, trapping copepods and plankton inside. Once caught, the prey is pulled into its body and digested.

It’s a strange combination:

An animal that walks with its lung and eats with its head-net.

Everything about it defies expectation.

Transparency and Camouflage

One reason Melibe viridis is hard to spot is its translucent body.

Its tissues are see-through, making it nearly invisible against sandy or rocky backgrounds. You might swim past one without ever realizing it’s there—just a whisper of movement across the seafloor.

This camouflage also helps with its foot-breathing strategy. A thin, exposed body makes for easier gas exchange—perfect for an animal with no shell or gills.

Not Just One Species

Interestingly, scientists have found several species of Melibe, all with similar features—flattened feet, translucent skin, and no visible gills.

Some live in tropical waters, others in colder seas. Some are large, others barely visible. But they all share the same foot-based breathing strategy.

This makes Melibe one of the few examples of convergent respiratory evolution in marine life—where different species independently arrive at the same unique solution.

What This Teaches Us

The story of Melibe viridis is a reminder that nature doesn’t always follow one path.

Where other animals evolved gills, this slug evolved a giant breathing foot.

Where others rely on speed or shells, it relies on stealth and stillness.

It doesn’t roar, chase, or sting.

It simply glides—silently breathing through the skin beneath it.

A quiet life, shaped by evolution, built on a surface most would ignore.