Snail Sculpts Bubbles to Make Life Raft

a snail with a purple shell has bubbles come from its shell on a beach

Biological Strategy

Common violet snail

Mary Hoff

Image: Douglas Cliff / Shutterstock / Some rights reserved

Modify Buoyancy

Buoyancy is an upward force exerted by air or liquid on a solid object that works against the object’s weight. A hawk glides through air and a duck floats on water due to buoyancy. Some living systems (such as fish eggs) can remain sedentary and therefore maintain the same buoyancy at all times. But most must adjust their buoyancy because they can’t survive long unless they change position. These living systems require strategies to not only be buoyant, but also adjust buoyancy level. They often modify buoyancy by adding or decreasing lift, or by changing their weight. For example, birds like condors that soar for a long time shift the directional orientation of their wingtip feathers to manage buoyancy. Fish alter the amount of air in their swim bladders to increase or decrease their weight, thus altering their buoyancy.

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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Gastropods

Class Gastropoda (“stomach-foot”): Snails and slugs

There are at least 40,000 known species of gastropod, with many more yet to be discovered, making it the largest class of mollusks. These animals exhibit torsion, meaning their internal organs are twisted 180° during development. The majority of gastropods have shells, and most open up on the right side. The largest gastropods are the size of a small child (3 feet, or 1 meter) while the smallest are barely visible to the naked eye.

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The foot of a Janthinid snail creates a bubble raft that allows the snail to float on the surface of the ocean.

Introduction

Some snails live on land. Others live under water. Members of the Janthinidae family of ocean-dwelling snails live at the interface of water and air, floating upside down at the surface of tropical oceans with the help of a raft of bubbles they build with air and mucus.

Image: rez242 / Public Domain - No restrictions

Janthinid bubbles are made of a polymer that helps them occupy the interface between water and air.

The Strategy

To make its flotational device, a janthinid snail needs four things: air, water, mucus that is compatible with both, and its muscular foot. It starts by reaching above the surface of the water with the front of its foot. Then it curls in the edges of the foot to create a pocket of air. Glands in the foot spread mucus around the air pocket, creating a bubble. Finally, the snail draws its foot below the surface, leaving the bubble attached to the back part of the foot. Each bubble takes about 10 seconds to make, and the snail can create up to 10 or so without stopping. It might take an hour to produce a multi-chambered raft buoyant enough to allow the snail to maintain its position at the intersection of air and water.

The mucus that forms the exterior of the bubble is a type of exopolymer—a complex molecule extruded by an organism to perform a function outside of its body. The exopolymer janthinids use to form their bubbles creates a sac that is firm and elastic but not sticky. Because it is amphiphilic—some parts of the molecule are water soluble and others repel water—it reduces surface tension, making it easy for the snail to move from one place to another with minimal expenditure of energy. That’s the same principle that enables soap to dissolve in water yet remove grease from your dirty hands (and it’s what causes the bar to scoot across the shower floor if you accidentally drop it).

The bubble raft benefits the snail by allowing it to occupy a part of the ocean that few other creatures do, minimizing competition for resources. By providing mobility, it also enhances access to food—mostly other invertebrate animals such as jellyfish that float or swim at the surface of the sea.

The Potential

The janthinid bubble raft provides inspiration for a wide range of human innovations. Its sturdy but lightweight nature could provide insights useful for designing easy-to-store personal flotation devices, inexpensive packing materials, or foam for cushioning and insulation. Even the concept of living suspended at the interface between water and air can offer inspiration. As our planet becomes more crowded, it might be helpful to think beyond solid ground and imagine other places and spaces we humans might occupy, whether on land, in the air, on the water—or all of the above.

Each bubble takes about 10 seconds to make, and the snail can create up to 10 or so without stopping.