Gelatinous sea snails ‘fly’ through water like a butterfly, say scientists.

Are these marine mollusks really honorary insects?

Courtesy of David Murphy
This is a time lapse picture of a sea butterfly at different stages of a wing beat.

New research suggests that sea butterflies swim through Arctic waters much like flying insects traverse the air – by flapping their wings.

A new study led by David Murphy, a postdoctoral researcher at Johns Hopkins University, examined the fluid mechanics used by a small marine snail called Limacina helicina. Dr. Murphy and colleagues found that these creatures, better known as sea butterflies, appear to mimic insect flight as they swim. The study was published Wednesday in the Journal of Experimental Biology.

L. helicina is just one species in the group known collectively as sea butterflies. These mollusks live exclusively in Arctic waters, feeding on phytoplankton and small zooplankton. Unlike most snails, which use a single fleshy “foot” to crawl, these sea butterflies are propelled by wing-like protrusions known as parapodia.

Aside from a calcium carbonate shell, L. helicina have entirely gelatinous bodies. This posed a challenge for Murphy and colleagues, who needed to ship these extremely fragile creatures from polar waters to their facilities in Atlanta. What’s more, sea butterflies can be difficult to find in the first place.

“They tend to be abundant for just a short time each year,” Murphy tells the Monitor, “so the experiment has to be ready and waiting for them when, and if, they arrive. We were amazingly lucky to get these in such good shape from collaborators on the West Coast. We were also really lucky to get such good data out of them. It's really hard to get the animals to swim right in front of the camera, but these behaved beautifully and gave us perfect data.”

Researchers used four high-speed cameras to capture the unusual movement of these creatures. They had only a few hours to film – any longer, and swimming conditions would no longer be ideal.

“We looked at the wing kinematics – how it moves its wings in a figure eight pattern – and it's very similar to how a fruit fly beats its wings,” Murphy says. “Then we measured the flow around the animal as it swims, and the sea butterfly uses one of the same tricks to generate extra lift that lots of tiny insects use. In this trick, called the 'clap and fling,' the animal claps its wings behind it and then flings them apart, sucking flow in between the wings. This creates tiny flow tornadoes, or vortices, at each wing tip, which helps to lift the animal.”

These results came as a surprise to Murphy, who didn’t expect to find such insect-like morphology in a marine mollusk.

“I've studied lots of other types of zooplankton,” Murphy says, “and almost all of them use their appendages as paddles to push themselves through the water. I thought we would find that the sea butterfly uses its 'wings' to do the same thing, but the more we looked into it, the more we found that the sea butterfly is an honorary insect.”

Genetically speaking, sea butterflies and insects are not closely related. But through convergent evolution, they came to develop a remarkably similar biological mechanism from disparate origins.

“Convergent evolution occurs when different, unrelated organisms display the same features – that is, solve a particular problem in the same way,” co-author Jeannette Yen tells the Monitor. “The structures that have the same function in the different organisms would be analogous – they don’t have the same developmental origin.”

Dr. Yen and co-author Donald Webster, both researchers at Georgia Tech, will continue working with sea butterflies – they recently traveled to Antarctica to study a related species known as Limacina antarctica. They will develop computer simulations, with the aid of Johns Hopkins researcher Rajat Mittal, in an effort to further understand the mechanics behind sea butterfly swimming.

You've read  of  free articles. Subscribe to continue.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to

QR Code to Gelatinous sea snails ‘fly’ through water like a butterfly, say scientists.
Read this article in
QR Code to Subscription page
Start your subscription today