A team of researchers say that members of a species of gelatinous snail known as sea butterflies flap their wings in order to swim through Arctic waters.
The study, led by postdoctoral researcher David Murphy of Johns Hopkins, looked at the fluid mechanics employed by marine snail Limacina helicina. The team found that the sea butterflies move in the same way as insects.
Snails flap “wings” to swim around Arctic Ocean
Results of the study were published this Wednesday in the Journal of Experimental Biology. L.helicina is one of a number of mollusks that live exclusively in Arctic waters, where they feed on phytoplankton and small zooplankton.
While most snails crawl around on one fleshy foot, the sea butterflies use wing-like protrusions called parapodia in order to get around. The sea butterflies are entirely gelatinous apart from their calcium carbonate shell, which made it hard for the scientists to ship the creatures from the Arctic to their research center in Atlanta. The snails can also be hard to find at the best of times.
“They tend to be abundant for just a short time each year,” said Murphy, “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.”
Scientists study movements using cameras
The peculiar movements of the snails were captured using four high-speed cameras.
“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.”
Murphy was surprised by the results as he had not expected to find such similarities to insect morphology.
“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.”
Evolution explains similarities between two very distinct animals
When you look at their genetics, sea butterflies and insects are not closely related. However they have developed a very similar biological mechanism thanks to convergent evolution.
“Convergent evolution occurs when different, unrelated organisms display the same features – that is, solve a particular problem in the same way,” said co-author Jeannette Yen. “The structures that have the same function in the different organisms would be analogous – they don’t have the same developmental origin.”
Further work on the snails will be carried out by Dr. Yen and co-author Donald Webster, both researchers at Georgia Tech. The pair have traveled to Antarctica to study a similar species called Limacina antarctica.
Rajat Mittlal, a Johns Hopkins researcher, will help the scientists develop computer simulations in order to improve our understanding of the mechanics involved in sea butterfly swimming.