When it comes to moving in the water, few aquatic creatures can swim as gracefully and delicately as sea snails (Limacina helicina). In fact, scientists have even compared their movements to the flight of butterflies flying through the air.
What makes these snails different from other sea creatures is that, instead of paddling to propel themselves from one point to another, they "fly" through the water. This is done by flapping their tiny wing-like appendages that protrude from the opening of their shells.
While researchers have had a general idea as to how these tiny sea butterflies are able to swim, they have remained largely in the dark — until now — as to how the creatures' movement is essentially done.
What Are Sea Snails?
The Limacina helicina is a species of tiny predatory sea snails often found living in the cold waters of the Arctic. It is regarded as a vital part of zooplankton populations in the region.
The creatures were first observed by German naturalist Friderich Martens in the waters near the island of Spitsbergen in Norway in 1675.
Marine researchers initially considered the sea snails found in the Antarctic to be the same as those endemic to the Arctic, but it was later discovered that these belong to an entirely different species. The sea snails living in the North Pole are called Limacina helicina, while those in the South Pole are called Limacina antarctica.
Mimicking Insect Flight
To find out how sea snails are able to propel themselves in the water, David Murphy and his colleagues at the Johns Hopkins University in Maryland photographed the creatures' movement using four high-speed cameras.
They focused the cameras on a cube of water measuring about 2 centimeters (0.79 inches) on each side. They also added small particles in the water that would allow them to track the flow of water around the sea snails' wings by using lasers.
The research team then placed 20 individual sea snails in the cube and observed how they would move in the water.
After a while, Murphy and his team were able to photograph one of the tiny creatures swimming through the water on three different occasions.
"We were lucky," Murphy said. "They were right in the field of view."
The researchers observed that, unlike most other types of zooplankton, the sea snails flapped their wings to generate lift similar to how fruit flies do.
Murphy said that he had recognized the motion when the creatures' wing tips moved in the same figure-eight pattern as those of flies.
Sharing the Same Ancestry
Insects and gastropods are known to have a common ancestor from which they both diverged some 550 million years ago. This is why the Limacina helicina are able to make use of the same clap-and-fling motion with their wings the way flies do.
The relatively small size of the sea snails allows them to balance the inertial and viscous forces they encounter in the water so that they can propel themselves.
By examining the motion of the sea snails' wings, scientists can gain better understanding of how insects are able to fly as well. Studying the science behind insect flight has been a challenge for researchers because of how fast the creatures' wings move, which is clocked at 200 beats per second.
Sea snails, on the other hand, move their wings only at 5 beats per second.
Another interesting observation was about how the sea snails tend to pitch their bodies forward and backward repeatedly with every stroke of their wings, whereas insects tend to keep their bodies still during flight.
The researchers believe this is done to help the sea snails maintain a manageable distance between each wing stroke so that they can have hydrodynamic advantage in their movement.
The findings of the Johns Hopkins University researchers are featured in the Journal of Experimental Biology.