The sea butterfly (Limacina helicina), a zooplankton snail that lives in cold oceans, really lives up to its name. Instead of paddling like most small marine animals, they fly like insects, flapping their wings to produce lift and move through the water.
“Snails evolutionarily diverged from flying insects 550 million years ago,” says Donald Webster, professor in Georgia Tech’s School of Civil and Environmental Engineering. “Hence, it is amazing that marine snails are using the same figure-eight wing pattern that is typical of their very distant airborne relatives.”
“Sea butterflies are honorary insects.”
Another amazing similarity between the 3-millimeter marine mollusks (pteropods) and insects is the use of a clap-and-fling wing motion. Each species claps its wings together, then rapidly flings them apart to generate enhanced lift.
“Almost all other plankton use their appendages as paddles, kind of like a turtle,” says David Murphy, who led the study that is published in the Journal of Experimental Biology, and received his Georgia Tech doctoral degree in 2012 and is now a postdoctoral fellow at Johns Hopkins University. “Sea butterflies are honorary insects.”
The researchers did find one major difference in sea butterflies and flying insects. Nearly two-thirds of the plankton’s body is its shell. When it’s not moving forward, it sinks to the ocean floor. To avoid sinking, the pteropod rotates its body up to 60 degrees with each stroke. The rotation puts its wings in the proper position to flap downward during every half-stroke (about 10 times per second) and move in an upward, zig-zag path in the water.
“Insects and birds don’t typically rotate their bodies in a similar manner to generate lift,” Webster says. “By rotating their shell during each stroke, sea butterflies put their wings in a position to always generate upward thrust and fly forward.”
The researchers study the plankton for two reasons. First, they play a vital role in the food web in the Pacific, Arctic, and Southern Oceans. Fish, seals, and sea birds eat them in massive quantities. Second, absorption of carbon dioxide increases the acidity of the oceans. As carbon dioxide levels increase in the future, so will seawater acidity, which breaks down the shells of pteropods.
The researchers wanted to better understand how they moved. The next steps in the research will be to see how changes in both shell composition and fluid viscosity affect its ability to rotate its body and “fly” upward.
Deepak Adhikari, a civil engineering postdoctoral fellow and Jeannette Yen, professor of biology, are study coauthors. They both traveled to Antarctica in 2014, in part, to study pteropods, which grow as large as six millimeters in the frigid Southern Ocean.
Not only do the findings help to better understanding the biomechanics of the animal’s movement, but scientists say they could someday help engineers build miniature autonomous robots that swim in the ocean.
Source: Georgia Tech