Within the frigid waters of the Arctic resides a tiny aquatic snail whose appearance is nothing short of surreal. Belonging to a group commonly known as sea butterflies, a study in the Journal of Experimental Biology reveals it is able to move through the sea by flapping a pair of wings in a remarkable display of “underwater flight.”
Only three millimeters (0.12 inches) in size, this minuscule mollusk’s precise swimming tactic had not been documented before. This novel method of locomotion shocked the researchers, who point out that most other, similarly sized aquatic animals use their appendages to paddle through the water, but they aren’t able to use them as wings.
In fact, this snail’s wing-flapping resembled that of a fruit fly, using a similar figure-of-eight movement. “It really surprised me that sea butterflies turned out to be honorary insects,” said Dr. David Murphy, a mechanical engineer at Johns Hopkins University and lead author of the study, in a statement.
The majestic sea butterfly moving through the water column. Murphy et al./The Company of Biologists via Youtube
Using high-speed video, the researchers filmed 20 of the Arctic species – known scientifically as Limacina helicina – “flying” within laboratory water tanks. Small particles that can be illuminated by lasers were mixed into the water, which allowed the researchers to clearly see the currents created by the snails as they flapped their wings.
Their analysis revealed that the wing tips used the same characteristic figure-of-eight pattern – which pushes the water beneath them out of the way, creating buoyancy and allowing them to float – that small insects use to fly. Additionally, this flapping action sucks fluid into the V-shaped gap that emerges as the wings open up; this produces small vortices at the tips of each wings, which generate additional lift.
It also pitches its body backwards by about 60 degrees with each wing flap, allowing it to carefully position its wings in the water so it doesn’t need to move them particularly far, reducing energy expenditure.
The last common ancestor between insects and snails lived over half a billion years ago, which means that this flapping mechanism evolved twice in two very distantly related evolutionary lineages. This phenomenon is known as “convergent evolution,” and this isn’t the first time a mechanism of flight has independently appeared in different types of animal.
Before becoming extinct, pterosaurs were able to fly over the heads of dinosaurs tens of millions of years ago. Today, birds and bats fly and glide in much the same way, despite both belonging to two separate evolutionary branches.
This curious sea snail is a predatory critter, using large webs of mucus to trap and feed on plankton suspended in the water. Being able to fly through the water in this way allows it to efficiently move towards sources of food, quickly escape predators and find potential mates.
Their bodies are gelatinous and fragile, and although they’re protected by their shells, this may not be sufficient in the future: Man-made climate change is causing the oceans to become increasingly acidic, which threatens to eat away at their shells.