http://jeb.biologists.org/content/219/4/465.1Snails usually lumber along on their single fleshy foot, but not sea
butterflies (Limacina helicina). These tiny marine molluscs gently flit
around their Arctic water homes propelled by fleshy wings that protrude
out of the shell opening. But little was known about how they move
through water. ‘Most zooplankton swim with a drag-based paddling
technique,’ explains David Murphy from the Georgia Institute of
Technology, USA, and even though one of Murphy's thesis advisors –
Jeannette Yen – had filmed one of the enigmatic snails swimming while it
was attached to a wire in 2003, it had not been possible to observe how
fluid flowed around the animals to explain how they move. So, when
Murphy built a new 3D system to visualise fluid movements around minute
animals, Yen and Don Webster were keen to test more sea butterflies as
they swam freely to discover more about their exotic mode of propulsion.
However, working with the delicate animals in land-locked Atlanta posed
a unique set of challenges. Murphy explains that sea butterflies are
scarce at the best of times, and that transporting the fragile
gelatinous creatures across the continent from their ocean home was
tricky. ‘You have to ship them overnight in an insulated cooler to keep
them cold and if the water is too dirty, particles will stick to them,
so the water has to be very clean’, he says. Yen then devised a cunning
V-shaped structure at the bottom of the tank to ensure that the freely
swimming snails repeatedly ascended through the middle portion of the
tank – where four high-speed cameras were focused to capture every
detail of the wings’ movements. Even then, Murphy admits that it was a
miracle that they were able to collect any data, and he recalls that the
team only had a few short hours to film the molluscs, explaining that
they could only be sure that the swimming conditions were ideal then.
However, by the end of the afternoon the snails had serendipitously
crossed the path of the cameras on four occasions. ‘In this sort of
free-swimming experiment, it's normal to take 30 passes to get three
usable ones, but we got really lucky! The animals even cooperated by
swimming in different orientations, so we could see different
perspectives’, chuckles Murphy, who then began visualising the snails'
wing beats and the fluid movements around their bodies with Deepak
Adhikari.
After months of painstaking analysis – interrupted by Murphy's
relocation from Georgia to Johns Hopkins University in Baltimore,
Maryland – the team was astonished when they realised that the snails
were swimming just like fruit flies fly. ‘I said to myself, “Its wing
stroke is just like what an insect is doing”’, recalls Murphy,
describing the snails’ characteristic figure-of-eight wing beat that was
only apparent after he took account of the molluscs’ extraordinary
bobbing motion. And when he investigated how water flowed around the
snails’ wings, he was impressed to see that the molluscs generated the
same low-pressure system that produces lift in flying fruit flies.
Murphy explains that the snails (and fruit flies) clap their wings
together at the top of a wing beat before peeling them apart, sucking
fluid into the V-shaped gap between the wings to create low-pressure
vortices at the wing tips that generate lift. He says, ‘No one has
actually been able to measure the flow around an insect doing this while
it is flying, and so that was kind of the holy grail of this area of
research’. And he adds, ‘It really surprised me that sea butterflies
turned out to be honorary insects’.
