American lobsters use "jet-assisted" walking to travel faster across the ocean floor, scientists have found.
The lobsters have small paddle-like structures on their abdomen, which they fan to create a wake which propels the crustacean forward.
These paddles can generate the same force underwater as some fish do with their pectoral fins.
The researchers who made the discovery describe the paddles as acting like "little auxiliary thrusters."
To investigate why the lobsters fan these paddles, known as pleopods, graduate student Jeanette Lim and Professor Edwin DeMont of St Francis Xavier University in Antigonish, Canada devised a mechanical model which replicates the moving parts of a lobster's abdomen.
"No one had actually measured how much force the American lobster's pleopods could produce," says Lim.
"We just took the abdomen of a lobster, emptied out the tissues, and hooked up eight mini servomotors bought from a robotic toy company in California to the pleopods."
The motors made the life-sized model fan the pleopods at the same rate as a live lobster would.
Pleopods fan to give extra thrust
The researchers then used a technique called particle image velocimetry to image and measure how fluid flowed from the pleopods, and the forces they generated.
They report their results in the Journal of Experimental Biology.
"Once we saw the flow visualisations, we were surprised with how large the wake was," says Lim, now studying for her PhD at Harvard University in Boston, US.
"The pleopods on American lobsters (Homarus americanus) are relatively broad and paddle-shaped compared to pleopods on crayfish, for example," she explains.
"But they are still fairly diminutive and rather flimsy appendages when you consider the size and toughness of the rest of the body."
"So we were surprised their beating produced a sizable wake with thrust that was on par with forces produced by the fins of some swimming fish."
Pleopods viewed from below
The model showed that the pleopods continually pull in water around the lobster's body and funnel it into a jet directed out behind.
Each lobster can produce 27 to 54mN of thrust, which is comparable to that produced by other proficient swimmers such the bluegill sunfish (Lepomis macrochirus) and surfperch (Embiotoca jacksoni), which use their pectoral fins to move at slow to moderate speeds.
Exactly how the lobster harnesses these forces is unclear.
The crustaceans beat their pleopods to swim, and the new research shows that the pleopods can provide substantial thrust while doing so.
But it also seems likely that the pleopods give the lobsters extra thrust while walking along the seabed or over obstacles. The researchers call it "jet-assisted walking".
"American lobster's pleopods are capable of producing forces that could potentially aid walking, by acting like little auxiliary thrusters," says Lim.
Robotic motors move real lobster pleopods
"We cannot say for certain whether the pleopods are 'significant' for locomotion in live lobsters as we measured forces from a model, but our results suggest they could be helpful when a little extra thrust is needed."
However, not all types of lobster would produce similar forces, as pleopod shape, which affects its ability to be an effective paddle, varies a lot among lobster species.