By Claudia Geib
For the lamprey, it turns out that swimming sucks.
Most fish swim by creating backward thrust, their tails pushing against the water. Last year, an MBL Whitman Center team made the surprising discovery that lamprey and jellyfish propulsion is just the opposite: Each undulation of the animal’s body creates a small, spinning area of low pressure that sucks the organism forward.
Yet the team didn’t understand how this movement works — knowledge that could help engineers design swimming robots.
“To make this discovery of suction thrust a practical finding, it’s important to understand what hydrodynamic features are associated with the negative pressure on the animal,” says Sean Colin, professor at Roger Williams University. “However, healthy animals move more or less optimally. It’s hard to understand why their movements work so well until you compare them to less optimal movements.”
“This project was born out of an informal discussion between labs, and putting our different areas of expertise together,” says Morgan. “For me, that was what was really exciting—to get people from disparate fields working on a complex problem.”
The team compared swimming in normal lampreys and in lampreys that had spinal injuries. In normal lampreys, they found that each vortex of water along its body strengthened the negative pressure regions around the lamprey body. However, in injured lampreys that exhibited passive tail movement, vortices along the transected area didn’t strengthen as the animal swam.
The team hopes to investigate how common suction thrust is among fish and in flying animals. “Engineers could benefit from understanding suction thrust to build flexible aquatic vehicles that capitalize on the efficiency of this type of propulsion,” Colin says.
Video of healthy control lamprey swimming through water seeded with hollow glass beads and illuminated with a green laser. Particle image velocimetry (PIV) software uses this video to quantify the fluid motions and to generate a field of velocity vectors that show how the water moves. Credit: Sean Colin
Brad J. Gemmell, Stephanie M. Fogerson, John H. Costello, Jennifer R. Morgan, John O. Dabiri, Sean P. Colin (2016) How the bending kinematics of swimming lampreys build negative pressure fields for suction thrust. J. Exp. Biol. doi:10.1242/jeb.144642
Photo at top: A lamprey, illuminated by laser light, swimming through the suspended glass beads used to capture its motion and the movement of the fluid. Credit: Sean Colin