Wriggling is a dependable swimming mode for a select band of aquatic species. From fish larvae and tadpoles to the gyrating motions of eels and lampreys, these sinuous swimmers appear to scythe through water effortlessly. ‘We are interested in why [swimming] animals bend, as opposed to being rigid like engineered vehicles,’ says Sean Colin, from Roger Williams University, USA. Yet, little was understood about how the writhing motions propel these animals forward until Colin and colleagues Jack Costello, Brad Gemmell and John Dabiri discovered that wiggling lampreys generate spinning regions of low-pressure water adjacent to their bodies that literally suck them forward. ‘However, we still didn't know what hydrodynamic features led to these negative pressure zones and how they were generated’, says Colin. It was only when chatting with Jennifer Morgan during a summer spent at the Marine Biological Laboratory (MBL), Woods Hole, USA, that he realised that sea lampreys may hold the key to answering these questions: the distasteful creatures are able to repair damage to their spinal cords. This could provide a way of temporarily disabling the rear end of lampreys to impair – but not prevent – swimming, so Colin decided to compare how healthy and temporarily semi-disabled lampreys move to find out how their flexible style generates suction thrust.
Working with Stephanie Fogerson, Morgan operated on several lampreys to sever the spinal cord halfway along the body to temporarily disable the rear portion of the body. However, Colin admits that working with the able-bodied and impaired lampreys could be enormously frustrating. Despite Gemmell's lamprey-wrangling talents, the fish rarely swam in a straight line through the sheet of laser light that was necessary to reveal the spinning vortex wakes produced by the fish. ‘We recorded any sequences where the lampreys were swimming through the laser correctly’, recalls Colin. Then, having discarded clips where the animals were accelerating instead of swimming steadily, Colin compared the swimming motion of the intact and partially disabled fish.
Tracing the body positions of both sets of lampreys, Colin could see that the writhing swimming movement travelled like a wave along the full length of the intact fish's body and became stronger, increasing in amplitude, as it moved toward the tail. However, the travelling wave failed to propagate beyond the point where the spinal cord had been severed in the partially disabled fish, so the tail simply waved passively from side to side.
Colin then calculated how fluid flowed around the fish's bodies and realised that the bending movement of the intact fish produces small spinning suction vortices that originate near the head and are then accelerated by the fish's weaving motion as they roll along the body toward the tail. In addition, the rippling movement ensured that the vortices were evenly spaced to generate maximum thrust. However, when he investigated the motion of the fluid flowing around the bodies of the partially disabled fish, the strength of the vortices failed to increase after passing the position where the spinal cord had been severed; ‘the hydrodynamic features kind of fizzled out’, says Colin.
Having identified the source of the novel propulsion mechanism, the team is keen to discover how commonly other species us suction thrust, which Colin says ‘[could] be useful for engineers in the development of novel and perhaps more efficient vehicles that can use flexible propulsors and rely on suction thrust’.
