What do a sperm, a nematode and a lamprey all have in common? Apart from being good swimmers, they all use a similar type of movement to get around:`eel-like', or anguilliform swimming, where swimmers propel themselves forwards by sending waves down the entire body. Scientists want to understand the link between body movements and the forces that propel the body forwards,as well as the efficiency of anguilliform swimming, explains Petros Koumoutsakos at ETH, Zürich, Switzerland. Together with colleague Stefan Kern he built a three-dimensional computer model of anguilliform swimming, and found that the computerised creatures swam in two distinctly different ways,depending on whether they are swimming to optimise efficiency of propulsion,or speed (p. 4841). First the team built their computer model of a swimming eel-like fish. The model estimated the forces generated along the body and how these forces propelled the animals forward. Koumoutsakos explains that they allowed their model fish to flex and deform, but didn't program the fishes' exact body movements into their model as they wanted it to mimic nature, wondering: `what kind of motion would evolutionary processes generate?' Instead, they programmed the model to select fish that swam with optimum efficiency, or optimum speed, and then examined the body movements that caused the fish to swim this way.
To select speedy or efficient swimmers, they put their computerised subjects through their paces by selecting `parents', letting the computer program alter the fishes' body movements to produce `children' with different swimming styles. The model then chose the children who swam the fastest, or the most efficiently, to be the parents of the next generation. The process continued through many cycles in the computer until swimming performance didn't improve any further.
The team found that swimmers `bred' to be efficient, and those bred to be speedy, developed individual swimming styles. The bodies of efficient swimmers undulated from side-to-side down the entire body length, and the tail and the middle of the body generated the thrust needed to propel the animals forward. While this swimming style was more leisurely, the propulsion was more efficient than fast swimming. When the model selected speedy swimmers, the computerised eels kept the front part of the body straight during swimming,generating most of the thrust in the tail. They swam 40% faster than efficient swimmers, but their propulsion was 60% less effective. The model's results suggest that there is a link between body movements and swimming for different outcomes: quickly to pounce on unsuspecting prey or escape from hungry predators; or efficiently to travel long distances.
Finally, the team found both types of swimmer shed vortex rings and lateral jets of water behind them as they swam along, similar to those measured by researchers studying live swimming eels. Not only does the model show that electronic anguilliform swimmers modify their swimming style according to whether they want to swim quickly or efficiently, but also that the model will help researchers studying the forces in live swimming animals, which are difficult to measure.
