Although a great deal is known about the synchronised movements fish use to propel themselves through water, and new techniques have recently allowed us to visualise the complex fluid flows that drive them, no studies had linked the two until Eric Tytell decided to put American eels through their paces. By filming eels as they swam over a range of steady speeds at Harvard University,Tytell was able to capture their undulating movements while also viewing the propulsive jets they generated with a sheet of laser light. Correlating the two, Tytell found that although the wake's flow increased as the fish swam at higher speeds, the structure of the wake changed little, even at the highest speeds tested (p. 3265). This is in stark contrast to the wake structures of other fish, which reorient significantly as the fish speed up.

Having visualised the fish's hydrodynamics, Tytell was able to estimate both the power shed into the wake and the change of momentum, as the fish moved through the water, and to compare his measurements with values derived from earlier theories. Surprisingly, the calculated theoretical change in lateral momentum was half the value measured by the hydrodynamic visualisation, while the power predicted by theory agreed well with Tytell's measurements. And when Tytell calculated the wake's cost, he realised that as eels speed up they `squander less power producing their wakes', although he adds that `they still waste more power at higher speeds, even if it's a smaller fraction than the amount wasted at lower speeds'.

Tytell, E. D. (
). The hydrodynamics of eel swimming. II. Effect of swimming speed.
J. Exp. Biol.