Sauntering along the road, it's hard to appreciate the complex interplay of forces that propel us along. And the situation becomes even more complex when trying to understand how fish swim. It is impossible to directly measure the forces that they exert on their surrounding fluid, which is why scientists have turned to vast and complex computational models to calculate how adult fish interact with their environment. However, no one had tried to tackle how tiny fish larvae propel themselves through water, which is relatively sticky on their diminutive scale. Working with collaborators from the Netherlands and the USA, Hao Liu from Chiba University, Japan, built a computational model that predicts the forces that propel the fish through water based on accurate measurements of the larvae's movements provided by Ulrike Müller (p. 4015). Simulating regular undulatory swimming and a specialised escape response, known as the C-start, the team were able to accurately reproduce the complex fluid flows produced by the fish in real life in their computational simulation and show that the larvae produce thrust with their rear ends. The team was also able to exaggerate the larvae's swimming style, simulating unnaturally large and small undulations, successfully reproducing how the tiny fish's speed increased. In addition, the team found that as the larvae's speed increased, the animals had to put in proportionately more effort – the mechanical power quadrupled – although their efficiency hardly improved, increasing their cost of transport dramatically.

U. K.
van Leeuwen
J. L.
Body dynamics and hydrodynamics of swimming fish larvae: a computational study
J. Exp. Biol.