Podcast with Maarja Kruusmaa on biomimetic fish and lateral line sensing

How does a dead fish swim upstream, and what does that reveal about the hidden intelligence of body design? Maarja Kruusmaa explores the surprising physics of fish locomotion, lateral line sensing, and why propellers may not be the last word in underwater engineering.

Subscribe for more from the Convergent Science Network podcast series.

Kruusmaa challenges naive biomimetics, the tendency to copy everything from nature without understanding which features actually matter. She draws parallels to early propellers with feathers and cars with horse compartments, arguing that the real engineering challenge is identifying which biological principles are worth extracting. While propellers remain a mature and powerful technology, fish outperform them in energy efficiency and acoustic stealth, leaving almost no wake behind them. The key advantage lies in distributed actuation across hundreds of muscle fibers, something current motor technology cannot replicate at practical scales.

The conversation dives deep into fish swimming mechanics. Kruusmaa explains how swimming speed relates linearly to tail-beat frequency, while amplitude remains an independent variable. Fish control their propulsion primarily through stiffness modulation, which shifts resonance frequency and thereby changes amplitude. At cruising speed, fish use remarkably few anterior muscles while the rest of the body remains passive, explaining their extraordinary endurance. The discussion of a dead fish swimming upstream in George Lauder's lab at Harvard illustrates how body morphology alone can generate propulsion in periodic turbulent environments, a striking example of morphological computation.

Kruusmaa introduces the concept of inverse biomimetics, using robotic fish as tools for biological discovery rather than just engineering products. Her work on artificial lateral line sensors demonstrates this approach: by selectively disabling parts of a robot's sensory array, researchers can generate hypotheses about biological function that are difficult or impossible to test in living fish. The lateral line's dual modality, measuring both flow velocity and pressure, enables fish to build complex hydrodynamic maps of their environment, a capability roboticists have barely begun to explore.