Synchronized locomotion of undulatory swimmers occurs ubiquitously in nature, ranging from microorganisms such as sperms to larger aquatic animals such as eels and knifefish. To develop a unified understanding of this phenomenon, we use Taylor's two-dimensional infinitely long undulatory sheet model to study the side-by-side locomotion of two swimmers at nonzero Reynolds numbers. Our quasi-steady-state analysis of the far-field hydrodynamics demonstrates that fluid inertia causes the swimmers to reach a stable antiphase synchronized state. Numerical simulations confirm this result for two swimmers separated by a relatively large gap distance. However, when the distance falls below a critical value, the two swimmers settle into a stable in-phase state. The antiphase configuration increases the coswimming speed, the power expenditure, as well as the energy efficiency of both swimmers.

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• Low Reynolds number flows
• Swimming