Vortex topology around American eel undulating with $λ*=0.50$ at $St=0.30$ when its tail is in the middle of its rightward stroke, where coherent structures are visualized through Q-criterion with its value of 2 for gray-shaded features and 15 for green-colored vortex cores. Multimedia view: http://dx.doi.org/10.1063/5.0040473.1