Movie S1.

Young adults migrating upstream in the presence of mild external flow (300 µL/h, 321 µm/s) in a 2.6-mm-wide conduit. Animals swimming along the side surfaces occasionally exhibit synchronized motion. The flow is to the left. Play speed: 2× real time. There are more animals near the top of the channel than its bottom, probably owing to the conduit inlet’s being closer to the channel’s top than to its bottom. (Scale bar: 1 mm.)