Fig. 2. Swim trajectories. During the 3 min in the flow chamber the medusae behaved differently depending on the visual appearance of the obstacle. When high contrast obstacles were present (A,B), the medusae swam against the current and in general kept at a distance from them. When transparent obstacles with or without a black top were present, the medusae did not respond to them and either bumped into them and passed them (T. cystophora, C) or kept swimming into them (C. bronzie, D). It should be noted that the tracks represent the movements of the top of the bell and therefore contacts with the obstacles by other parts of the bell can have taken place without the track showing it.