Fig. 1. Experimental set-up. The animals were forced to encounter obstacles of different color (black, red, blue, green, transparent, transparent with black top) and size (1 or 2 cm diameter) in a flow chamber (A,B). In the case of T. cystophora, three evenly spaced obstacles were present, and when using C. bronzie, two obstacles were present (A). The brilliance of the obstacle and the wall of the flow chamber was measured with a spectrophotometer and weighted by the relative sensitivity of a 500 nm opsin (C). The colors of the lines correspond to the colors of the obstacles; the yellow line represents the wall of the flow chamber. The weighted brilliance was turned into contrast between the obstacles and the wall of the chamber (D).