Fig. 1

Experimental setup and overview of experiments. a, b Blueprints of the Y-maze seen from different perspectives. Inflow nozzle from peristaltic pump not shown. c Calibrating the water flow using methylene red and standard pen ink. Note that the nets were removed prior to experiments. d Night vision shot of stimulus treatment experiment. Only three of four setups are shown. e Overview of control and odor treatment experiments. Note that the combined relative choice frequency and standard deviation from all four parallels are shown. There are no significant differences between the two branches of the Y-maze in the negative-control experiments for both, Saduria entomon and Procambarus fallax. P. fallax showed a highly significant preference for the odor. Only four experiments with S. entomon yielded a significant response. The combined stimulus (IV), herring (VI_a+b) and the amino acid taurine (XIII) had a significantly repellent effect, and only in one experiment (XIV, threonine) the stimulus was attractive (see text for details). Random chance (50 %, based on the area of branches vs. total area of the maze) is indicated by the gray solid line. Asterisks denote significance levels (*p < 0.05; **p < 0.01; ***p < 0.001)