Fig. 2. Amount of time Gnathonemus petersii needed to find the aperture. Data represent mean times ± S.E.M. per trial-day. At some points, the error bars are smaller than the symbols. Trial-days 1-10: water depth was held constant at 35 cm. H₂O up: water level increased by 10 cm and time was recorded for the first 3 min; aperture height was maintained at 25.5 cm from aquarium floor; medium landmark size was maintained for large and small fish. All fish significantly decreased the time to find the aperture over the first 10 trial-days. After the water level was raised, large fish with a landmark significantly increased the time to find the aperture over the time taken on trial-day 10. Values for the first 3 min of each group in trial-day 10 were nearly identical to the 10 min values and were not plotted because they would obscure the other data.