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Landmark use and development of navigation behaviour in the weakly electric fish Gnathonemus petersii (Mormyridae; Teleostei)

Peter Cain^* and Sapna Malwal

Department of Psychology, St. Lawrence University, Canton, NY 13617, USA

View larger version (17K): [in a new window]   Fig. 1. The experimental tank with landmark and aperture shown (not to scale). The wooden cover, lined with aluminum screen and grounded to reduce electrical interference, was removed to show the placement of the four lamps (L). The aperture, in black, was centred 25.5 cm from the floor, and the landmarks (white circles) were centred 6 cm from the floor and extended 12.5 cm on both sides of the divider wall. The electrolocation boundaries are marked on both sides of the divider wall and the depth is marked in 5 cm increments.

View larger version (15K): [in a new window]   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.

View larger version (21K): [in a new window]   Fig. 3. Height of contact with the divider wall for each group. The centre of the aperture (arrow) was maintained 25.5 cm from the aquarium floor. Data represent the mean heights ± 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, height of contact was recorded for the first 3 min; medium landmark size was maintained for large and small fish. The controls increased the height at which they contacted the divider wall, while the fish with a landmark did not.

View larger version (8K): [in a new window]   Fig. 5. (A) Amount of time for large and small fish to find the aperture with different landmark sizes. The control fish had no landmarks so they were not included. Data represent the mean time ± S.E.M. of each group for the first 3 min of trial-day 10 (medium landmark size; 6.8 cm) and the first 3 min after the landmark either increased (to 8.3 cm) or decreased (to 3.9 cm) in size. Both the large and small fish increased the amount of time to find the aperture after the landmark size increased. When the landmark size decreased, the large fish did not change the amount of time to find the aperture, the small fish took longer. (B) Height at which large and small fish made contact with the divider wall in the presence of different sized landmarks. Data represent the mean height ± S.E.M. that each group made contact with the divider wall in the first 3 min of trial-day 10 (medium landmark size) and the first 3 min after the landmark increased or decreased in size. The height of the aperture (25.5 cm above the aquarium floor) is marked. (C) Mean number of times per trial-day that fish crossed from one compartment to the other for the first 3 min of trial-day 10 (medium landmark size) and the first 3 min after the landmark increased or decreased in size. Both the large and small fish decreased the number of times that they crossed through the aperture after the landmark changed size.

View larger version (21K): [in a new window]   Fig. 4. Number of times per trial-day that fish crossed from one compartment to the other. Data represent the mean number of crossings ± S.E.M. per trial-day. Trial-days 1-10: medium landmark for large and small fish; water depth was held constant at 35 cm. H2O up: water level increased by 10 cm and the number of crossings was recorded for the first 3 min; aperture height was maintained at 25.5 cm from the aquarium floor; medium landmark size was maintained for large and small fish. The controls increased the number of crossings after trial-day 4; the large and small fish did not.

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