African clawed toads (Xenopus laevis) sense the distance of lateral line stimuli
Sighted African clawed toads use their lateral lines to detect stimulus distance, although accuracy and precision are poorer than for stimulus direction. Single surface wave trains elicited discrete turns and/or swims towards the wave origin. Most responses were brief, ending with the toad stationary (70 % overall; 54–86 % individual toads) or pausing before turning away (11 %; 1–24 %). Lunges or capturing movements with the arms (13 %; 10–22 %) also indicated where toads expected to find prey. Overall, 94 % (88–100 %) of oriented responses had well-defined endpoints. Swim distance—measured as means, medians, and upper and lower quartiles—and the number of bilateral leg kicks increased with stimulus distance. Swim distance also depended upon stimulus angle due to features of turning. Most responses (81 %; 62–92 %) ended short of the wave origin. Regression slopes were 0.45 ± 0.04 mm/mm for stimulus distances up to 85 mm (ca. 2–3x body lengths), 0.16 ± 0.07 mm/mm for distances of 85–130 mm, and non-significant for larger distances to 220 mm. Slopes were steeper for responses that included lunges or capture movements. In only 15 % (3–26 %) of responses were both turn direction and swim distance sufficiently accurate for the toad to sweep through the wave origin.