Behavioral discrimination of water motions caused by moving objects

We tested whether goldfish, Carassius auratus, discriminate hydrodynamic stimuli caused by moving objects. Blindfolded goldfish responded to a passing object with changes in inter-gill-movement intervals. To learn whether goldfish can discriminate water motions caused by different moving objects we habituated them to a certain object stimulus. If the stimulus was altered, e.g., by altering speed, direction of motion, or size or shape of the object, fish again showed a temporary suspension of breathing when the object passed by. If animals failed to respond to an altered stimulus, we paired this stimulus with a weak electric shock during training. Goldfish discriminated object motion direction. In addition, in two choice experiments goldfish discriminated water motions caused by objects which moved with different speeds (e.g., 5 cm s(-1) versus 6 cm s(-1)), or by objects which differed in size (e.g., 1 cm x 1 cm versus 1.4 cm x 1.4 cm cross section), or shape (e.g., a round versus a triangular object). If object size and/or shape was varied quasi-randomly such that the faster moving object not always caused the greatest water velocities, fish still discriminated object speed.