Refraction and the spitting behavior of the archerfish (Toxotes chatareus)

Abstract

1. Archerfish (Toxotes chatareus) spit droplets of water at aerial insect prey, knocking them onto the water surface to be eaten. Since the fish's eyes remain completely below the water surface during sighting and spitting the fish must deal with potentially severe refraction effects at the air-water interface. High speed (200 f.p.s.) motion picture films of 480 spitting sequences were analyzed to determine the magnitude of the refraction effect and to suggest how the fish compensate for it. 2. T. chatareus do not shoot from a position directly below the prey, but can correctly set their spitting angle to compensate for the refraction unique to a variety of positions (Fig. 6). The fish can correct (Fig. 14) for large refraction effects on the prey's apparent elevation (Fig. 12) or apparent height (Fig. 13). They may be enabled to do so by a rather precise linear relationship between the real elevation of the prey from the nose and the apparent elevation from the eye which exists during sighting (Fig. 15) and spitting. However, spitting accuracy decreases with increasing prey height (Fig. 7) or range (Fig. 8). 3. The archerfish must also correct for significant curvature of the water droplet's trajectory (Fig. 10). Since shot velocity is relatively constant (Fig. 9) the fish must make this correction via their spitting angle, but the stage in the spitting process at which this occurs is unknown.