Prey catching in the archer fish: angles and probability of hitting an aerial target.

When trying to squirt down aerial arthropods, archer fishes (Toxotidae) have to cope with the displacement of the image of the target by refraction of light at the water surface. It has been suggested, and accepted, that Toxotes jaculatrix would avoid the effect of refraction by squirting vertically. In our previous study, however, Toxotes chatareus was observed to squirt under a wide range of angles, including angles much smaller than 90 degrees. The aim of the present study was to asses in T. chatareus (N=15), the frequency distribution of angles used, the relation between angle and probability to hit, the relation between target height and angle, and the smallest angles the fishes can use. The outcome was a distribution with a range of 102-58 degrees and a median of 74 degrees, no relation between angle and probability to hit, no relation between target height and angle and a smallest angle of 45 degrees. The results clearly indicate that T. chatareus does not evade the refraction effect by squirting only vertically (90 degrees ), but masters a considerable refraction effect thanks to a mechanism that is not investigated here. The capacity to hit prey under a wide range of angles is seen as an important adaptation to the natural habitat where the visibility of prey is restricted by vegetation. The constraints of the range of angles are discussed.