Prey catching in archer fish: the role of posture and morphology in aiming behavior

Archer fishes (Toxotidae) bring down aerial prey with a squirt of water from their mouth. How they aim, however, is not clear. Measurements of the angle between the squirt and the fish's body axis (the mouth angle) already yielded agreement that pointing the body plays a part in aiming, and it has been suggested that more accurate aiming would occur by means of lips and tongue. In a previous study, however, we found that the mouth angle is an individual property, with a much smaller range than reported by others. Moreover, we found that archer fishes do not possess movable lips, and that their tongue seems too short to direct the squirt. Presently, we found that the tongue cannot even reach the rostral part of the mouth cavity, that affects the squirt's direction, and that the mouth angle is an individual property remaining constant, even across a wide range of target distances. These findings suggest that aiming the body is the only way of aiming the squirt. We also found that during aiming, the body rotates around the eyeballs which remain fixed in space. The position of the eyeballs in the head (eye angle) at the moment of squirting was also found to be an individual property. However, the eye angle is less constant than the mouth angle. The systematic variance of the eye angle probably is related to the refraction effect.