Rapid depth perception in hunting archerfish. I. The predictive C-starts use an independent estimate of target height.

Abstract

Archerfish dislodge aerial prey with water jets and use their predictive C-starts to secure it. Their C-starts turn the fish to the later point of impact and set the speed so that the fish arrive just in time. The starts are adjusted on the basis of information on speed, direction, timing and horizontal start position of prey movement - sampled during less than 100 ms after prey starts falling. Presently, it is unclear whether one essential parameter, the initial height of prey, can also be determined during this brief sampling time. Shooters and probably also observing bystanders already know target height - used to hit and to shape their jets - and would simply have to feed this information into their C-start circuitry. We challenged archerfish by launching initially invisible prey objects either from the expected height level, at which the fish were looking and at which they fired shots, or from more lateral positions and a lower or higher initial height. The arrangement was designed so that an analysis of the direction and the linear speed chosen by the starting fish could determine whether the C-start information is based on the expected height or on the actual height, which can be detected only after hidden prey has begun falling. Our findings demonstrate that the fish quickly estimate initial height during the initial falling phase of prey and do not simply use the expected height level to which they were cued.