Light has long been recognized as the main exogenous factor controlling Chaoborus diel vertical migration, but the characteristics of light which affect larval behavior and the behavioral responses of larvae to light are not clearly understood. We measured larval photobehavior
in the laboratory in conjunction with field measurements of light and vertical migration to investigate the role of light in Chaoborus vertical migration. In the laboratory fourth-instar Chaoborus punctipennis larvae exhibited a light-induced positive geotaxis at high light intensity
(about about 1.3·10−5·Wm−2) and a light-induced negative geotaxis at low light intensity (below about 3·10−6·Wm−2) when stimulated with light filtered to match their spectral sensitivity. Phototaxis
was not observed under conditions which simulated the natural underwater angular light distribution. In the field, larvae ascended near the time of sunset and descended near sunrise. At dusk the population peak occurred at each sampling depth when the average light intensity there was 3·10−7·Wm−2.
At dawn the population peak migrated down just ahead of the threshold intensity for positive geotaxis (1.3·10−5·Wm−2). This relationship between larval migration and underwater light was essentially constant over the 7-day sampling period in
October 1984. There was no evidence that the larvae remained at a constant light intensity during their migration and no relationship between the rate of relative change in light intensity and either timing of the migration or migration velocity. The migration pattern of fourth-instar Chaoborus
larvae was consistent with the larval photobehavior we measured in the laboratory and can be explained as a simple response to absolute light intensity. Upward and downward movement of the population occurs at ambient light intensities just bclow the respective “threshold” intensities
for negative and positive geotaxis. Light is not a controlling cue for the migration. The initiating cue is absolute light intensity and the orienting cue is gravity.
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