DIEL VERTICAL MIGRATION AND PHOTORESPONSES OF THE CHAETOGNATH<i>SAGITTA HISPIDA</i>CONANT

Abstract

In estuarine waters at Beaufort, North Carolina, adult Sagitta hispida perform a diel vertical migration. By day, few adult chaetognaths are found in the 7 m water column. Shortly after sunset their numbers increase rapidly at all depths. We hypothesized that the evening ascent is dependent on photoresponses elicited by some aspect of the change in light intensity occurring at sunset. In the laboratory, S. hispida's upswimming increases markedly whenever light intensity drops below 1016.7 photons m–2·s–1. Animals adapted to light intensities below this level, and to darkness, show strong upswimming. This indicates that continuously decreasing light intensity is not required to maintain the upswimming response. In the field, in the afternoon and evening, downward irradiance values of 1016.7 photons m–2·s–1 are found only at sunset. These findings suggest that the daily ascent of S. hispida occurs as an all-or-none phenomenon, dependent only on exposure to light intensities below approximately 1016.7 photons m–2·s–1. This threshold intensity for ascent lies above the threshold for photoreception in this species, suggesting that the ascent begins as a positive phototaxis. However, experiments in which light direction was reversed indicated that the ascent is geotactic or photokinetic, since changes in light direction had little bearing on the orientation of the upswimming response.