THE HORIZONTAL‐VERTICAL DISTRIBUTION HYPOTHESIS: LANGMUIR CIRCULATIONS AND DAPHNIA DISTRIBUTIONS1

Abstract

The individual positions of a population of Daphnia magna Straus in a plastic chamber were recorded by infrared photography. The distribution of the Daphnia in the chamber was first observed during a 12‐hr light : 12‐hr dark regime and then during the photoperiod portion of the regime when spiral currents were introduced into the chamber. Observations during the 12L : 12D regime revealed a vertical migration tendency in the chamber while the horizontal position of the population oscillated from left to right.The currents introduced into the chamber were analogous to natural Langmuir spirals: horizontal components at the top and bottom, a downwelling at the left, and an upwelling at the right of the chamber. A slow current system, of velocity range 0.8–2.4 cm/sec, and a fast current system of velocity range 2.0–8.8 cm/sec were studied. During the photoperiod (light intensity above 70 ergs cm−2 sec−1), the Daphnia shifted to the left side of the chamber in the slow current system and to the right in the fast current system. The population shifts were caused by Daphnia’s steady swimming position heading into the current and counterbalancing a current velocity of 1–2 cm/sec by its swimming efforts. When the light intensity was below 70 ergs cm−2 sec−1, at the beginning and end of the photoperiods, the Daphnia populations drifted with the currents, unable to oppose them. The dorsal light reaction functions above a threshold light intensity of 70 ergs cm− 2 sec−1, so the current orientation must be made possible by visual detection of currents. Vertical displacements decreased in both experiments.From these experimental relations a model of active aggregation of Daphnia in Langmuir spirals has been constructed. The current velocity range within Langmuir spiral circulations will determine whether the Daphnia are aggregated in upwellings, downwellings, or in‐between portions of a spiral during a photoperiod. In darkness, passive aggregation tendencies described by earlier models will prevail. The presence of the dorsal light reaction in planktonic crustacea allows the generalization of the new model to account for at least the aggregation tendencies of planktonic crustacea.