PERSISTENT, VERTICAL-MIGRATION RHYTHMS IN BENTHIC MICROFLORA. VI. THE TIDAL AND DIURNAL NATURE OF THE RHYTHM IN THE DIATOM HANTZSCHIA VIRGATA

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Abstract
The diatom, Hantzschia virgata, appears on the surface sands of Barnstable Harbor, Mass., during daytime low tides. Surface accumulations of this organism reach such concentrations that the sand takes on a golden-brown color. As the tide returns the cells re-burrow into the sand. This vertical-migration rhythm will persist in the laboratory in constant Illumination, constant temperature, and away from the influence of the tide for as long as 11 days. During this time the cells remain in approximate synchrony with the feral cells in nature. In nature, when the times of low tide approach sunset, the cells rephase their rhythm to the early morning hours of daylight. Cells collected during late afternoon low tides and returned to constant conditions in the laboratory undergo a similar rephasing in an interval of just 3 days. To explain the various unique properties of this rhythm, it is postulated that the rhythm is a manifestation of an interacting dual-clock system: a lunar-day clock which measures periods of 24.8 hr. and is responsible for a bimodal migratory rhythm; and a solar-day clock responsible for the suppression of the night-time supra-surface phase of the migration rhythm.