Abstract
High- and low-shore populations ofBathyporeia pilosaLindstrÖm, kept in the laboratory under natural day-night illumination and constant dark conditions, show distinct swimming rhythms of tidal frequency (12–4 h) with activity maxima on the early ebb tide. A diurnal component is evident, activity during thenight being greater than during the day. The tidal rhythm deteriorates under laboratory conditions, but swimming activity recurs after a semi-lunar period. Under natural illumination, both populations show a night tide swimming rhythm of semi-lunar frequency, activity occurring mainly over the 'falling' spring tides. Plankton sampling on night tides over the area of the beach occupied by high-shoreB. pilosashows that the animals emerge and swim on ebb tides which are in phase with darkness. The importance of exogenous factors such as wave action, pressure, temperature and light is discussed. Ecological implications and the possible function of the swimming rhythm are considered.
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