RHYTHMS OF VERTICAL MIGRATION AND MOTILITY IN INTERTIDAL BENTHIC DIATOMS WITH PARTICULAR REFERENCE TO PLEUROSIGMA ANGULATUM

Abstract

The surface population densities of epipelic algae inhabiting the intertidal mud of the Menai Straits were studied in the field during low water exposure of the sediment. Navicula cf. miniscula, Pleurosigma angulatum and Gyrosigma sp. migrated onto the surface of the sediment following tidal exposure and burrowed back into the mud prior to inundation by the incoming tide during the day; decrease in incident irradiance at dusk during periods of tidal exposure of the sampling sites also resulted in movement of the diatoms back into the sediment. In conditions of constant irradiance and temperature in the laboratory away from the influence of tides, the rhythm of vertical migration in and out of the sediment was maintained for eight days. This rhythm was therefore endogenous and it remained tidal: the rhythm of vertical movement remaining in phase with the tidal exposure of the field sites. As the time of low tide approached the evening, the migration of P. angulatum became rephased, involving movement of the cells onto the sediment surface in the morning. There were also persistent endogenous tidal rhythms in the speed of movement of the cells and in the net distance travelled per unit time in the laboratory. The rhythms of motility and distance moved were biphasic, rapid rates of movement corresponding to periods of tidal exposure, when diatoms would have been found on the sediment surface in the field; minimum rates were found during periods of tidal cover, when the cells would have been buried in the sediment in the natural environment. Consideration is given to the environmental factors which may be implicated in the migration rhythm, and the ecological significance of such rhythms is discussed.