The Activity Rhythm of the Amphipod Corophium volutator (Pallas) and its Possible Relationship to Changes in Hydrostatic Pressure Associated with the Tides

Abstract

A rhythm of tidal (1274 h) frequency with activity maxima during the early ebb was observed in the swimming activity of Corophium volutator, collected intertidally and kept under constant conditions in the laboratory. This rhythm persists for about 3 days. It is independent of temperature changes within the range of those encountered on the shore during different stages of the tide. Corophium collected from generally non-tidal brackish pools at Oxwich were arhythmic, indicating that diurnal variations in light intensity are not responsible for the entrainment of the rhythm. Corophium collected from M.H.W.S. and M.L.W.S. tide levels showed synchronous activity peaks, which indicate that the rhythm is not phased to the times at which the animals are covered or exposed by the tide. Rhythmic swimming activity, phased to the tides in the usual way, was induced in arhythmic laboratory Corophium by returning them to the shore or shallow sub-littoral in containers covered with boulting cloth. Others kept in containers floating from a buoy did not develop this typical response. This indicates that tidal changes in hydrostatic pressure, rather than variations in intensity of wave action of water currents, may be responsible for entrainment of the rhythm. Corophium collected intertidally generally responded in the laboratory to cyclical pressure changes of 20 min frequency, by swimming more actively following a pressure decrease. This response cannot be depth-regulatory, being opposite in sense. On the other hand, Corophium collected from generally tideless pools at Oxwich showed a depth-regulatory response to pressure changes. Corophium collected intertidally and subjected to cyclical pressure changes of tidal amplitude and frequency, but opposite in phase to the naturally occurring tides, rapidly came into phase with this artificial cycle. It therefore seems that the rhythm of activity is probably entrained by the decrease in pressure as the tide ebbs. Field observations indicate that, although Corophium may be induced to leave the mud by the incoming tide, they do not swim for long before seeking the bottom again, and that still greater numbers swim during the ebb tide.