The activity of G. zaddachi from a non-tidal environment (the brackish lake “De Putten”, prov. North Holland, The Netherlands), has been studied in a current chamber in which tidal cycles can be simulated, in order to make a comparison with estuarine populations. The animals show a clear nocturnal activity pattern in stagnant water, which is preserved after transfer to running water, but with a reduced amplitude. Like in estuarine G. zaddachi, a very strong increase in swimming activity is provoked by a so-called complete high tide simulation, i.e. a decrease in current velocity, followed by a short period of standstill and a slow current in the opposite direction, and a rise in salinity (which resulted also in a rise in pressure of maximally 0.1 atm. due to pumping) and in temperature. Contrary to what was concluded for estuarine populations, these non-tidal G. zaddachi show a much stronger response to changes in salinity than to changes in current. In fact, high tide simulations consisting of changes in salinity only result in an increase in activity, comparable with that at complete high tide simulations. A strong response is also noticed to a short increase in pressure of 0.3 and 0.5 atm., but the construction of the current chamber only allows tests of longer duration with an increase in pressure of 0.15 atm. This merely causes a small increase in swimming activity. Remarkably enough, simulations in the light have the same effect on activity as simulations in the dark. An (endogenous) circatidal activity rhythm with peak swimming at the expected time of simulated high tide is entrained in non-tidal G. zaddachi by subjecting them to a series of complete high tide simulations at tidal intervals. A series of high tide simulations, consisting of changes in current and salinity (but without changes in temperature) also entrains the rhythm, but neither changes in current nor changes in salinity are able to entrain the circatidal rhythm when applied alone (the same holds true for the small increases in pressure, due to pumping). The result of a combination of factors is therefore more than simply the sum of their individual effects. The endogenous tidal rhythm does not show any sign of inhibition at expected high water in the light. Earlier fieldwork on the migration of estuarine G. zaddachi suggests that the tidal migrations of this species are confined to the dark. The activity of freshly caught estuarine G. zaddachi from the river Slack (France) shows a circatidal rhythm in the laboratory with equal peaks in the light and in the dark. The absence of tidal migrations during spring tides in daytime is therefore difficult to explain on the basis of the behaviour observed under laboratory conditions. Possibly the influence of light in the laboratory is different from that in the field, and further fieldwork will be necessary to determine the effect of light on emergence, height of swimming, etc., of estuarine G. zaddachi.

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Journal Bijdragen tot de dierkunde

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Looijenga, P. J., & Dieleman, J. C. (1980). Effect of tidal simulations and entrainment of an endogenous tidal rhythm in a non-tidal population of Gammarus zaddachi. Bijdragen tot de dierkunde, 50(1), 35–51.