Ecophysiological responses to salinity changes in selected euryhaline amphipods with special reference to Gammarus duebeni

Abstract

Haemolymph and external medium sodium concentrations have been investigated in three euryhaline amphipods, Gammarus duebeni, Chaetopammarus marinus and Cammarus locusta. These were subjected to various salinity regimes, including sudden and cycling salinity changes. The permeability to water of these three species was investigated in various salinity regimes using THO as a marker, and of the three species, only G.duebeni demonstrated large and significant changes in apparent permeability to water. Combining the haemolymph and medium sodium concentrations with the permeability to water, the bulk flow into and out of the animals was calculated showing a variation of responses in the three species. G. duebeni exhibiting the smallest bulk flow and G.locusta the largest. The heart rate of G.duebeni was investigated, demonstrating that the heart rate of G.duebeni was not correlated to the observed permeability changes. The urine clearance rates in G.duebeni were measured using ⁵¹Cr E.D.T.A. in order to calculate the water fluxes into the animals. Using the haemolymph and medium sodium concentration results and the water fluxes found using ⁵¹Cr E.D.T.A., the theoretical water permeabilities of G. duebeni were calculated and compared with the values obtained using THO. This comparison showed a good similarity in the changes of permeability to water using the two techniques and hence supported the hypothesis that the changes in permeability noted in G.duebeni using THO are indeed real. In order to investigate possible mechanisms for such large changes in permeability, the potential difference (P.D.) across the body wall of G. duebeni was artificially controlled and the permeability to water monitored. These results demonstrated that changing the P.D. across the body wall does not appear to alter the permeability to water. Two species of arctic amphipod, Gammarus setosus and Onisimus litoralis were studied and compared with the three British species. These two species showed large changes in apparent permeability to water, although they did not demonstrate the same pattern of change as found in G.duebeni. The ecophysiological responses of G.setosus and O.litoralis to oil was also studied, showing that these animals appear sensitive to physical contact with oil while remaining apparently unaffected by the presence of dispersed oil. In conclusion, the five amphipod species studied have demonstrated that osmoregulatory responses are correlated to the ecology of the species, and that in the more euryhaline species, a varying permeability to water appears to be an important facet of the osmoregulatory mechanism.

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