Some Aspects of Ionic and Osmotic Regulation inTisbe[Copepoda, Harpacticoida] in Relation to Polymorphism and Geographical Distribution

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Abstract
Ability to tolerate low salinities has been compared in homozygous adult females of the trifasciata and violacea forms of the polymorphic copepod Tisbe reticulata from Chioggia (lagoon of Venice). Both forms appear to be equally capable of tolerating sea-water concentrations of between 100 and 33 %. Further comparison with the trifasciata form of T. reticulata from Plymouth and with three different geographical populations of Tisbe furcata has shown that these copepods are rather less tolerant.A study of ionic regulation using the isotopes 22Na, 42K and 137Cs has shown that in diluted sea water T. reticulata does not control the body Na concentration but does control the K concentration. Exchange rates for 42K and 137Cs are higher in the violacea than in the trifasciata forms of T. reticulata from Chioggia and the difference may have an adaptive nature although its significance is not clear. With regard to 42K, the difference between the two polymorphic forms from the same population of T. reticulata is greater than that between the three different geographical populations of T. furcata. This serves to add more emphasis to the fact that colour polymorphism in T. reticulata can be associated with other processes.

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