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

Ability to tolerate low salinities has been compared in homozygous adult females of thetrifasciataandviolaceaforms of the polymorphic copepodTisbe reticulatafrom Chioggia (lagoon of Venice). Both forms appear to be equally capable of tolerating sea-water concentrations of between 100 and 33 %. Further comparison with thetrifasciataform ofT. reticulatafrom Plymouth and with three different geographical populations ofTisbe furcatahas shown that these copepods are rather less tolerant. A study of ionic regulation using the isotopes22Na,42K and137Cs has shown that in diluted sea waterT. reticulatadoes not control the body Na concentration but does control the K concentration. Exchange rates for42K and137Cs are higher in theviolaceathan in thetrifasciataforms ofT. reticulatafrom Chioggia and the difference may have an adaptive nature although its significance is not clear. With regard to42K, the difference between the two polymorphic forms from the same population ofT. reticulatais greater than that between the three different geographical populations ofT. furcata. This serves to add more emphasis to the fact that colour polymorphism inT. reticulatacan be associated with other processes.