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Journal of Experimental Biology 48,141-158 (1968)
Published by Company of Biologists 1968

Ionic Regulation of the Baltic and Fresh-Water Races of the Isopod Mesidotea (Saduria) Entomon (L.)

P. C. CROGHAN ¹ and A. P. M. LOCKWOOD ²

¹ School of Biological Sciences, University of East Anglia
² Department of Oceanography, University of Southampton

1. The isopod Mesidotea entomon has colonized the Baltic and certain Swedish lakes since the end of the last Ice Age.

2. The ionic regulation of Baltic animals and fresh-water animals (L. Mälaren) has been compared.

3. It has been possible to adapt Baltic animals to very dilute media, but 5% Askö sea water (5.5 mM/l. Na) appears to be the limit of adaptation. The haemolymph sodium concentration of Baltic animals from the very dilute media was considerably lowered.

4. The haemolymph sodium concentration in Mälaren animals is high (250 mM/l. Na) and comparable with that in Baltic animals in much more concentrated solution. The haemolymph ionic ratios of the Baltic and freshwater animals are similar. The Cl:Na ratio rises slightly in the more concentrated haemolymph samples.

5. From the concentration of ions in the haemolymph and in the total body water, the relative volume of the haemolymph was calculated. Mälaren animals appear to have a much larger haemolymph volume.

6. The permeability of the animals was determined from the rate of loss of sodium into de-ionized water. The permeability of the Mälaren animals is considerably reduced compared to the Baltic animals. Permeability is not related to the medium to which the animals had been adapted.

7. The sodium influx was determined using ²²Na. The rate of active uptake was calculated from this. The maximal rate of active uptake was similar in Baltic and Mälaren animals. The sodium concentration of the medium at which active uptake was half maximum (K_M) was considerably lower in Malaren animals than in Baltic animals.

8. The evolution of Mesidotea as a fresh-water animal is interpreted as a result of a reduction in permeability of the external surfaces to NaCl and an increase in the affinity of the active transport mechanism enabling the animal to maintain the haemolymph NaCl concentration in a steady state in fresh water.

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