SENSITIVITY TO STRESS IN THE BIVALVE MACOMA-BALTHICA FROM THE MOST NORTHERN (ARCTIC) TO THE MOST SOUTHERN (FRENCH) POPULATIONS - LOW-SENSITIVITY IN ARCTIC POPULATIONS BECAUSE OF GENETIC ADAPTATIONS

The stress sensitivity, determined in copper exposure experiments and in survival in air tests, and the genetic structure, measured by means of isoenzyme electrophoresis, were assessed in populations of the Bal tic clam Macoma balthica (L.) from its southern to its northern distri bution limit, in order to test the hypotheses that near the distributi on limit the clams would be more stress sensitive and would have a low er genetic variability. The populations in west and north Europe show a strong genetic resemblance. The populations in the sub-Arctic White Sea are genetically slightly different, and show a low stress sensitiv ity. The populations in the Arctic Pechora Sea are genetically very di stant from the other populations, and show the lowest stress sensitivi ty. Near the southern distribution limit, in agreement with the hypoth eses, genetic variability is low and stress sensitivity high. On the o ther hand, in contrast to expectation, near the northern distribution limit, in the populations of the Pechora Sea, the genetic variability was higher, thus not reduced, and the stress sensitivity was low compa red to ail other populations. Yet, it remains a question if such is du e to gradual physiological acclimatization (and ongoing differential s election) or to genetic adaptation.