GROWTH IN THE BIVALVE MACOMA-BALTHICA FROM ITS NORTHERN TO ITS SOUTHERN DISTRIBUTION LIMIT - A DISCONTINUITY IN NORTH EUROPE BECAUSE OF GENETIC ADAPTATIONS IN ARCTIC POPULATIONS

The hypothesis was tested that towards a species limit of distribution its performance, such as growth or fitness, decreases. To this end, l atitudinal changes in growth, maximum attainable length and genetic co nstitution were assessed for the Baltic clam, Macoma balthica (L.), at stations ranging from the most southern distribution limit (France) t o its most north-eastern limit in the Arctic Pechora Sea (North Russia ). Growth was analyzed by means of growth-rings on the shells, the gen etic constitution by electrophoretic isoenzyme analysis. Growth patter ns and the genetic constitution of populations from West Europe, North Europe and the White Sea were similar, whereas the populations from t he Pechora Sea are distinct from the other populations. The performanc e of clams in the Pechora Sea populations, with a relatively low annua l growth but high maximum length, was, in contrast to the hypothesis, not decreased. It is concluded that the Pechora Sea populations form a separate group, genetically different from other European populations and adapted to the Arctic conditions. (C) 1998 Elsevier Science Inc. All rights reserved.