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, latitudinal changes in growth, maximum attainable length and genetic constitution were assessed for the Baltic clam, Macoma balthica (L.), at stations ranging from the most southern distribution limit (France) to its most north-eastern limit in the Arctic Pechora Sea (North Russia). Growth was analyzed by means of growth-rings on the shells, the genetic constitution by electrophoretic isoenzyme analysis. Growth patterns and the genetic constitution of populations from West Europe, North Europe and the White Sea were similar, whereas the populations from the Pechora Sea are distinct from the other populations. The performance of clams in the Pechora Sea populations, with a relatively low annual 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.