VARIATION IN GENETIC-TRAITS OF THE BALTIC CLAM MACOMA-BALTHICA FROM ATIDAL GRADIENT IN THE SUB-ARCTIC

In a subarctic tidal gradient, strong heterogeneity in genetic traits of the Baltic clam Macoma balthica was found. The heterogeneity was st ronger within the intertidal gradient, over a distance of only about G O mi than along a horizontal gradient over a distance of 1200 km in cl ams from the west European coast. For the locus Idh1 and the average h eterozygosity, a tidal dine was found. The frequency of allele Idh1-B decreased with tidal level, whereas the frequency of allele Idh1-C, as well as the average heterozygosity, increased. The possibility is dis cussed that the strong genetic heterogeneity and tidal dines are cause d by differential selection related to the (subarctic) temperatures to which the higher tidal zones are more exposed.