ICY HERITAGE - ECOLOGICAL EVOLUTION OF THE POSTGLACIAL BALTIC SEA REFLECTED IN THE ALLOZYMES OF A LIVING FOSSIL, THE PRIAPULID HALICRYPTUS-SPINULOSUS

Genetic variation of 16 allozyme loci in 397 Halicryptus spinulosus (P riapulida) revealed overall polymorphism of P = 0.438 and Hardy-Weinbe rg expectations for heterozygosity of H-e = 0.060 for Baltic Sea stock s, H-e = 0.143 for the White Sea and H-e = 0.121 for Iceland. Maximal unbiased standard distances of D = 0.0693 separated Baltic and White S ea populations. Nordic and Baltic populations could be distinguished b y allozymes, but Baltic subsamples proved cohesive. Gene flow amounted to effective exchange values per generation of N-m = 2.94 over 650 km of continuous habitat, N-m = 10.65 over 175 km, and N-m = 13.85 over 20 km. Gene flow started to decrease with geographic distance beyond a dispersal threshold of 20 km, but hierarchical G(ST)-statistics indic ated light isolation by distance beyond a minimum of 8 km. Gene flow i s high for a benthic worm assumed to lack dispersal by pelagic larva, a paradox which cannot be resolved now. Baltic populations are charact erized by lower heterozygosity than Nordic stocks. In the Baltic Basin , temporally continuous brackish-water conditions have only existed fo r the past 7000 years. The two possible colonization routes of H. spin ulosus to the geologically young Baltic Sea imply genetic drift, wheth er by founder effect (sweepstake colonization from Iceland) or by refu gial bottlenecking during the Ancylus phase of the Baltic Basin after a direct connection to the White Sea had been sequestered. Continued g enetic drift is emphasized by lower heterozygosity in the ecologically unstable Belt Sea compared to the central Baltic. Allozymes falsify t he reduced-mutability hypothesis to explain bradytelic evolution of Pr iapulida. Regional genetic homogeneity, ample polymorphism, and prefer ence for anoxic black mud qualify H. spinulosus populations as indicat ors of microevolutionary responses to water circulation regimes or pol lution in the Baltic Sea.