Icy heritage: ecological evolution of the postglacial Baltic Sea reflected in the allozymes of a living fossil, the priapulid Halicryptus spinulosus

Abstract

Genetic variation of 16 allozyme loci in 397 Halicryptus spinulosus (Priapulida) revealed overall polymorphism of P=0.438 and Hardy-Weinberg expectations for heterozygosity of He=0.060 for Baltic Sea stocks, He=0.143 for the White Sea and He=0.121 for Iceland. Maximal unbiased standard distances of D=0.0693 separated Baltic and White Sea populations. Nordic and Baltic populations could be distinguished by allozymes, but Baltic subsamples proved cohesive. Gene flow amounted to effective exchange values per generation of Nm=2.94 over 650 km of continuous habitat, Nm=10.65 over 175 km, and Nm=13.85 over 20 km. Gene flow started to decrease with geographic distance beyond a dispersal threshold of 20 km, but hierarchical GST-statistics indicated light isolation by distance beyond a minimum of 8 km. Gene flow is high for a benthic worm assumed to lack dispersal by pelagic larva, a paradox which cannot be resolved now. Baltic populations are characterized by lower heterozygosity than Nordic stocks. In the Baltic Basin, temporally continuous brackish-water conditions have only existed for the past 7000 years. The two possible colonization routes of H. spinulosus to the geologically young Baltic Sea imply genetic drift, whether by founder effect (sweepstake colonization from Iceland) or by refugial bottlenecking during the Ancylus phase of the Baltic Basin after a direct connection to the White Sea had been sequestered. Continued genetic drift is emphasized by lower heterozygosity in the ecologically unstable Belt Sea compared to the central Baltic. Allozymes falsify the reduced-mutability hypothesis to explain bradytelic evolution of Priapulida. Regional genetic homogeneity, ample polymorphism, and preference for anoxic black mud qualify H. spinulosus populations as indicators of microevolutionary responses to water circulation regimes or pollution in the Baltic Sea.