The effects of spatial habitat structure on the evolution of density-dependent growth and reproduction in freshwater snails

We examined the growth and reproductive rates of freshwater snails, Physa acuta, in two habitat types. In the Asabata habitat, snails lived in isolated water pools, which occasionally joined to form a single large pool; in the Kakegawa habitat, they lived in a slow-running water way. Genetic structure assessments using three microsatellite loci supports the idea that a stable panmictic population occupies the Kakegawa habitat. The Asabata habitat, however, is occupied with an alternate mixing population as revealed by microsatellite data. The Asabata population might alternate between localized mating within isolated pools (as revealed by high F _IS and F _IT values) when the water levels are low and panmixia (as revealed by the low F _ST values and AMOVA analysis) when the habitat is flooded. Laboratory experiments, using snails collected from the two habitats, showed that juvenile snails grew faster, laid more eggs, and laid them earlier in the Asabata habitat than in the Kakegawa habitat. Growth rates were lower at high density than at low density in the Kakegawa habitat; the inverse was true in the Asabata habitat. Density-dependent response of individual snail reproduction was higher in the Kakagawa habitat than in the Asabata habitats. The results support the hypothesis that spatial structure affects the evolution of density-dependent growth rates and of timing for reproduction.