Character displacement and coexistence in mud snails (Hydrobiidae) (original) (raw)

Populations of coexisting and of allopatrically occurring species of hydrobiid snails (Hydrobia ulvae, H. neglecta, H. ventrosa and Potamopyrgus ]enkinsi) have been studied in 90 localities within three different areas. When H. ventrosa coexists with H. ulvae they show character displacement, i.e., the average body size of the former is smaller and that of the latter is larger. When these species live alone they are of approximately the same size. It is shown here that the size ratio between the coexisting species usually found (1.3-1.5) allows stable coexistence based on food particle size selectivity alone. Variation in the degree of character displacement from locality to locality is explained by different degrees of genetical isolation of the populations. Coexisting H. ventrosa and H. ulvae have shorter, more well-defined periods of reproduction than they do when they occur alone. H. negIecta is larger than coexisting H. ventrosa and smaller than coexisting H. ulvae. Due to the patchy distribution and the fluctuating populations of this species, and due to the fact that pure H. neglecta populations are rare, data on this species are difficult to interpret. Potamopyrgus ]enkinsi shows a different food particle size selection than the Hydrobia spp. of identical sizes. Competitive interactions between P. ]enkinsi and coexisting Hydrobia spp. are therefore probably weak. In accordance with this, P. ]enkinsi does not show character displacement when coexisting with, e.g., H.ventrosa.

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