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Figure 4

From: Dynamics of natural populations of the dertitivorous mudsnail Potamopyrgus antipodarum (Gray) (Hydrobiidae) in two interconnected Lakes differing in trophic state

Figure 4

Field data of the two populations in meso-oligotrophic lake I and eutrophic lake II. Upper panel: brood pouch data; means ± S.E.; Nsnails = 25; open dots denote total numbers of embryos; solid dots denote number of nearly-neonates (not counted in 1984); middle panel: monthly relative shell width (in mm) frequency distribution of snails; bottom panel: total snail densities (lake I: N = 20; lake II: N = 10). There was no significant difference in average number of eggs per brood pouch between both lakes (ANCOVA: F = 0.036, P = 0.85). There was also no significant difference in average number of neonates per brood pouch between the lakes (ANCOVA: F = 1.18 =, P = 0.28). A significantly higher contribution of larger (>1.75 mm) snails was observed in the eutrophic lake compared with the meso-oligotrophic lake (GLM-ANCOVA, Tukey-HSD post hoc, P < 0.05). Snail densities in the meso-oligotrophic lake were significantly lower than the eutrophic Lake (t-test; P < 0.05; bottom panel).

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