Effects of Acid Water on Shells, Embryos, and Juvenile Survival ofPlanorbella trivolvis(Gastropoda: Pulmonata): A Laboratory Study

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Abstract
Adults of the freshwater pulmonate snail Planorbella trivolvis were maintained in aquaria at four pH levels of approximately 4.9, 5.9, 6.8 and 7.8. Adult mortality was moderate over 82 days (10–32%) and not related to the magnitude of pH reduction. Decrease in both shell calcium per unit tissue dry weight and shell calcium per unit maximum shell diameter were significantly related to both time of exposure and pH level. Both measures were significantly reduced after 53 days of exposure to acid water such that for mean Ca mg−1, pH 4.9=5.9 < 6.8 < 7.8 and for mean Ca mm−1, pH 4.9=5.9 < 6.8=7.8. Empty (ashed) shells without a periostracum lost CaCO3 about 4.6 times faster than did shells of live snails with periostracum intact. Fecundity per snail did not differ greatly from one pH treatment to another although at pH 4.9 the embryos developed slowly and showed a high incidence (38.5%) of abnormality compared to embryos reared at pH 7.8 (4.8% abnormality). Juvenile survival at pH 5.1 was poor with 100% mortality by day 43. At pH 6.6 and 7.5, 9 (=4.1%) and 15 (=8.9%) individuals respectively were alive at day 93, some of which were functional adults laying eggs. These experiments demonstrate that under laboratory conditions acidified natural water (pH 4.9–5.1) causes measurable snail shell erosion in adults, as well as embryonic abnormalities and increased juvenile mortality. Extrapolation to natural populations under stress due to acidification suggests that adult shell erosion, although unlikely to be a direct cause of death, provides an indicator of stress. A more serious threat to snail populations may be recruitment failure due to mortality of eggs and juveniles.