Abstract
The density of Pomacea canaliculata (Lamarck) with shell lengths > 25 mm and of its egg masses were estimated in three habitats: rice field, canal and river. The mean densities were 3.72 ± 1.31, 2.12 ± 0.90 and 3.20 ± 1.85 snails/m2, respectively: no significant difference was detected by ANOVA. In contrast, the mean densities of egg masses, 1.84 ± 0.40, 0.72 ± 0.24 and 0.39 ± 0.19 eggs/m2, differed significantly between rice field and canal and between rice field and river, though not between canal and river. Correlation of these densities with water depth, current velocity, chemical oxygen demand (COD), and predator fauna (Carassius spp., Zacco temmincki, Procambarus clarki and Potamon dehaani) were examined by Kendall's coefficients of rank correlation. Water depth showed a significant negative correlation with snail density but not with egg density. Depths > 50 cm were likely to reduce both snails and eggs markedly, though neither were significantly correlated with depth. There was no significant correlation with current velocity, although velocities > 50 cm/sec reduced the densities nearly to zero. Both snail and egg densities were positively correlated with COD, indicating that this snail is adapted for more polluted water. Neither density was correlated significantly with the predator fauna. These correlations are discussed with special reference to the life history of the snail in these habitats.
