Reproduction and juvenile growth of the invasive apple snails Pomacea canaliculata and P. scalaris (Gastropoda: Ampullariidae) in Taiwan

The South American apple snails Pomacea canaliculata (Lamarck, 1822) and P. scalaris d'Orbigny, 1835 were intentionally introduced to Taiwan in 1979. Pomacea canaliculata is now widely distributed in Taiwan and adjacent islands but P. scalaris occurs only in southern Taiwan, where it coexists with P. canaliculata. We conducted a comparative study on the reproduction and juvenile growth characteristics of these 2 invasive species. Pomacea canaliculata reached a greater maximum size than P. scalaris. Sexual dimorphism in shell size, with females being larger than males, was found in both species. Without seasonal peaks, the percent gonad coverage varied monthly in males from 46% to 76% and 60% to 80% in P. canaliculata and P. scalaris, and in females from 19% to 52% and 22% to 32%, respectively. In the laboratory, the hatching period of P. scalaris (10.4 ± 1.3 d) was shorter than that of P. canaliculata (12.2 ± 2.3 d), and its size at hatching was also smaller. The 6-mo growth equations for P. canaliculata and P. scalaris were y = 0.426 + 0.353x - 0.001x² (R² = 0.93, p < 0.001) and y = 1.428 + 0.203x - 0.001x² (R² = 0.86, p < 0.0001) when fed dry fish feed, and the final shell lengths were 15.8-43.5 and 10.2-31.0 mm, respectively. Significant differences in growth between the 2 species were observed 2 wk after hatching and onwards. Additionally, the 1st laboratory oviposition by P. canaliculata and P. scalaris occurred on days 175 and 163, respectively. Our results indicate that a smaller hatching size and inferior growth performance of P. scalaris may also have played a role in shaping its distribution in Taiwan.