The golden apple snail Pomacea canaliculata (Lamarck, 1822) was introduced into Taiwan intentionally in the early 1980s and has become a recurring pest that seriously threatens aquatic crops. In this study, a field description of imposex with a developed penis sheath and penis in female golden apple snails from crop/domestic wastewater drainage sites and a six-order river is presented for the first time. Based on the five field collections and the aquarium group, the vas deferens sequence (VDS) of P. canaliculata in imposex development was categorized into four stages, i.e., stage 0: without male genital system; stage 1: with rudimentary penis; stage 2: with rudimentary penis and penis sheath; and stage 3: the rudimentary penis developing into penis pouch and penis. The VDS indices varied between 1.07 and 2.82 and were lowest in the aquarium group and Yuanlin2. Regarding the severity of imposex, the aquarium group was less pronounced, as illustrated by the length of penis sheath and penis length, than the field collections (p < 0.05). In respect of the penis length, males of the most imposex-affected site were up to 15% shorter than that of the aquarium group. Negative correlations between male penis length and female imposex characters (i.e., penis length and penis sheath length) were also observed.
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