ABSTRACT
In molluscs, sexuality includes gonochorism, parthenogenesis and hermaphroditism; the latter includes Simultaneous Hermaphroditism (SH), protandric, serial and Marian hermaphroditism. This would imply the presence and operation of diverse mechanisms of sex determination. Sex ratio represents the cumulative end product of sex determination and differentiation processes. In the Japanese ampullarid rice field pest snail Pomacea canaliculata, Yusa and Suzuki (2003) reported that the ratio was highly variable and ranged from almost exclusive daughters to almost exclusive sons. Further, they also observed that the ratios were constant between successive broods of the same parent. This observation suggests the operation of polygenic sex determination mechanism (Bull, 1983), in which environmental factors like temperature play a role in sex determination. Yusa (2004b) investigated the role of (i) presence of adult males or females, (ii) age of parents, (iii) temperature (20-25ºC), (iv) food availability, (v) position in the egg mass, (vi) aquarium size and (vii) indoor (14 L : 10 D) and outdoor (14 L : 10 D to 11 L : 13 D) aquaria. He found that none of these factors had any significant effect on sex ratio. Further, the brood ratios were significantly different among different parents in all these experiments. The results reported in the representative apple snail clearly indicate that environmental sex determination does not occur and both the sex determination and sex ratio variations are under genetic regulation in molluscs.
