カワニナ属 3 種の産仔頻度, 産仔数と新生貝の大きさ The Birth Frequency, Number and Size of Newborns in the Three Species of the Genus Semisulcospira (Prosobranchia : Pleuroceridae)
The birth frequency, number and size of newborns in the three species of the genus Semisulcospira, S. libertina, S. reiniana and S. kurodai, were studied under the laboratory condition where parent snails were reared individually. Births of newborns continued during 4 to 7 days and then stopped during a few days. This birth pattern was repeated. Peak of daily birth of newborns in the three species occurred once or twice during a month. Maximum number of daily births of newborns in S. libertina, S. reiniana and S. kurodai was counted 69, 18, 10, respectively. Most frequent number of daily births in S. libertina, S. reiniana and S. kurodai was 5-15, 1-3, 1-3, respectively. No synchronized birth was observed among parent females in all these three species. Average number of newborns in S. libertina, S. reiniana and S. kurodai during one year was estimated 607-858 (max. 1535), 93-149 (max. 268), 253-370 (max. 464), respectively. Birth of newborns in these three species occurred at a water temperature between 12℃ and 24℃. Large number of newborns in S. libertina were produced during the upward period of water temperature. Mean size (shell diameter) of newborns in S. libertina, S. reiniana and S. kurodai was estimated 0.60-0.99 mm (max. 1.22 mm), 1.04-1.24 mm (max. 1.63 mm), 1.23 mm (max. 1.56 mm), respectively. Two types was distinguished in the shell size of newborns in S. libertina. Mean size (shell diameter) of small and large type of newborns was estimated 0.60-0.68 mm (max. 0.80 mm) and 0.88-0.99 mm (max. 1.22 mm). Maximum size of shell height of newborns in these three species was 1.73 mm, 2.40 mm, 2.44 mm, respectively. Number of whorls of newborns in these three species was 2.0-3.5 (mode 2.5-3.0), 2.0-4.0 (mode 3.0), 2.5-4.0 (mode 3.5), respectively. Mechanism of birth in the genus Semisulcospira was discussed in relation to birth frequency, size of newborns and water temperature.