Seasonal reproductive cycle in relation to tolerance to high temperatures in the terrestrial slug Lehmannia valentiana.

Seasonal maturation of the gonad (hermaphrodic gland) was examined in the terrestrial slug Lehmannia valentiana in Osaka, Japan. The ratio of the gonad weight to body weight was low in June–September, rapidly increased in October, and gradually decreased thereafter. In May–September, most slugs had only spermatogonia and spermatocytes, and all slugs had mature sperms in October–April. The oocyte size increased in September and large oocytes were observed in late October–April. Under natural temperature and photoperiodic conditions in Osaka, slugs laid eggs in early November–May. Therefore, this species reproduces from late autumn to spring, in contrast to many terrestrial slugs. In January–early February, however, the number of laid eggs was small and oviposition activity showed a bimodal seasonal pattern. In contrast, the hatching pattern was unimodal and most hatching was observed in spring. After a 1-h exposure to 33°C, the survival rate was 100% in juvenile slugs but 0% in eggs. Although hatching success was >85% at 15°–20°C, no eggs hatched when they were maintained at 25°C. The heat susceptibility of eggs in this species may be one of the key factors restricting oviposition from late autumn to spring. [ABSTRACT FROM AUTHOR]