Abstract
Reproductive traits of three species of caprellids attached to the annual Sargassum horneri were studied by monthly sampling throughout the year in Amakusa, west Kyushu. For three species (Caprella tsugaruensis, C, danilevskii and C, decipiens), some ecological characters such as individual growth and number of molting, sexual dimorphism, 50% maturity size of female, the clutch size - female body size relation etc., and their seasonal variations were examined. Seasonal fluctuations of host algal biomass and that of caprellid populations on the algae were followed. Accompanying the rapid growth of host algae, caprellid populations increased several thousand times during colder months and reached their peak in March. Most of them were lost with decline of host algae in early summer, only a few population survived on perennial Sargassam and red algal undergrowth in summer.
