Common moon jellyfish, Aurelia aurita s.l. has occurred abundantly in Korean coastal waters such as Gyeonggi and Masan Bay since latter half of 1990s, and damaged commercial fisheries and beach recreation. The common jellyfish A. aurita s.l. has a complex life cycle, involving an alternation of a sexual pelagic phase (medusa) and an asexual benthic phase (polyp). The polyp stage is an important component of the life cycle because the polyp is able to increase its population size through asexual reproduction (budding) and produce many ephyrae through an another asexual reproduction (strobilation). Ephyra develops into metaephyrae, and grow adult medusa. Hence, understanding of the asexual reproductions of polyp is essential to reveal the cause of recent mass occurrences of jellyfish.
Present study is focused on elucidating the two different types of asexual reproductions of A. aurita polyp at colony and individual level, and extracting the relationship of somatic growth with asexual reproduction. The increase of sea water temperature has been raised as a plausible cause of recent mass occurrence of jellyfish, and indeed, it was revealed that seasonal changing seawater temperature was related with development in each life stage. Therefore, of various factor such as temperature, salinity and prey, present study revealed the effect of temperature on two asexual reproductions (budding and strobilation) and somatic growth of polyps. Particularly, polyp asexual reproduction via budding was conducted at colony and individual level, and polyp strobilation in changing water temperature.
Aurelia aurita polyps rapidly increased their abundance via budding in high water temperatures (22℃ and 26℃), while their somatic growth (increase of oral disc) was insignificant. On the contrary, increase of abundance was slow in cold water (10℃ and 14℃), while their body size grew faster than that of high water temperatures. Polyps cultured in 10℃ produced pelagic larvae (ephyrae). All results suggested that polyps increase their abundance in warm seasons, and become larger in cold seasons, and the cold water temperature is favorable for strobilation.
Although trends in increase of abundance and somatic growth by temperature variations between individual level and colony level were similar, the daily asexual reproduction of polyps between them were significantly different. The daily asexual reproduction rates in the individual level was constant, but decreased exponentially at colony level suggesting that the asexual reproduction rate of polyp was density dependant. Temperature variation at the 10℃ accelerated polyp strobilation, but the large variation of temperature limited number of strobilae.