Effects of water temperature and food concentration on polyps of Aurelia malayensis around the Ogasawara (Bonin) Islands, Japan

Abstract

This study conducted laboratory experiments to investigate the relationship of polyps to water temperature and food concentrations in Aurelia malayensis from a subtropical zone. The growth of polyps was found to be unrelated to water temperature, while significant differences were observed between feeding and no-feeding experimental rearing schemes. Polyp size increased in all feeding schemes and decreased in all no-feeding schemes, suggesting a direct positive relationship between feeding and polyp growth. Polyp asexual reproduction was suppressed at a lower temperature (20°C) under a low food concentration and strobilation was observed even under constant water temperatures. These findings suggest that A. malayensis is adapted to an environment of high water temperature with low levels of fluctuation and low food concentrations. Ephyrae were produced at water temperatures of 25°C and 30°C at both high and low food concentrations in all four rearing schemes. At water temperatures of 30°C and with low food concentrations, the size of liberated ephyrae was significantly smaller than in the other three schemes. These results suggest that at high water temperatures and low food concentrations, small ephyrae are liberated. This study reveals the effects of water temperature and food concentration on the polyp stage of A. malayensis, suggesting that this species is particularly adapted to high water temperatures. This also suggests that in the future, as global warming progresses, medusae blooms of A. malayensis may occur. From this perspective, it is necessary to monitor the population dynamics of A. malayensis and focus intently on its trends.