Effects of duration at low temperature on asexual reproduction in polyps of the scyphozoan Nemopilema nomurai (Scyphozoa: Rhizostomeae)

In recent decades, massive outbreaks of the giant jellyfish, Nemopilema nomurai have been appearing frequently in coastal seas of China. In the life cycle of N. nomurai, asexual reproduction by its benthic polyps may largely determine the jellyfish population size, and it was thought that asexual reproduction leading to medusae might be affected by differences in the duration at low winter temperature on different parts of the coast. To test this hypothesis, polyps were first stored at low temperature for two different durations, 5°C for either 40 or 117 days. They were then maintained at seven temperatures, (maintained at 5°C, and separately elevated to 10, 13, 16, 19, 22, 27°C) reached by raising the temperature 1°C every 2 days from 5°C, as well as a control temperature: 5°C. Polyps were fed with three frequencies (once per 5 days, once per 10 days, and unfed, prey concentration, 0.1 mgC l⁻¹, being supplied once). The percentages of polyps strobilating, strobilation duration, ephyra and podocyst production were recorded. The results showed that longer duration at low temperature accelerated strobila formation at warmer temperature, as well as significantly enhancing podocyst production, but not for the strobilation percentage or ephyra production. Temperature had a marked influence on strobilation percentage, as well as on ephyra and podocyst production. The optimum temperature for strobilation by N. nomurai polyps ranged from 10 to 13°C. The maximum numbers of ephyrae appeared after polyps had been stored at longer low temperature then warmed to 10°C and fed once per 5 days, while the maximum production of podocysts occurred in polyps warmed to 27°C and fed once per 5 days. We conclude that longer duration at low temperature in winter, a continuous 10–13°C period in spring, and increasing food supply all accelerate the strobila formation, increase jellyfish production and are tend to increase blooms of N. nomurai medusae.