No abstract

To determine the reproductive season of the giant jellyfish Nemopilema nomurai, we investigated gonadal maturity in specimens collected from the East China Sea, Korea Strait, Wakasa Bay, and the Shonai Coast of Yamagata Prefecture. After the sex of the samples was determined, the long axis of at least 256 oocytes from each female was measured. In specimens collected from the coast of Japan in 2005 and in 2006, all gonads were sufficiently developed to determine sex. However, 18 of the 20 specimens from the East China Sea collected in July 2005 were immature, and sex could not be determined. The maximum and third quartile of oocyte length had a significant correlation with days elapsed from 30 June, but they were not related to bell diameter. Observations of gonad tissue sections of specimens collected in Wakasa Bay in 2006 confirmed that oocyte length was a good proxy for female maturity. Male maturity could also be determined. In conclusion, the sex of all of the small-sized medusae collected along the coast of Japan was determinable, and their gonads were at various stages of development up to fully mature. Therefore, the occurrence of small-sized jellyfish during the autumn in Wakasa Bay is not caused by recruitment of young population from the nearby coast.

Biomass and body elemental composition of the giant jellyfish Nemopilema nomurai in the southwestern Japan Sea were studied using specimens collected with mid-water trawl nets during September and October 2006, 2007. The average biomass of N. nomurai was calculated as 0.26-0.93 g wet weight (WW) m . Body elemental composition of N. nomurai was water=95.8% of WW, carbon=6.6% of dry weight (DW), nitrogen=1.7% of DW, and phosphorus=0.14% of DW. Carbon requirements for growth and respiration were estimated in per unit area using biomass data. For growth analysis, size-frequency distributions of bell diameter (BD) were examined from specimens collected using set-nets in Wakasa Bay during August-January in 2005-2007 and the growth rate was calculated as 1.5 cm d . Biomass and carbon requirement data of N. nomurai were compared to the biomass of the common squid Todarodes pacificus and copepod production, respectively, in the Japan Sea. In the present study, it was demonstrated that when mass incidences of N. nomurai occurred in the Japan Sea, the biomass of N. nomurai was substantial and equivalent to that of the common squid. The amount of zooplankton consumed as prey by N. nomurai was not large enough to influence the biomass of zooplankton as a whole; however, it was critical for dense aggregations in a small area.

No abstract