노무라입깃해파리(Nemopilema nomurai Kishinouye)의 생식과 섭식에 관한 연구

Abstract

Massive occurrence of jellyfish is frequently reported worldwide recently. In Korean and Japanese waters, various jellyfish (i.e. Aurelia aurita, Chrysaora melanaster, Cyanea capillata, Crambionella orsini, Nemopilema nomurai) showed big blooming events frequently, and N. nomurai is one of the biggest nuisances. In the past, massive blooms of this species have been reported intermittently (i.e. in 1920, 1958, and 1995), but events in frequent blooming have been reported since the early 21st century (i.e. in 2002, 2003, 2005, 2006, 2007, and 2009). However, little is known about biology and ecology of this species. Understanding the characteristics of early stage (polyp) and feeding behavior is necessary to elucidate the causes of massive occurrence and its impact on marine ecosystem. Therefore this study focused on reproduction and feeding ecology of N. nomurai. The sexual maturation process, release of spermatozoa/eggs, oocyte diameter, and gonad color of the rhizostomeae medusae N. nomurai were investigated using samples collected from the waters around Korea and Japan in August 2006 to June 2008. Immature medusae were observed from June to October only in the Korean waters. The onset of release of spermatozoa and eggs occurred in September and October, respectively, and the peaks occurred in December to January. Medusae migrate eastward from the source areas to the Tsushima Warm Current, where they form gametes and spawn. No fertilized eggs or embryos were found in the gonads. The bell diameter at which medusae matured varied, but there was a clear trend for more mature female medusae to have larger maximum oocyte diameter, which was correlated negatively with surface water temperature. Sex could not be distinguished clearly by gonad color, but light pink gonads were more prevalent in males, whereas various deep colors such as orange and brown were more frequent in female medusae. Benthic polyps of N. nomurai asexually produce further polyps and ephyrae, and the abundance of medusae depends on the success of asexual reproduction of the polyp stage. In this study, laboratory experiments were carried out to investigate the effect of water temperature (10, 15, 20, 25℃) and salinity (12, 17, 22, 27, 32psu) on asexual reproduction rate of N. nomurai polyps. Survival rate was high in all treatments indicating that polyps have a wide tolerance to environmental conditions. Polyp produced two kinds of cysts; the most frequent was podocysts (92.6%) and the other one was planulocyst (7.4%). Number of podocysts increased at warmer temperature (20℃) and number of ephyrae was high at colder temperature (15℃). Temperature affected on strobila formation and starting time of strobilation as well as production rate of podocysts. However, salinity did not affect significantly. Feeding apparatus, mechanism and passage of ingested prey were described for N. nomurai. Besides, predation rate by various conditions and diets by analysis of oral arm contents were investigated. N. nomurai medusae without a central mouth have developed complicated canal systems connecting from the tip of the tentacle and oral arm to the gut cavity. The number of junctions in the canal system increases with the bell diameter. The prey is gathered by paralyzing nematocyst at the tentacles and by adhering cirri at the oral arms and scapulets. They are engulfed into the terminal pore located at the oral arms and scapulets, and entered into the gut cavity via the canal system. The estimated digestion time is approximately 2 hour (±0.5). The diameter of terminal pore is always about 1mm, implying that they could not eat prey larger than that pore size. On the other hand, ephyrae have a central mouth and could swallow prey as large as adults could. Predation rate increased with increasing prey concentration (4,000~13,000indsL-1), predator size (15~43cm), and salinity (24~33psu). Predation rate was higher at 23℃ than 26℃. Various zooplankton were observed in oral arm contents of N. nomurai, and calanoid copepod was predominant (>50%) in the Yellow Sea. Prey size was in accordance with diameter of terminal pore (0.2~0.93mm). Thus, N. nomurai could be considered as a non-selective feeder. In conclusion, N. nomurai reproduced seasonally clear cut. The number of polyp may increase during summer-autumn and ephyrae may be produced during winter-spring. In winter, sexual reproduction occurred, and consequently new polyps are provided for next year's asexual reproduction. These results indicate that N. nomurai from Northwestern Pacific Ocean adapt to a wide range of temperature. And we suggest that N. nomurai has an ability of high predation rate on zooplankton and the ecological role of N. nomurai as potential predator on various zooplankton might be important in coastal ecosystem.