THE DIET OF THE COMMON MEDUSA SPECIES IN THE EAST CHINA SEA REVEALED BY FATTY ACID AND STABLE CARBON AND NITROGEN ISOTOPIC ANALYSIS
基于脂肪酸标记法和碳氮稳定同位素比值法的东海水母常见种的食性分析

Fatty acid compositions and stable isotope ratios of carbon and nitrogen were used to identify the diet of six common medusa species in the East China Sea,Ocyropsis crystalline,Bero cucumis,Liriope tetraphylla,Diphyidae spp.,Aequorea coerulescens,and Nemopilema nomurai.Analysis of the fatty acid composition showed that the total fatty acids of three medusae were low,which reflecting their lower lipid nutritional values.O.crystalline,B.cucumis and L.tetraphylla showed similar fatty acid compositions.Firstly,they all had a higher content of 16:1ω7 and 18:4ω3,indicating feeding on phtytoplankton(mostly diatoms and dinoflagellates) more than the other medusae.Secondly,the higher contents of 20:1+22:1 reflected herbivorous copepods are parts of their food items.In addition,they were characterized by high contents of Σω3,accumulated via the grazing food chain.By contrast,A.coerulescens was characterized by an increase in Σω6 of macroalgal origin,which was transferred via the detritus food chain.Otherwise,it also fed on herbivorous copepods in view of the fatty acid 20:1+22:1.The higher Σω6 contents also indicated that Diphyidae spp.accumulated via the detritus food chain,and it fed little on herbivorous copepods.The δ13C of O.crystalline,B.cucumis,L.tetraphylla and N.nomurai were similar to C4 plants,indicating significant contribution of C4 plants to the nutritions of four medusae.The δ15N of the six medusae indicated their higher trophic levels in plankton food web.Although N.nomurai is big,it located at a similar trophic level as O.crystalline,B.cucumis and L.tetraphylla,because of its smaller terminal pore.In addition,the marker of Bacteria FA was evident in N.nomurai.