||Temporal and spatial distribution in species composition and abundance of hydromedusae in relation to hydrographic variables in the waters off southern and northern Taiwan were investigated from January 2005 to July 2006. In total, 46 hydromedusae species belonging to 26 genera and 18 families were identified in the waters off southern Taiwan, with mean abundance of 88±14 inds./1000m3. The abundance of hydromedusae showed distinct seasonal change, higher in January 2005 and July 2006 compared with other seasons. The predominant species were Aglaura hemistoma, Amphogona apsteini and A. apicata, together they contributed to 71 % of the numerical total of hydromedusae. In the waters off northern Taiwan, 36 hydromedusae species belonging to 24 genera and 19 families were identified, with mean abundance of 235±68 inds./1000m3. The abundance of hydromedusae showed no apparent seasonal change. The 2 most dominant species, A. hemistoma and Eutima levuka, comprised 70.3 % of the numerical total of hydromedusae.|
The mean species number and species diversity were higher in the southern waters than the northern waters of Taiwan, but the overall mean abundance was about 3 times higher in the northern waters than that in the southern waters, particularly in the northwestern waters off Taiwan. Different dominant species showed different distribution patterns.
In total, the species number and species diversity index of hydromedusae in the present study areas showed significantly positive correlation with water temperature, while the abundance and species richness of hydromedusae and the dominant species showed significantly positive correlation with zooplankton. The dominant species, E. levuka, showed significantly negative correlation with water salinity. The species diversity was always higher in the waters off southern Taiwan, probably due to warm and stable hydrographic conditions. The abundance of most dominant species were positively correlated to zooplankton abundance in the southern waters, suggesting that the abundance of food availability might also be a key factor to determine the distribution pattern of hydromedusae. While in the waters off northern Taiwan, seasonal variation of hydrography was more obvious than that in the waters off southern Taiwan, and so as the species number and the species diversity index of hydromedusae, implied that the distribution of hydromedusae was closely related to the seasonal variation of water masses.