Community Structure Of Pelagic Tunicates In The North Yellow Sea

In this paper, seasonal changes of distribution patterns and abundance of pelagic tunicates were examined in the North Yellow Sea, China in2006-2007. Zooplankton samples were obtained by vertical towing from the bottom to the surface using a WP2plankton net (200μm mesh size, mouth area:0.25m2). Five species belonging to two classes were identified:Oikopleura dioica, O. longicauda and Fritillaria borealis belonging to class Appedicularia; Salpa fusiformis and Doliolum denticulatum of class Thaliacea. O. dioica and O. longicauda were the dominant species, occurring in the samples of all four seasons, with different distribution patterns. Their maximum abundance were1664.7ind./m3(spring) and1031.7ind./m3(spring) respectively. Following Oikopleura spp. was D. denticulatum, which was found only in autumn, with average abundance of149.6ind./m3and S. fusiformis, detected all the year long except for autumn with low abundance (max. abundance289.4ind./m3in summer). Only a very small amount of F. borealis was detected in summer samples, with average abundance of2.7ind./m3. The relationship between tunicates abundances and the environmental factors was analyzed using the stepwise regression model for each species. The variation of appendicularian abundance showed a significant correlation with the surface water temperature and with the concentration of chl-a. No relationship was found between tunicates abundance and salinity, likely due to the slight changes in surface salinity of the studied area during the four seasons. Salps and doliolids abundance was significantly correlated to bottom water temperature proving that these two species (S. fusiformis and D. denticulatum) migrate vertically in the water column. In particular D. denticulatum, known to be a warm water species, not only showed an important correlation with water temperature, but also a spatial distribution connected to the warm currents in the North Yellow Sea. The occurrence of D. denticulatum represented an interesting result, never experienced in past research works. Water temperature, algal distribution and currents were the most relevant environmental factors influencing the tunicate abundance and distribution in the North Yellow Sea. Further research is needed in order to get more information on the ecology of these organisms and to better understand their role in the ecosystem and in the oceanic food web.