Biology and life cycles of pelagic tunicates in the Lazarev Sea, Southern Ocean

Four grid surveys were carried out in the top 200 m layer of the Lazarev Sea during fall 2004, summer 2005–06, winter 2006 and summer 2007–08 onboard the R.V. Polarstern as a part of the German SO-GLOBEC. The distribution, abundance and biology of two species of salps, Salpa thompsoni and Ihlea racovitzai, were investigated. With the exception of fall 2004, I. racovitzai dominated the salp community although being represented by low densities (<20 ind. 1000 m−3). S. thompsoni was scarce during the summers of 2005–06 and 2007–08 and almost absent from the region during winter 2006. Nevertheless, it was modestly numerous during fall 2004 reaching densities of up to 33 ind. 1000 m−3 in the south-western stations of the grid. The data on the seasonal population structure and life cycle of I. racovitzai showed that this species followed the generalized pattern typical of S. thompsoni, i.e. sexual/asexual reproduction and spawning during fall. I. racovitzai densities were the lowest during summer, increased during fall and peaked in during winter. Numerous offspring were produced by I. racovitzai during fall, just before the area became ice-covered. Conversely, S. thompsoni was not able to complete its life cycle in the Lazarev Sea, with a high occurrence of stage X (unfertilized) aggregates present. Highest S. thompsoni densities in summer and fall, and its disappearance in winter are indicative of a population of the expatriate origin that is sustained by advection.