Long-term population variability and reproductive strategy of a northward expanded ctenophore Pleurobrachia globosa Moser, 1903 in a temperate bay, China

A ctenophore Pleurobrachia globosa Moser, 1903, recognized as a warm water species, showed abrupt population rise since the year 2000 in a temperate bay–the Jiaozhou Bay (JZB), a coastal bay in northern China, >1100 km north from the reported native habitat. Using hundreds of surveys in the JZB and the adjacent Yellow Sea (YS), we examined the population fluctuations over the recent 20 years, possible links between abundance and environmental factors, and the reproductive strategies. In the JZB, P. globosa mainly appeared from August to December each year, reaching the maximum abundance of 170 ind. m−3 in August 2008. A general warming trend continues in the JZB during the recent two decades, with the alternating of warm and cool periods. Temperature is recognized as a primary driver of abundance variations, with significantly higher abundance levels in warmer years than in cooler years. The population uses either larval reproduction or adult reproduction almost exclusively in the JZB, with unknown mechanisms. High abundance can result from both reproductive strategies. Larval reproduction distinguishes itself from adult reproduction by significantly smaller parent size, egg size and clutch size, explaining the short-size dominated population in several surveys. Clutch size is positively related to body size and temperature. The geographical range of the YS population reached to ~39°N in June 2014, indicating a northward expansion since the late 1990s, together with other evidences from the JZB and the Bohai Sea. While the successful establishment could be driven primarily by warming, lack of competitors and predators as well as the particular life history strategies also contributed. Issues of the vectors for introduction, local overwintering population, and regulatory mechanism of reproductive strategy need to be addressed in further studies.