Seasonal occurrence of the alien freshwater shrimp Palaemon sinensis (Sollaud, 1911) in lower reaches of a river in western Japan.
Palaemon sinensis, a shrimp native to China, has now been recorded in numerous freshwater zones in Japan. The present study is the first report of the seasonal occurrence of P. sinensis in a river environment. Monthly field research was conducted from December 2015 to November 2016 at three sites in the lower reach of the Seno River, Hiroshima Prefecture, western Japan. At Stn. 1, in a side pool of the main river, P. sinensis was found throughout the year. Ovigerous females were present there from April to July, and undifferentiated juveniles from July to September. At Stn. 2, at a side bank of the main stream, the species was collected in December, January, and August to November; water velocity when P. sinensis was collected ranged from 3 to 8.5 cm/s. At Stn. 3, in a tide pool in the estuary zone, the shrimp was detected during months with relatively heavy precipitation (April and June) and in September-November; the range of salinity when P. sinensis was found was 0.1-22.7 ppt. No ovigerous females or undifferentiated juveniles were collected in the main stream or in the estuary zone. Together, these findings suggest a pattern wherein P. sinensis eggs hatch in riverside pools from April to July, with individuals then growing to a sexually differentiated size before dispersing and entering the main river stream, from approximately August to November, after which time they are probably only passively transported with the current into the estuary zone.