Daphnia pulicaria, a zooplankton species that suddenly appeared in 1999 in the offshore zone of Lake Biwa

Daphnia galeata has been the sole Daphnia species and one of the dominant zooplankton species in Lake Biwa for the past 30 years. In 1999, however, another Daphnia species suddenly appeared in the lake. Based on morphological characteristics, the species was initially identified as Daphnia pulicaria. This identification was supported by mitochondrial (mt)DNA sequences of the 12S rRNA and NADH-5 (ND5) genes. In addition, comparison of the latter sequences showed that D. pulicaria in Lake Biwa is genetically very similar to populations distributed across the western regions of North America rather than those in other regions, including Europe. Thus, it is likely that D. pulicaria in Lake Biwa is a zooplankton invader from North America. Through routine sampling, we first detected D. pulicaria in winter 1999. This species gradually increased in abundance and reached a maximum density of 10 000 individuals m^-2 in mid-May 1999. Thereafter, it decreased in abundance and almost disappeared from the water column after August 1999. D. pulicaria are much larger than D. galeata and are distributed in deeper water. They showed a marked diel vertical migration, although substantial numbers of individuals stayed above and within the thermocline, both during the day and at night. Based on seasonal and vertical distributions, we discuss the ecological implications of the sudden appearance of D. pulicaria in Lake Biwa.