The colonization process of non-indigenous Chydorus brevilabris (Crustacea: Cladocera) into Lake Biwa revealed by long-term collection samples

Abstract Although the genus Chydorus Leach, 1816 is benthic, their abundance suddenly increased in the pelagic Lake Biwa north basin in the 1990s, according to a previous paleoecological study. To understand what exactly happened in the lake, we examined collection samples (Center for Ecological Research, Kyoto University, and Lake Biwa Museum) from Lake Biwa from 1980 to 2002. Specifically, we re-identified Chydorus individuals in offshore and shore collection samples with updated taxonomy and examined their daytime vertical distribution in the offshore north basin. We confirmed that all individuals in the offshore north basin samples were non-indigenous Chydorus brevilabris . This species was first collected in November 1993 and was collected intermittently in subsequent years, with the highest abundance in 1998. On each sampling date, most C. brevilabris individuals were concentrated in the 0–5 m layer. In contrast, we identified an East Asian native Chydorus species (previously believed to be C. sphaericus ) in the shore samples collected in 2001–2002, along with a smaller amount of C. brevilabris . We consider that C. brevilabris is more durable offshore than the native Chydorus because the former species was exclusively found offshore. We conclude that the aforementioned paleoecological study depicted an increase in non-indigenous C. brevilabris in Lake Biwa in the 1990s. The offshore durability of C. brevilabris could mitigate the founder effects of native Chydorus . As such, historical collection samples provided detailed insights into the colonization process of non-indigenous C. brevilabris .