Possible origins of planktonic copepods, Pseudodiaptomus marinus (Crustacea: Copepoda: Calanoida), introduced from East Asia to the San Francisco Estuary based on a molecular analysis

The calanoid copepod Pseudodiaptomus marinus Sato, 1913 is native to the coasts of East Asia, but has occurred in the San Francisco Estuary (SFE) as a non-indigenous species since 1986. Genetic analysis of the mitochondrial cytochrome b locus (400 bp) was used to infer the possible origin(s) of the introduced SFE population by comparison between the SFE population and populations from potential donor areas in Japan, Korea, and China. We detected 39 haplotypes from 131 individuals from 7 localities. The SFE population showed a high haplotype diversity and significant Fst values among the localities, suggesting that multiple introductions from several localities in Japan may have occurred, although introductions from Korea could not be ruled out. However, based on the morphological and molecular data the population from Xiamen, China, previously identified as P. marinus, was found to be a separate species. Possible vectors for P. marinus include ship ballast water or aquaculture. Our results also suggest the origin of P. marinus in the Asian continent and subsequent isolation and expansion of populations in Japan.