Spread in The Netherlands and across Europe of a mitochondrial haplotype of Eriocheir japonica (De Haan, 1835) in the invasive Chinese mitten crab, E. sinensis H. Milne Edwards, 1853 (Decapoda: Brachyura: Varunidae)

The Chinese mitten crab (Eriocheir sinensis H. Milne Edwards, 1853) is a widespread aquatic invader in Europe that causes significant ecological and economic damage. Some European specimens carry an Eriocheir japonica (De Haan, 1835) mitochondrial haplotype, although this has gone unnoticed for at least three decades. This haplotype is likely a result of an introgression in the overlapping native ranges of both taxa. Its presence in Europe is probably a result of a secondary introduction, but the ecological impact of introgressed Eriocheir De Haan, 1835 individuals remain unknown. Analysis of mitochondrial data collected over time in Europe revealed a turnover of Eriocheir genetic composition, with the E. japonica haplotype increasing in frequency over time. Consistent with this pattern, two Dutch Eriocheir populations (Den Helder and Nieuwe Statenzijl/Polder Breebaart) showed some of the highest frequencies of this haplotype recorded to date. The increasing dominance of this mitochondrial lineage underlines the impact multiple introductions can have. In light of this, two morphometric approaches and a newly developed (NaK) nuclear marker were evaluated to assess their utility for monitoring ongoing spread of mitten crab haplotypes and possible new introductions. The morphometric approaches proved insufficient to distinguish European E. sinensis specimens (regardless of haplotype), even from Japanese E. japonica individuals, highlighting the need for molecular monitoring tools. NaK proved effective to separate E. sinensis and E. japonica and thus provides a first step for wide-scale genetic monitoring of invasive populations. Integrating mitochondrial and nuclear genetic monitoring will be essential for the early detection of ongoing Eriocheir introductions and for understanding the invasion dynamics of E. sinensis in Europe.