Evidence for self-sustaining populations of Arcuatula senhousia in the UK and a review of this species' potential impacts within Europe.

The invasive Asian date mussel (Arcuatula senhousia) inhabits diverse global coastal environments, in some circumstances posing significant ecological and economic risks. Recently recorded in the Greater North Sea ecoregion, an established population has not previously been confirmed. Combining historical and field data, we provided baseline information from the UK and recorded colonisation in a variety of habitats. Gonadal development was assessed using the gonadosomatic index (GSI) to determine if an intertidal soft-sediment population is self-sustaining. Arcuatula senhousia records from subtidal muddy/mixed-sediment within a major estuarine system from 2007 to 2016 were also analysed. First detected in 2011, spatial distribution was variable across the years within the subtidal, with individuals found at 4-9 out of 25 sites, and densities per site varying from 10 to 290 individuals per m. The intertidal population was, in part, associated with seagrass (Zostera spp.) and attached to bivalves. In marinas, individuals were attached to concrete tiles, associated with live Mytilus edulis, and to dead Ostrea edulis. Mean GSI from the intertidal population differed across months, peaking in July before declining in September/October, but with high inter-individual variability. Arcuatula senhousia is reproducing and maintaining viable populations. Using a natural capital approach, we identify the potential impacts on Europe's functionally important habitats, fisheries and aquaculture if its spread continues.