Figure 1

(a) Map of the UK denoting the Solent survey region. (b) Locations across the Solent region where A. senhousia has been detected. (c) Distribution of A. senhousia in the Solent focusing on Southampton Water and its tributaries as determined by EA benthic surveys. Dashed rectangle in (b) denotes area (c). Black fill indicates presence of A. senhousia, white fill indicates absence. Overlapping symbols are layered in order of year (most recent at the top). Symbols without numbers are Environment Agency (EA) survey sites (site numbers excluded to maximise clarity of map); EA site locations and associated A. senhousia densities can be found in Supplementary Table S2. Numbers 121–130 refer to surveys by other organisations (see Table 1). Mean densities for all surveys can be seen in Table 1. Site 128 is not a specific location but represents one individual found in Chichester harbour. Map created using ArcGIS Pro 2.6 https://pro.arcgis.com/. The intertidal shore at Brownwich (Fig. 1; site 122) was comprehensively surveyed in 2019. Compared to the subtidal sites in Southampton Water, the population density was low, with only 169 individuals recorded equivalent to 0.06 m⁻² (Table 1). Single individuals were found mainly on the higher part of the shore partially buried in the sediment. None were attached to seagrass (Zostera spp.), however, when removed from the sediment a number were attached by their byssal threads to dead cockles (Cerastoderma edule) (empty shells) and living individuals. Arcuatula senhousia shell lengths ranged from 9 to 32 mm (mean = 20.1 + /− 3.9 SD).