Sample collection
22 ditch water samples were collected from ditch surface water by Natural England staff at Stodmarsh NNR between the 16th and 27th November 2020 (Figure 2). 10 ditch water samples outside the known range of S. nitida were collected by ADAS staff between the 10th January and the 4th February (sample information can be found in supplemental file 1, table S1 and S2). 20x 30 mL water samples were collected along the length of each of the ditch and pooled into sterile sampling bag. Up to 500 mLs of this water was filtered through a 0.22 µM PES sterivex filter (Millipore) using a 50 mL luer-lock syringe (Table 1). After filtration of water, 95% ethanol was added as a preservative.
Snail specimens were collected during an invertebrate survey carried out at the same 22 ditches at Stodmarsh NNR by Dan Bennett of The Ecology Co-op, contracted by Natural England between December and January 2020/2021 (specimen information and detailed sampling methodology can be found in supplemental file 1, table S3). Manual surveys of the 22 ditches were performed using an adapted invertebrate sampling protocol from Drake (2007). The emphasis was on the free-style netting of suitable looking micro-habitats (e.g. emergent vegetation stands) that are likely to be most productive for this assemblage. Effort was deliberately not divided in proportion to the extent of features nor length of ditch, since species are not distributed in this fashion. Snail specimens from S.nitida and nine other similar ram’s-horn shaped species, from the 16 species of snail ) found during invertebrate survey were identified and preserved in 95% ethanol prior to shipment to the laboratory: Anisus vortex, Bathyomphalus contortus, Gyraulus crista, Gyraulus albus, Hippeutis complanatus, Planorbarius corneus, Planorbis carinatus, Planorbis planorbis, Segmentina nitida, Valvata cristata.