IMIS

48 snails were collected in the field (Centinela, Baiona, Spain; coordinates: N 42° 4' 38.06", W 8° 53' 47.47") in the wave, crab and hybrid zone respectively (total sample size: 144). Prior to collection, the habitat and position of each individual was recorded with a survey instrument (total station) to <10cm resolution. Specimens were transported to the lab where they were marked with individual waterproof labels and non-toxic nail polish along the margin of the aperture (to track growth); shell thickness was measured with a gauge; shells photographed for morphology; and DNA collected from mucus (non-destructive DNA sampling protocol using TFA cards). Individuals were then released in the field: each collection group (crab/hybrid/wave) was randomly split in three subgroups and released in the hybrid, crab and wave habitats, respectively, recording the precise release position and habitat characteristics. One day later, the positions and habitats of marked snails was recorded for movement. This round of collection-mark-release was repeated four times (one release included only 48 snails from the hybrid zone due to not permissive weather), for a final number of 480 released snails. One month later, in June, surviving snails were recaptured, with positions and habitats recorded, phenotyped (growth, weight) and dissected for sex determination and tissue collection for additional DNA analysis.