The upper limit of distribution on a shore is increased by shade and exposure. In some situations seasonal variations in sunshine causes a downward migration in spring/summer and an upward migration in autumn/winter, though the upward movement is not confined to individuals which originally occupied the high levels (Lewis, 1954). Spat settle in pools and damp places, becoming vagrant after emergence for the first 1-3 years and then settle down to a homing lifestyle, although swapping of home does occur. Adult Patella vulgata generally return after feeding to their so-called 'home scar', a depression in the rock formed by abrasion of the rock surface by the shell, resulting in a tighter fit to the rock and reduced risk of desiccation. Patella vulgata is tolerant of low salinities extending into the mouth of estuaries, surviving in salinities down to about 20 psu (Fish & Fish, 1996).