Why would edible periwinkles be found on the middle shore rather than the upper shore?

One reason could be the time they can survive out of sea water. On the upper shore they are going to be exposed for the longest period, 12 hours, whereas on the middle shore they will be exposed for only 6 hours.





Edible Periwinkle Littorina littorea





This is the most well known of all of the periwinkles as they are commonly eaten. They are an adaptable species as they thrive in different salinities and exposure levels; and across most beach zones, making them very abundant.





At least in northern Britain Littorina littorea migrates down shore as temperatures fall in autumn (to reduce exposure to sub-zero temperatures) and up shore as temperatures rise in spring; migration depends on local winter temperatures. When exposed to the air, the species usually remains inactive unless conditions are very moist.





Also don't forget their food, algae. They are going to concentrate where this is most abundant.

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