The winkle (or periwinkle to some), Littorina littorea.
As a small boy, my mum would take me to the rock pools around Canvey Island and we would harvest the winkles. On the same trips we would walk out to the mudflats and go cockling. We'd take the winkles and cockles, Cardiidae, home and they would be our tea.
Winkles are very small and fiddly to eat, but if you gather enough of them it's a cheap way of feeding a family. All you need to add is a little vinegar and pepper, some bread and butter to accompany them and you're done.
These didn't come from Canvey they came from nearby Shoeburyness and we didn't eat them. It's a tradition that's died with my generation.
These were gathered because Cliff, who keeps marine fish, had a spare tank and wanted to use it to keep local sea creatures. The winkles, that always look so immobile when secured to a rock actually turned out to be a lively little bunch. We had to put them back after a couple of hours, because they kept escaping the tank and crawling around our house!
I respect nature's inventive ways of extracting energy from the void, so I won't scream in disgust of these, but... PLEASE TAKE THEM AWAY! PLEASSSSSEEEE! ;-)