What does a winkle eat other than crabs?

Winkles or periwinkles don't usually eat crabs, they tend to graze on algae in the intertidal zone with their scraping mouthparts, but do sometimes eat small barnacle larvae. It would mostly be the other way around - crabs eating periwinkles. Both are somewhat opportunistic feeders and crabs are known to be omnivorous, as well as mollusks like the periwinkle will eat worms, other crustaceans, fungi, detritus, and also algae. Some crabs specialize, and some eat fish.