The oyster is a marine bivalve eaten live on the half-shell, a glistening, silver-grey morsel cradled in seawater that is among the purest expressions of the ocean any food offers. The Pacific oyster (Crassostrea gigas) is the workhorse of the world's farms, cup-shelled and frilled, while the prized European flat oyster (Ostrea edulis) is rounder, scarcer and more minerally. A raw oyster smells of clean sea air and tastes briny, deeply savoury and metallic, lifted by a startling cool note of cucumber and melon. Because the animal filters its food straight from the water, its flavour is shaped by where it grew — its "merroir" — so two oysters of the same species can taste of utterly different seas.