Crab paste is the shellfish member of the paste shelf seen from the jar rather than the bread, and it is governed by the same inverted ratio as the rest of that row: the filling is the thing you use least of. Crab is cooked, cured, reduced, and pounded with fat and seasoning into a smooth, salty spread that keeps in a sealed jar at the back of a cupboard, and the entire design follows from that keeping. A paste built to survive months unrefrigerated is concentrated to a point where a thin scrape across buttered bread is already a full-strength mouthful. Spread it like a filling and it is too salty and too strong to eat. Spread it like a stain and it is a sandwich. It is the economical way to put crab between bread when fresh crab is neither cheap nor to hand.
The craft is the scrape and the butter beneath it. Butter is not optional lubrication here; it is the carrier that drags the paste's salt evenly across the slice and stops a thin smear reading as a smear rather than a flavour. Crab paste keeps the sweet, faintly briny register of the shellfish but loses the flake entirely in the pounding, so it brings flavour and no texture of its own, which is why the bread under it is always soft and plain: a crust with real chew would have nothing to chew against. There is no heat, no second layer, no acid in the jar itself. It is built to be assembled cold, cut thin, pressed, and left in a tin until lunch, and it sits well because the paste works into the crumb rather than weeping out of it. Its whole logic is that a very small amount of something concentrated turns bread and butter into a meal.
The variations are a row of jars rather than a row of recipes. Salmon paste carries the same potted idea into another fish; bloater paste pushes it to the strong, smoked end with herring; beef and chicken paste swap the shellfish for cured meat on the same engineering. The made sandwich, the crab paste read from the bread rather than the jar, is its own consideration. Each deserves its own article rather than being crowded in here.