The crab sandwich is the dressed-crab build at its most restrained, and what defines it is how little it does. Fresh crab, both white and brown meat, goes on bread with butter and a squeeze of lemon, and that is the whole sandwich. There is no mayonnaise bind, no herb, no second component. The discipline is to get out of the way of a good crab and let it be the entire statement, which is why a seaside crab sandwich made with the morning's catch needs almost nothing and a poor one cannot be rescued by what is added to it.
The craft is the balance of the two meats and the restraint around them. White meat is sweet, mild, and flaked; brown meat is soft, rich, and intensely savoury, closer to a paste than a flake. Used alone the white is delicate to the point of thin and the brown is too strong to carry a sandwich; the build works because they are layered or lightly combined so the brown deepens the white without drowning it, a ratio judged by how rich the sandwich is meant to read. The crab is handled gently and never beaten smooth, because the flake is the texture that makes it crab rather than paste. Lemon is applied as a squeeze, not folded through, so it lifts the sweetness without slackening anything, and butter on soft brown bread is the only lubrication, bridging the meat to the crumb and sealing it against the brown meat's moisture. The bread is plain because a good crab is loud enough alone and an assertive loaf would only mask it.
The variations are the rest of the coastal shellfish shelf, each the same restraint met in a different catch or a different single note. Crab and lemon folds the acid into a mayonnaise bind; crab and chive adds a mild onion herb through that bind; crab paste takes the shellfish to the concentrated, shelf-stable end of the larder; the prawn and crayfish builds swap the catch entirely. Each deserves its own article rather than being crowded in here.