Hokkaido is crab country, and the Hokkaido crab sando is the sandwich that leans hardest on that fact. The filling is the island's prized crab, usually taraba (red king crab) for sweet, meaty legs or kegani (horsehair crab) for a richer, more delicate flavor, and the entire build is arranged so that crab is what you taste first, last, and most.
Crab is a difficult sandwich protein for one reason: it is watery and fragile, and it will turn a soft loaf to paste if handled carelessly. A good version respects that. The meat is picked carefully to keep recognizable chunks rather than shredding it to a slurry, gently squeezed of excess moisture, and bound with a restrained amount of kewpie mayonnaise, just enough to hold the pieces and carry seasoning without burying the crab's clean sweetness. The bread is almost always a fine-crumb milk loaf, crusts off, inner faces buttered to build a moisture barrier between a wet filling and a delicate crumb. Some shops add a thin layer of cucumber or lettuce for crunch and as a sponge. The bind you want is a cohesive crab layer that holds large flakes, mayo present as a thin slick rather than a sauce, and bread that stays dry to the cut edge. The failure mode is a pale, over-mayo'd salad with no discernible crab texture, the bottom slice already soaked and slumping; at that point you are eating an expensive seafood-flavored mayonnaise sandwich.
The eating experience should be light and clean despite the richness of the crab. The meat is sweet and faintly briny, the mayo adds a little acidity and egg-rich body, and the buttered milk bread is deliberately quiet so it frames rather than competes. A squeeze of lemon or a few capers is common to lift it; without some acidity a whole sandwich of rich crab and mayo can go heavy fast.
Variations track the crab and the binder. A taraba build is meatier, sweeter, and reads more substantial; a kegani one is more delicate and prized for flavor over volume. Some shops fold in a little crab miso (the tomalley) for depth, which pushes it richer and more savory. A version with sliced avocado adds fat and turns it creamier. The other two Hokkaido-produce sandos, the salmon and the butter sando, share the regional logic but behave nothing alike in bread, and each deserves its own article rather than being crowded in here.