Kani sando is the crab entry in the Japanese deli-salad sandwich family: crab, bound with mayonnaise, between slices of soft shokupan. The crab is more often canned crab or kani-kama, the imitation surimi crab stick, than fresh leg meat, and that is a feature of the genre rather than a compromise, since the sweet, mild, slightly springy surimi takes the mayonnaise cleanly and slices into a uniform pale-pink filling that holds its shape against white bread. It sits beside the egg sando and the tuna sando as a convenience-store and bakery staple, the cold, creamy, tidy kind of sandwich Japan does with particular care.
The build is a study in restraint. The crab is shredded or flaked fine, sometimes with a little cucumber or shredded lettuce folded in for a cold crunch and to keep the filling from reading as one soft note, then dressed with a Japanese mayonnaise that is sharper and more egg-rich than the western kind. The bread is milk-loaf shokupan, soft and faintly sweet, usually with the crusts trimmed so the cross-section is clean. The dressing wants to be enough to bind and gloss the crab without drowning its delicate sweetness, and a touch of salt or a squeeze of lemon keeps it from going flat. A good one has the filling spread thick and even all the way to the crust line, the crab still distinguishable as strands rather than a paste, the bread soft and dry-bottomed, a clean chilled cut. A sloppy one is over-mayonnaised to a featureless slick, the filling thin and pushed to the middle so the edges are bare bread, the surimi bland and watery and the loaf going gummy underneath.
The variations stay within the cold-salad logic. Some shops use real snow-crab or blue-crab meat for a richer, sweeter, more expensive version; some fold in corn, cucumber, or a little chopped egg for texture and color. A squeeze of lemon, a dab of wasabi, or a few capers sharpens the higher-end ones. Push the format into a hot, breaded interpretation and you arrive at the crab cream croquette sando, which is a different texture entirely and deserves its own article rather than being crowded in here.