Tuna crunch takes the bound tuna baseline and answers it with structured crunch, and the name is doing real work. The tinned tuna and its measured mayonnaise bind are the constant; the variable is a deliberate set of hard, raw, finely chopped vegetables, classically celery and pepper, added not for flavour but for texture. Where sweetcorn pops and cucumber cools, the crunch build aims at a sustained, brittle bite running all the way through the filling. The word is also a shelf word: it is the name supermarket sandwich ranges give to this particular tuna, so the sandwich carries a faint branded identity, a tuna mayo that has been sold as an upgrade rather than a base.
The craft is keeping the crunch present without unbalancing the bind. The celery and pepper are diced small and even so the texture is distributed through every bite rather than landing in occasional stringy lumps, and celery in particular is patted dry, because it carries water that would slacken a filling that depends on a firm bind. The vegetables are folded through at the end so they stay crisp and audible rather than softening into the mayonnaise. Proportion is the decision the whole thing turns on: enough hard vegetable that the sandwich earns the name, not so much that the tuna is reduced to a binder for a vegetable salad. The tuna is bound to the same just-cohesive point as the baseline, the bread is soft and plain, and the filling is spread evenly so the crunch is consistent corner to corner.
The variations are the rest of the tuna family, each leading on a different counter to the same fixed bind: cucumber for cool water-crisp, onion for raw bite, sweetcorn for the sweet pop, plain mayo for the baseline. Crunch is the one that made texture the whole proposition and put a brand name on it, and adding sweetcorn or red onion to it edges it back toward the other members. Each deserves its own article rather than being crowded in here.