A Sandwich aux Fruits de Mer is a mixed-shellfish build, and the word that does the work is mixed: not one creature but a small assortment, picked and chilled, bound just enough to stay in the bread. The usual range is shrimp, a little crab, sometimes a few small mussels or whelks out of the shell, dressed with a thin mayonnaise or a squeeze of lemon and a turn of pepper. It reaches coastal France, where the catch is varied enough that a sandwich can be an assortment rather than a single species.
The build is governed by perishability and by texture. Shellfish goes flat and weeps water if it sits, so the assembly happens close to service and the dressing stays light, there to carry salt and acid rather than to mask anything. A split crusted loaf holds the mound loosely so the pieces keep their separate bite; pack it tight and the assortment turns to paste. There is no warm element and no reason to wait. The work was done by whoever sorted and cooked the shellfish; the sandwich just keeps it cold and intact to the bite.
The Sandwich aux Fruits de Mer sits with the cold-water builds the catalog groups under Baguette Poisson. Its specific contribution is the assortment itself: a sandwich whose character is a small mixed catch rather than a single fish, decided by what came in fresh that morning.