A feeding walrus works the seafloor with its muzzle, using the dense array of vibrissae to locate buried bivalves and jetting water from its mouth to excavate them. Once a clam is found, the walrus seals its mouth around it and retracts a large muscular tongue, generating suction strong enough to draw the soft tissue free of the shell.
The evidence is on the plate. Walrus stomachs are typically full of clam siphons and feet with almost no shell fragments, a signature of suction feeding rather than crushing. The palate is vaulted and the tongue piston-like, both consistent with that mechanism.
The tusks, meanwhile, are used mainly in social display and dominance contests and to haul the animal out onto ice, which is what the genus name Odobenus, tooth-walker, records. The idea that walruses rake the bottom with their tusks is a persistent misconception; tusk wear patterns are more consistent with dragging along the substrate than with active digging.