Tufted ducks dive up to 5 m of depth to feed. They eat mostly zebra mussels where they are available, whereas in places where zebra mussels are not widespread, their diet is more varied, with crustaceans, insects, seeds and some molluscs. The mussels, which are of small size are swallowed whole and crushed in the muscular gizzard. The small mussels are sucked and filtered, while the larger ones require more manipulation.
and down it goes!More information
Olney, P. J. S. (1963). The food and feeding habits of tufted duck Aythya fuligula. Ibis, 105(1), 55-62. here.
Joep J. De Leeuw and Mennobart R. van Eeerden 1992. Size selection in diving tufted ducks Aythya fuligula explained by differential handling of small and large mussels Dreissena polymorpha. Ardea 80:353-362. here