A good question. My Fx BGxRES have demonstrated the wide variation of easily observable BG and RES traits (coloration, mouth size, body form) that one would expect. Given this, I strongly suspect that some of them have good enough pharyngeal teeth to eat snails.
I can confirm that I never see any snails in that (or, FWIW, my other) pond. But then both ponds should/could have a population of pure RES (that are not targeted and very seldom caught).
| 
