Catching dinner in water isn't easy. If you lunge forward you'll most likely push the food out of range on your bow wave. Carrie Carreño and Kiisa Nishikawa explain that pipids, an entirely aquatic species of frog, cannot feed like other frogs because they lack a tongue, so how do they do it? One possibility is that they slurp food in by suction. However, it wasn't clear whether some or all pipids feed by suction. Some observers had found that some pipids appear to suck while others sweep food into their mouths with their forelimbs. The duo decided to film four pipid species and measure the pressure in their mouths as they fed to lay the controversy to rest (p. 2001).
Filming Surinam toads, African clawed frogs, dwarf African clawed frogs and Merlin's frogs as they fed, the duo could see that the food began moving before the frogs were close enough to bite. The animals must be sucking it in. And when Carreño and Nishikawa analysed the pressure profiles in the animal's mouths, they found that the pressure did drop and they could suck morsels into their mouths. In addition the African clawed frogs and Surinam toads usually swept their forelimbs in front of them to sweep the morsels towards their mouths.
So all four pipid species can, and do, suck food into their mouths, explaining how pipids have overcome the loss of their tongues when feeding.