How would the "first fish" with a grooved mouth structure "know" that it could spew water that way to hit a target insect? As far as I know, no other fish shoots jets of water for any reason, dolphins and whales not being fish. Therefore, the whole evolution precept would seem to fall flat. A "learned skill" would have to be passed down from generation to generation via outright teaching or observation. If another fish that did not have this "grooved mouth structure" had the opportunity to watch and learn, it still could not do the same function............
"As far as I know, no other fish shoots jets of water for any reason,"
Hmm, slow changes over time to develop a specialty that insures survival. Sounds a lot like.... evolution.
It wouldn't "know." When it happened to be facing in the right direction it would get the extra food.