[Celtjew Libertarian]

How would the "first fish" with a grooved mouth structure "know" that it could spew water that way to hit a target insect?

It wouldn't "know." When it happened to be facing in the right direction it would get the extra food.

[Red Badger]

When it happened to be facing in the right direction it would get the extra food.

And water just happened to come forth out of its mouth for no obvious reason?...................