As I understand it birds deposit them as part of their life cycle in their droppings. Same with mussels.
Then you have guys like me that like RES and actually inoculate a
pond dedicated to raising RES with snails and snail eggs attached to water primrose I transferred from one of my other ponds.
I looked just yesterday and I have a very good population of snails along the bank line out to a couple inches deep water, then nothing. I have often speculated that the RES keep the snails "cleaned up" up to as shallow water as they dare to go. Where the water is too shallow for the RES to go the snails thrive. Out in deeper water you have to turn over a rock or dead tree leaf to find snails where they can hide from the RES. At least where there is a large enough RES population to utilize all the snails.
In absence of enough snails the RES will eat just about anything BG will eat including FHM, fish larvae, invertebrates, etc. Some of them will eat pellets if exposed to them but they do not pellet train nearly as easily as BG. From my observation in an aquarium of small RES that I was feeding they would be "interested" in the pellets as long as they were sinking or moving. But as soon as the pellet hit the bottom and was still they would stare at it then leave it alone. If another fish would make some water movement that caused the pellet to move then the RES would get interested again and sometimes eat the pellet. It appeared to me the RES keyed into movement of the prey. When I put live snails in the RES would see a snail move, race to it, stare at it a short time, then when it moved again would eat it. As long as the snail was still they did not seem to bother it. I surmised this need to see movement in their prey is one reason they do not pellet train easily. That is my speculation based on anecdotal observation.
