I think it may be a case of a non-native species showing up. Maybe they weren't bucket stocked into Canyon's pond by a neighbor, but somewhere, sometime, those snails became a part of the ecosystem.
Maybe they migrated from pond A to pond B, without a bucket. Or maybe an animal carried them. But if they are non-native, then how did they end up in pond A in the first place?
If we cared to chase it back, I'm betting we would encounter a human presence somewhere.
