Where do snail abbies come from?

[raisemyrent]

Hi,

I found a snail looknig thing eating off the rock sonce, pictured it, never saw it again. It had no shell. Like a month later I noticed it around, it's a snail alright... it's just growing its shell slowly... It looks A LOT like my other 2 astreas...

question: where did it come from? did my astreas get personal under a rock? I certainly didn't buy it and it was months and months since I last added anything to the tank...

oddly enough, my sexyshrimp disappeared (read: died) and my nassarius passed away too...