We have a small plastic pond on our 2nd floor deck (we put it up there to keep it away from predators since we have a turtle in there and we still get an occasional raccoon trying to heft her out of the water). Last year the kids caught a crayfish and we put it in the pond. Then I think they caught another and put it in there, too. But in May, husband drained it to clean out the algae and there was only 1 crayfish, so we decided to put it in our goldfish tank, where it's been ever since. husband drained and cleaned the pond again this past fall -- no sign of crayfish still. Yesterday, difficult child 2 was cleaning out the pump filter and he comes in to tell me about these tiny silvery creatures burrowing in the filter mesh. He thinks maybe they're water lice. I go out to look at them and then go online to find out what they are -- which was NOT water lice! They almost looked like baby pill bugs/roly poly's/sow bugs whatever you want to call them. I got a small glass dish and put some rubbing alcohol in and dropped one of the critters in to get it to uncurl for a closer look since it's about the size of a large grain of rice. I'm about 95% sure they are crayfish larvae! But how in the world did they get in there???