Collonista snails have a pelagic stage. Probably a short one since they breed so well in tanks, but they do broadcast in to the water, so there's always the chance that if your timing was unlucky you could wind up with them in the water you use. No idea if Asterina stars broadcast spawn. They're starfish, so somewhere along the way there's sexual reproduction, but I think most people get over-run by them from splitting.
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