Is it this?
"It might sound odd that invertebrates such as these will often reproduce in aquariums, but I've seen it happen on many occasions. Most have separate sexes, but many are hermaphroditic, some being simultaneous hermaphrodites, and they sometimes spawn in aquariums, which is often a tank-wide event.1,6 I've seen dozens of them simultaneously emerge from their hiding places in rocks and such, climb up the tallest thing they can find, and then start releasing tiny puffs of gametes into the water. Some can also brood their young, which are kept in pouches in their bodies and released as miniscule juveniles from the parent."
From: http://www.advancedaquarist.com/2014/6/inverts
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