I haven't witnessed the hermits spawning. Snails and urchins I have seen firsthand. I know it happens though, and would consider it a sign of good health
Back when this tank was up and this was happening i could not seem to find any info of others witnessing it. At night a crab. Blue leg. Scarlet. Red leg. Dos not matter. But almost every other night one would climb to a high flow top rock and expose their tail section from out of the shell and snap it repeatedly. Tiny critters would fly around. They were mobile and be attracted to light.