When do molters pick a new shell?

[Gail]

I've never really paid attention to when my crabs move into a new shell. They are all close to the same size and the shells I have are mostly the same.
I do however have one crab that has stubbornly refused to change shells. If he molted, he put the same shell right back on. Lately he has been eating like mad, practically living in the food dish. Three days ago he finally gave up his beloved magpie for a turbo then promptly disappeared in the substrate.
I always assumed they would choose a new shell after coming up from a molt but he seemed to take a new one with him.

[LadyJinglyJones]

Nah - crabs change shells when the mood strikes them and it's common for them to switch before a moult.