I have a couple of old hermits that are spawning. I don't remember when I got them but it is at least 12 years or longer ago.
The male in the foreground has her pushed up against that bubble coral. He is pushing her so hard that he is lifting her [i]and[i] the 8" bubble coral. I am surprised that he can lift all of that with his right claw being he is a lefty.
How do I know they are spawning?
I will ask him when he comes back from having his cigarette.

I souldn't put this picture on here, but here they are in a "friendlier" pose

Did you get them to spawn successfully and were you able to rear the young?

I have had crabs spawning for years and I never raised the young. They swim around as plankton for a few molts before they settle down to the bottom. It would be very difficult to raise these things.
To see the babies hatch you need to watch at night with a flashlight. You can see them swimming but just barely.
The eggs are held inside the borrowed crabs shell and can't be seen. You have to tell by watching them for years to get to know their behaviour.
I am sure I am wrong many times.

Thanks for the extra info.
I had peppermint shrimp babies in my tank recently and it was awesome watching them before they all got eaten by my wrasse and probably by my corals also.