HCA: Hermit Crab Association

Just a question. I have one crab who is missing both upper antennae from the base up. The hard exo bases of both are there, but nothing else. It doesn't seem to hamper him at all, nor is it causing any problems that I have noticed. My question is do they regrow the upper antennae only during a molt? Is it possible for them to grow something along the lines of a gel limb without fully molting to regrow them? Just curious; this is the first crab I've had with missing antennae.

I had a crab with two and half missing antennae when I bought her. She didn't grow back any until her first molt several months later, and then they were perfect.