Sexing porcelain crabs

[COReeferCO]

Anyone know how to sex porcelain crabs? I have 3 rainbow anemones and bought 3 porcelain crabs only to find out they pair up. 2 paired up and kicked the 3rd out. I moved the 3rd nem way across the tank and the 3rd crab moved in. I'd like to get it a mate but I'm not sure how to tell male from female or if it matters. Can they switch sexes?

 

[James M]

Flip them upside down. Females have a broader flap than the males

 

[Jmas4]

I'm pretty sure sexing is the same with all crabs. Males have a light house shaped underside while females have a dome shaped one.
https://goo.gl/images/QnTih8