Clarkii Clownfish are getting ripped fins

[basket440]

I have two clarkii clowns and I have started to notice that their pectoral fins are getting mangled. Even the ones face (by the gill) looks messed up the white stripe looks almost like its getting chewed off. I have a royal gramma and a yellow tail damsel and they look perfect! I read it could be a fight for dominance however, I am not seeing the fighting I just see the damage. I also have 3 peppermint shrimp and an arrow crab. It's a 55 gallon tank so its not like they have to be by each other (although most of the time the fish stay on one side of the tank). Any help would be great I don't want to lose my fish!

 

[Durango]

I think it can be the arrow crab!