Crabs fishing or mating?

I have several thin-legged hermits in my tank. The medium one has been here 3 years and other 3 were added last month.

Today I saw the large one and the medium one fighting. When I looked closer the large one was not trying pinch the smaller one but trying to move his/her legs from what I could tell. Are they fighting/establishing dominance or is this a mating behavior?

 

Possibly or feeding behavior.

 

Add some empty shells of varying size & see what happens. They can be picky though, about what shell they will use. In my experience, they like a shell significantly bigger than they are, but lightweight. Some prefer right or left side opening, as well. Your medium looks big enough to be coveting the big one's shell LOL.

 

hermits will deff kill each other over shells even with extra shells in the tank.

 

Yes, apparently there are "primo" and "not-so-primo" shells LOL.
BTW, in my previous post I meant this type of crab in particular seems to like a really big shell. One that I had was so well hidden in the 1 1/2" shell the store gave me that both of us thought there was nothing in it, and we looked really hard. They have really long legs & like to be able to pull them all the way into the shell. After 2 years he was in a shell that the body of was the size of my fist. They get very large and are very clumsy, knocking stuff over and falling off the rocks LOL. Never picked at anything, though, but I always hand fed when it got large...just watch your fingers!

 

Doesn't look like fighting behavior to me. Whenever my hermits fight the smaller one is usually attempting to run away, this looks like the smaller one cooperated in getting their shells in that position.

Also I have a couple of these crabs and they really seem to dig nerite snail shells.