Hermit crabs cannibalism?

[1st reef]

What are these guys doing here? 

[DaJMasta]

Could be fighting for food, or fighting for a shell, or just messing with each other.  They're somewhat ornery little creatures, so even if they're eating plenty you'll see this or similar sometimes.

[1st reef]

Still going at it. Been like 5 hrs. 

[Jakesaw]

I don't have blue crabs, but my Red Scarletts do this from time to time.  Maybe shell envy, maybe fighting for Alpha position in tank.  entertaining to watch.

 

Best I've seen was a crab 1 woke up from nap and was looking to pick a fight with another crab.  Crab 2 which was also napping saw him coming and started to crawl to side of tank and around the rock to back.  Crab 1 chased ( at crab speed ) him around the tank til he got tired out and gave up.  Was pretty funny.