Beauty or war?

I mentioned the other day that a teeny, tiny crab was living in the kelp holdfast. I was mistaken; there's a whole family hiding in those roots. I've caught three, so far.

The largest is 5 mm. across the widest part of the carapace. The smallest is less than half that, a mere crab-walking speck, even in a white bowl.

I took a few photos of the two largest in a bowl, and then in the holdfast, dodging behind the roots.

Three "teeth" are visible at the corners of his carapace.

This one looks pinkish. The little striped "flags" he waves constantly flash bright red when they catch the light. 

Then tonight, doing routine maintenance on the tank, I found a freshly molted carapace; a good chance to get a look at those black claws!

Top view. He's lost some of his colour; the carapace is actually translucent, so much of the colour of a crab is from the body underneath.

And bottom view, showing the pincers and black claws.

This is the black clawed crab, Lophopanopeus bellus, aka Xanthteo bella. The species name, "bella" or "bellus" comes from the Latin, "beauty", although it has also been mis-translated as "war", from "bellum". Not without reason, in this case; these are aggressive crabs.

The crabs are variably coloured, ranging from whitish or grey to purple, but they all have black claws. They may grow to about an inch and a half across the carapace, and are known to give quite a hearty pinch for their size. (This one drew blood!) They live in sand under rocks, or in the holdfasts of kelp, eating a bit of everything, from algae to other crustaceans.

The holdfast also is home to a fair number of worms. Food for the crabs, or competition for the food available? Or both?