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?