darks aren't too much at all! you could probably even push them a lil more - don't forget about core shadow and reflected light! :) gotta make those shapes feel really rounded and volumous (which is a word i am making up right now.) also, even though it doesn't look like you are going to put in a background, i feel like i need to see how he stands -- the points of his legs make it look like they are too sharp to have any actual wear from walking, and it makes the ground plane vague. showing those other two legs a bit more might help.
I think it is getting, there! As far as your darks go, I think they are doing fine, but it looks like you are using a lot of black. It is making the crab look more brown than albino. Try to remember than when things are in shadow, they are more saturated. Try shading with some pinks, even. I like the growths on his right claw, but i don't see it much in other places. And the way he is posed, it looks like he only has four legs. I think we might see a hint of the other two. Overall, I think he is looking pretty mean, but the overall shapes are still really round, I would play with some straights in your design.
Very nice design. I think darks are just about right, but maybe highlight should be brighter. the light source (direction) is not very clear to me. I like the surface of his right hand. It emphasizes its evil looking. Maybe you want put it little more?