With that crab, i think you've shaded with in mind that the futher from the light source, the darker is gets in a linear (or quite close) line..
This makes it feel, that it's flat.. Also the pallete is horrible, there isn't enough Contrast (search if need be), and the lightest colour is huge in Saturation (again search if needed).
Try, and think how the light would affect by shape, by looking at it more as a shape than art. this will help you decide how light would act on the shape