Re: 5x 7 sandcrab
Oh very nice! I love the blue accents, the shading on the crab's back, the bright contrast of those black eyes. Very cheerful crab! The warm background and the crab's lighter limbs harmonize well. Good highlights and definition on all those legs. Well done!
The only suggestion I'd make is possibly to harden some edges on the crab's shell, either with a stick or by using the hard edge of a sponge tool - the wedge sponge for example, cut in close to the edge you want hard and pull outward using background color. To define hard edges or make a thin line across a joint, tap the end of a wedge sponge into color and then tap the line straight down where you want it. This may take a little practice but mistakes can easily be erased on Pastelmat.
For speed, pastel sticks or hard pastel or pastel pencil along the edge of the back at its rear, around the big claws, and with a mid value neutral where the two sides of the big claw are separated (a mid value gray-brown would be good, like umber and its tint or a mid gray) would help give that impression of stiff armor. Some of the soft edges can stay. You don't need to detail serrations or anything. But that line of the top and bottom pincer on the big claw is the main thing, a thin line in gray or grayish brown but not distracting black to compete with the eyes.
I love the white highlights on those shiny eyes! Really great!