I know you have many questions about above Render....
But the truth is I have not used any composition...
STEPS how I did.
1>Modeled the mesh.
2>textured it( I first baked normals in blender , then I created diffuse and other maps)
3>I just used one lamp to set up lighting setup .
4>for background I added a basic plane and inputed the textures.
5>I used PBR shader to render the crab
Hope u like it,,,,,cheers for Blender