Your work shows a lot of evidence of working from outlines and adding shadows and highlights to edges. This is not a good way to work, because generally areas of shadow and light describe form far more than outlines do. Aside from that I don't think the black interior lines are working and they look noisy, like these black lines running along the animal's back crystal do not seem to help showing the planes of the crystal. Black lines like this come across as very dark, nonsensical shadow.
I'm trying to show you what I mean with my edit here. In addition to generally using more contrasting shades, I am portraying your animal as if it were made of actual physical shapes like a cylinder and a sphere. My rendering is not the most accurate and I may have misinterpreted the shape of his limbs, but do you catch my drift? If there is a single light source one side of a form must always be in light and another in shadow. I think you owe it to your character design to try and make it appear solid, and not flat like a cookie.
I just read that the crystal is not a crystal but a fish spine. Silly me!
Like this?
If so I think your anatomy could use some work, but you can also suggested these spinelings running along the fin just by using the dark and light shade of the fin.
