The shading is actually pretty good for starters. You defined shapes within the shape, giving it a very good 'blob' feel. The problem is, though, that you've given it no specific lightsource. The middle is lit, but that's the only part it seems the light is hitting from. I mean... it is a lightsource, but not a very productive one. So what you have now seems almost pillow-shaded, where you shade it from the outline in.
Also, other things I think you need to change is the dithering, and outlines. You've dithering an object that is gelatin in nature, very slimy, slick and glossy. Dithering completely kills this vision and gives it a rough texture. I think you need to go back and remove the dithering to smoothen things out a bit. It might also help to give it some nice highlights to define the gloss of the blob. About the dithering, black is strong and hard to look at around sprites most of the time, especially without AA. It would most likely look better with a dark green outline, or perhaps even without one at all(probably stretching it), and this goes for the rest of the sprite too, like the teeth and such.
ps. don't worry about posting 2x, the forums have a built-in zooming feature(click on an image, you'll see), so you might wanna go back and post them 1x.
