Very interesting macro. It has a lot of potential. The compositing is good the problem is the lighting was just to poor and ruined a beautiful photo. I'd love to see this done over with better lighting.
It meets the challenge, just watch out for shadows. Good attempt.
The dark background and the framing really help to make the shell pop out of the picture. Love the great ripples in the shell. Playing with the lighting you would get some really vivid dark patterns and make the picture a lot more interesting.