That is a cool picture for Cloyster. The spikes would make it harder to draw. You still did great though. I have noticed that Cloyster does look more like a mollusk bivalve. One important characteristic is that it has its shells on the left and right sides of the body. Shellder has shells on the top and bottom sides of the body. It is like a brachiopod. Bivalves and brachiopods look alike but are not related. It is very ironic. Shell position is one thing that distinguishes them. I take that into account when designing creatures based on them.
The comments are hilarious! I already know about what Cloyster looks like to people with their minds in the gutter. The most disturbing part in my opinion is that the shiny form is blue. It is best not to put that in the daycare, especially with a Ditto. It could infect all your other Pokemon. I am kidding though.