Cute game, funny too! Also nailed the aesthetic really well!
I also really really like the shell mechanic! I think it definitely has a lot of potential. I think it would also be cool to have more interaction that forces you to use/find a particular shell, like a puzzle lever shot you can only make with the bullet shell or a super particular jump that only works with the dash shell.
(I'm also wondering why jumping up into spikes hurts if I'm wearing a shell >:T )
Really though it's a perfect idea and nice execution for 10 days!
Also, crab dance is *chefs kiss* mwah perfect