Neat game! The core mechanics are solid, and the sound effects are funny as others have mentioned.
My thoughts on specific parts of the game:
- The artwork is pretty nice, but I would've liked more stylistic consistency. The sprites differed a lot in amount of shading, amount of detail, presence or absence of outlines, etc. Consistency can make even incidentally poor art feel like a conscious choice in art direction.
- The sound effects are pretty loud (especially that spitting noise) and aren't universally applied. Would've loved SFX for UI and player/enemy movement. The ones that are there are pretty fun though.
- The controls are good! The movement system is responsive enough to give you control but not so tight as to make dodging enemies trivial. I would suggest some sort of buffer for shooting though so that you can buffer a shot if the cooldown is just about over.
- The gameplay mechanics are solid as aforementioned, but there isn't a whole lot to them. Once you've played the game for a couple minutes, you've seen all there is to it. Of course, it's a 3.5-day jam, but a full version of this game would do well to have more enemy variety, more upgrade variety, and some use of the shell theme beyond just visuals.
But all and all, solid stuff, especially given the time frame. Good work!