For the turtles death I think the turtles whole shell should just go black and show his bones, the turtles spine goes across the top of the shell.
Also maybe you should close the sharks mouth.
That's probably a good idea with the turtle, the full shell looks out of place. And I plan on closing the shark's mouth once I get a chance to.
Also, I just finished this as you were posting:
Thoughts?
I'll start working on the turtle death animation and the shark in the meantime.
EDIT: Okay, I got the shark done pretty fast:
Just need to make a new death animation for that now.
EDIT 2: @ Ben - Before I forget, here's a quick comparison of the original sprites and modified sprites without dithering:
Personally I don't know which I like better yet. I think I still kinda like the dithered ones though.