It has potential
Nothing really too new, but fun.
My recommendations:
More animation- animate the tanks, sharks, sheep, the spaceship. In it's current form it seems sterile.
Background story: Why is the spaceship trying to kill the evil animals and save the good ones (seems borrowed from Sonic, as well)
More audio: It's got good music and sound effects, but additional sound tracks would help prevent monotony.
More upgrades: In no time I had enough money to buy the three upgrades. Maybe add a movement speed increase, more gun upgrades, and/or misc modification.
All in all, good game. I like the backgrounds, the painted style of the art, the branching levels, the hidden sheep and slow time ability.