For as little depth as the game has, I found myself playing it for an hour at a time.
Note: This game just for the time I put into it was worth the price.
You take your creature through each enclosed (invisible barriers) area eating, avoiding, attacking other microbes or bacterias on each level. When you complete a level, you can ascend or descend to a previous or next level of water. Stages don't seem to respawn when you clear them.
The deeper you go into the sea, the more aggressive bacteria you find. You can eat/destroy them just the same as they can do to you.
I haven't played cooperative, and I haven't kept playing yet to see if there is an end to the game.
Graphically - the game is beautiful, the sound was pleasant as well.
This game is advertised as simple, and I agree - just don't hop into it competitively as if to beat it. Just play it to experience it. Just flow....