The creatures I created are quite simple, they get born, swim around randomly, rest sometimes , eat green dots, breed and finally they die. With my first tests, the creatures didn’t do well, they breed to much and soon there where to many, or they died to fast and got extinguished. So I added 2 simple rules:

1. You live longer if you eat.
2. You have to eat before you breed.

And that did the trick. The result is a fine balanced ecosystem.

You can download the source as an FDT project, but you have to compile it with the Flex SDK, and use the SWC to access the graphics (I lost most of the graphics by a Flash crash, the last saved version of the fla is  in the source )