Another Rotifer

Here's another rotifer from the scum in Calvin's fish tank, along with his friend the worm. The video below the picture shows the two of them in action. You can see the rotifer mouth sweeping in food, the jaws chomping it up, and the toes wagging like the tail of a happy puppy.






Why are there so many rotifers (three found so far) in this tank? I read that rotifers are added to fish tanks because they eat up debris and keep it clean. Were these added to the pet store fish tanks? Have they continued to propagate and survived the multiple tank cleanings and water changings?