Pearling is when the plants give off streams or bubbles of oxygen. It happens under optimal conditions when the plants are growing quickly and there is enough co2, fertilizers, and light. It is not necessary for good plant growth but it looks pretty cool though.
Here is a picture of some riccia pearling from one of tanks from the 2002 AGA Contest:
Here is a picture of some riccia pearling from one of tanks from the 2002 AGA Contest:
