If you were to examine a little wet soil from your rain garden, and put a drop on a microscope slide, you'd probably see a lot of rotifers. Barely bigger than amoebas, they still consist of many cells.
What's interesting is how these aquatic creatures survive the dry spells between rains...
"Some rotifers can withstand drying" better than most microscopic critters. "In this almost completely dried state they may live for years. As soon as moisture appears, they swim about and feed actively. Because of this capacity to resist drought, rotifers can live in places that are only termporarily wet, such as roof gutters, cemetery urns, rock crevices, among moss, and" in rain gardens.
"When the water evaporates, the animal contracts to a minimum volume and loses most of its water content. Somtimes the animal itself dies but its contained eggs survive until moisture returns."
So, whether it's wet or dry, they are always ready to help break down the pollutants that wash into your garden.
* Degenerate means smaller and more simplified. Why am I reminded of Homer Simpson?
Quotes from Animals Without Backbones, by Ralph Buchsbaum, 1948.