Sponges are sessile organisms that attach to a firm substrate. Sponges that inhabit freshwater are found in lakes and slow streams. Many have the appearance of brown or greenish brown jellie on rocks. They vary in size from microscopic to possibly a foot across. They have no nervous system, nor discernible organs, and their bodies consist, basically, of two layers of cells sandwiched around a layer of a jelly-like substance.

Physical Characteristics:
They have simple bodies with no organs or differentiated tissues, but different cells in the colony play different roles. Freshwater sponge colonies have numerous microscopic holes through which water passes into the sponge, and a few large holes through which water leaves. Sponges serve as food for a variety of other aquatic invertebrates, including caddisflies, midges, and spongillaflies.

Reproduction:
Sponges can reproduce sexually but are highly variable in the sex they choose to be. One may produce only male gametes one year and female gametes the next year. They can also multiply by starting a number of new colonies if fractured by disturbance, and they have a strong ability to regenerate.

Feeding:
They feed on microbes that they can strain from the water. Therefore their presence indicates the biological health of a water with abundant micro-organisms.

There are about 30 species of freshwater sponges in North America, all belonging to the family Spongillidae.

In the Smokies, they tend to inhabit the lower end of streams flowing out of the park, attaching themselves to rocks and logs, and filtering the water for various small aquatic organisms such as protozoans, bacteria, and other free-floating pond life.