Sponges are sessile organisms that attach to a firm substrate. It is difficult to determine how many sponges are present in a sample because one colony may be broken into smaller pieces. For example, when sponges are encountered in samples in California, each piece is recorded as an individual; most Northwest labs include a comment that sponges were found in a sample rather than counting the number of individuals.

There are about 30 species of freshwater sponges in North America, all in the family Spongillidae. Sponges typically live in still waters, and are found regularly in larger rivers, lakes, wetlands, and in streams near lake outlets. Their bodies are simple, having no organs or differentiated tissues, but different cells in the colony play different roles. Freshwater sponge colonies have numerous microscopic holes by which water passes into the sponge and a few large holes by which water leaves the sponge. Sponges serve as food sources for a variety of other macroinvertebrates, including caddisflies, midges, and spongillaflies (order Neuroptera, family Sisyridae).

Sponges can reproduce sexually but are highly variable in what 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 some disturbance. Similarly, they have a strong ability for regeneration. Sponge nodules are sometimes encountered. in urban streams and eutrophic lakes; these nodules are resting buds called gemmules and are comparable to finding an egg in a sample.