Sponges are simple multicellular animals. There are about 6000 species worldwide, they are mostly marine but there are about 100 freshwater species. Some have distinct upright shapes, some form thin encrusting layers, others assume either form depending on their environment.

Sponges are filter feeders, so prefer sites with strong currents or wave action.
A few specialised nudibranchs, crustaceans, echinoderms and fish feed on sponges, but generally the have few predators because they produce a range of toxic chemicals, contain unpleasant spicules and are poor quality nutrition.

Sponges are hermaphrodite. Fertilisation occurs between neighbours, the resulting larvae are motile and settle away from the parents. Many sponges also reproduce asexually.

Some sponges look similar to compound ascidians. The openings in sponges don't contract when touched as the openings in ascidians do.