Sponges are an exclusively aquatic and, with a few exceptions, a filter-feeding group of animals.The group consists of approximately 15,000 extant species in three distinct groups:
- the glass sponges (Class Hexactinellida)
- the calcareous sponges (Class Calcarea)
- the demosponges (Class Demospongiae)
Adult sponges can be asymmetrical or radially symmetrical and come in a variety of sizes, colors, and shapes, including arboresecent (tree-like), flabellate (fan-shaped), caliculate (cup shaped), tubular (tube shaped), globular (ball shaped), and amorphous (shapeless) among others. Sponges occupy both freshwater and marine environments, from shallow to abysmal depths, and are common in coral reef, mangroves, and seagrass ecosystems. In some places (e.g., Lake Baikal, Russia) sponges dominate benthic communities and along the pacific cost of North America they form modern sponge reefs.