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Sponges (phylum Porifera) are an exclusively aquatic and, with a few exceptions (Vacelet and Boury-Esnault 1995), a filter-feeding group of animals. The group consists of approximately 15,000 extant species in three distinct groups (Hooper and van Soest 2002):
- 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.
The body plan of a sponge is simple (e.g., De Vos et al. 1991): a single outer layer of cells (the pinacoderm) separates the inner cellular region (mesohyl) from the external environment. The pinacoderm lines the internal canals and is eventually replaced by the choanoderm, a layer of characteristic flagellated collar cells (choanocytes) grouped in chambers. Choanocytes make up the principle ‘pump’ and’ filter’ of the system, driving water through the sponge, trapping and phagocytizing suspended bacteria and other particulate food, which is then digested and nutrients distributed among the cells of the mesohyl that facilitate the functions of feeding, respiration and reproduction. The flow of water inside a sponge is unidirectional: the water is drawn in through tiny pores (ostia) in the pinacoderm and exits through one or more larger openings (osculae). The aquiferous system of a sponge is usually supported by a combination of two types of skeletal elements: mineral spicules (either calcareous or siliceous) and special protein fibers (spongin), although either one or both of these elements can be absent.
The 19th-century discovery of a remarkable similarity between porifera-specific choanocytes and free-living choanoflagellates led to a proposition that sponges are the most primitive metazoans, evolved from choanoflagellate-like protist ancestors (Clark 1866; Clark 1868). The ancient origin of sponges is corroborated by the existence of a poriferan fossil record going back to the Early Vendian (~580 Mya) (Li, Chen, and Hua 1998), and by sponge biomarker record going back to the Cryogenian period (~750 Mya) (Love et al. 2009).