Sponge reefs are reefs produced by sea sponges. All modern sponge reefs are formed by hexactinellid sponges, which have an endoskeleton made of silica spicules and are often referred to as "glass sponges", while historically the non-spiculed, calcite-skeletoned archaeocyathid and stromatoporoid sponges were the primary reef-builders.
Sponge reefs were once a dominant landscape in the Paleozoic and Mesozoic sea, but are now very rare, and found only in waters off the coast of North America's Pacific Northwest region, more specifically southern Alaska, British Columbia and Washington. Sponge reefs were reported in 2018 within the strait of Georgia and Howe sound close to Vancouver. Although still common in the late Jurassic period, reef-building sponges were believed to have gone extinct during or shortly after the Cretaceous period, until the existing reefs were discovered in Queen Charlotte sound in 1987–1988 – hence these sometimes being dubbed living fossils.
