1. A reef is a chain of rocks or ridge of sand or coral, at or near the surface of the water, or submerged. Living reefs can be made of coral, sponge, or of a combination of organisms that make rigid skeletons.
2. Sponge reefs: Occur on the NE Pacific continental shelf and are made by generation after generation of glass sponges growing on top of each other. Unlike most other sponges, reef-forming glass sponges are able to make a rigid skeleton of nearly pure glass (SiO2) that remains after the animal has died. Young sponges attach to the skeletons of previous generations; in time, the skeletons become buried in sediment and cemented in place.
3. Uniqueness of the reefs. Reefs of glass sponges have not been known since the Jurassic (200 mya) when they occurred throughout the Tethys Sea (what is now southern Europe). They diminished during the Cretaceous (160-90 mya), and today the Canadian reefs are the only ones known. Both trawl fisheries and potential oil and gas exploration endanger the reefs.
4. Glass sponges: a) They are mostly inhabitants of the deep ocean (500-3000 m), and are only found shallower than 500 m in four places: Antarctica, fjords of New Zealand, some caves in the Mediterranean, and the waters of Canada’s Pacific coast; b) they make a skeleton of glass – one type can fuse the glass together to form a rigid scaffolding over a meter in height and width (these are the reef forming sponges); c) the majority of the sponge soft tissue is multinucleate; d) this tissue is responsive to touch or sediment – it can conduct electrical signals that stop the sponge feeding.
5. Location, form, size and age of the sponge reefs.
a. Sponge reefs are only known from the continental shelf of western Canada. Four are in Hecate Strait, Queen Charlotte Sound.
b. The reefs are at 150 – 200 m depths; the largest is 30 km long and 15 km wide. Thickness ranges from 5 to 21 m.
c. Reefs first formed on mounds of boulders that line the edges of troughs gouged out by the base of icebergs 8-10,000 years ago.
d. Glass sponges grow between 2 and 7 cm in a year. The largest sponges (1-2 m in length) are estimated to be between 50 and 220 years old.
e. Carbon dating has aged one 5 m thick reef in Queen Charlotte Sound at nearly 6,000 years.
6. Conditions for growth of glass sponges, and sponge reefs: The conditions where glass sponges live are: a) low temperature (7-12°C); b) low light; c) high silica (30-120 µM); d) low sediment. Reef forming sponges need a sediment-free surface for attachment.