Where Glass Sponges are Commonly Found
Glass sponges are relatively uncommon and are mostly found at depths from 450 to 900 metres in all oceans of the world, although they are particularly common in the cooler Antarctic and Northern Pacific waters. The shallow water occurrence of hexactinellids is rare world wide.In the Antarctic two species occur as shallow as 33 meters under the ice. In the Mediterranean one species occurs as shallow as 18 meters in a cave with deep water upwelling (Boury-Esnault & Vacelet (1994). Another species occurs in shallow water in a southern New Zealand fjord.
Glass sponges use silica (glass) that is naturally dissolved in seawater to construct their skeletons, which have the consistency similar to baked meringue. Glass sponges thrive in BC's Strait of Georgia due to high concentrations of silica. Here, living glass sponges grow on top of dead glass sponge skeletons to form reefs as tall as eight storeys and covering an area of 700 square kilometers. Glass sponges can be found in several locations around the world, particularly Antarctica, but they don’t build reefs there. In fact, B.C.’s reefs are the only known living glass sponge reefs in the world, and scientists have only been aware of them for the past 25 years.
Sponge reef in Hecate Strait east of the Queen Charlotte Islands in British Columbia.
Five species occur in depths of 15-35 meters in a range of locations in fjords and in the Strait of Georgia in British Columbia. The shallowest record was communicated to us by Neil McDaniel in 1978. This was a Cloud Sponge at a depth somewhere between 2 and 5.2 meters in depth in Seymour Narrows where current speeds reach 16 knots resulting in major vertical mixing of water. In November 2003 members of the Victoria Dive Club & other divers (Mike Miles, Carole Valkenier Pope and Ian Pope, Mike Kalina, James Dranchuk) surveyed some of the Seymour Narrows area. They found Cloud Sponges at several locations along the SW side of Quadra Island as well as in Seymour Narrows. The shallowest (found by Ian and Carole) was 2.0 meters corrected for tide height in Seymour Narrows. To the best of our knowledge 2 meters is the shallowest record for a hexactinellid sponge since the Cretaceous.
Most of the glass sponges in British Columbia reach sizes of ½ to 3 meters. In northern British Columbia they occur in particularly dense populations which suggested the name “sponge garden".
These sponge reefs were discovered in 1987-1988 by the Geological Survey of Canada at depths of about 220 m. They form mounds up to 18 m high and beds several km in width. Some of these reefs are 8500 to 9000 years old based on core samples (Conway et al 1991). Glass sponge reefs were common during the “Age of Dinosaurs” (e.g. Krautter 1997) but were unknown since that time. Some were being destroyed by trawling. They have recently been designated as no trawling areas by Fisheries and Oceans Canada; but full Marine Protected Area status is needed to ensure that they remain intact.
Charts showing silicate concentrations in various seas and depths. The shallowest record (2m depth) is in Seymour Narrows where high silicate levels occur throughout the water column due to vertical mixing (Thomson et al. 1980 & unpublished info 2003. The highest silica concentration, near the surface, is in the Strait of Georgia (magenta line).
Excerpt from Memoirs of the Queensland Museum. Brisbane :Queensland Museum,1912- http://www.biodiversitylibrary.org/bibliography/61449
Bill Austin has suggested (Austin 1984, 1996) that the shallow occurrence of glass sponges in British Columbia is related to the high concentration of glass in the water as silicates.
Indeed, the deeper waters of the Strait have been known for 40 years to support large populations of Cloud Sponges. More recently, SCUBA divers have directly observed populations in the Strait and adjacent fjord. The graph to the right shows silica concentrations of 33 µM (green) throughout the year at the dotted line. This line represents the shallowest occurrence of glass sponges (15 m) in the area. While the data were obtained in 1931, there is no reason to think that the levels have increased or decreased substantially since then.
The other shallow water area with high silicates is the Antarctic. However Norwegian fjords, for example, have low silicate concentrations and have no shallow water glass sponges although many of the other species are identical or related to those occurring in British Columbia fjords (Bjørn Gulliksen pers. comm.)