The Role of Glass Sponges in Creating Habitat for Other Species
Northern British Columbia sponge reefs have dramatically altered the habitat from soft mud to hard glass. The reef builders include Cloud Sponge Aphrocallistes vastus, the Fingered Goblet Sponge Hetereochone calyx, and the Lace Sponge Farrea occa. These sponges belong to a group, the hexactinosans, which have fused mainframe skeletons.
When these sponges are alive, nothing settles on them and only armored fish and crustaceans rest or move about on their surface.
when a sponge, or portion of a sponge, dies the fused skeleton becomes covered with silt and is habitat for many hard substrate species including glass sponges. It has been noted that as sponges are dying their soft tissue moves upwards, away from silt. This may demonstrate that the sponge knows the silt is damaging.
We compared species on the reef sponges and skeleton with those on/in mud within and adjacent to the reefs at a conference of spongologists (Austin et al. 2002 unpub.)
This table is based on analyses of videos and direct observations during the 14 sub dives, as well as, sorting and identification of material from 7 grab samples. With the exception of fishes, no species occurred on both sponges and on mud. The fish included a greenling, 7 species of rock fish, and a sculpin. Eight of these fish spend much of their time swimming above the bottom and so may not be expected to be associated with a particular bottom type. The great majority of species on sponges (about 90%) occur only on dead skeletons. 36 species were identified on/in soft substrate while 87 were identified on sponges. We still don't know how long sponge skeletons remain intact, but recent evidence published by Manuel Maldonado and colleagues (2005) indicates that dissolution rates of siliceous sponge spicules are much slower than previously thought.
Conway et al. (2007) have reported on other sponge bioherms in the Strait of Georgia. Those surrounded by mud would serve as oases of hard substrate, perhaps comparable to artificial reefs. However, those adjacent to hard substrate as bioherms might amplify existing rock habitat. Known examples include areas adjacent to the east side of the Gulf Islands, an area at the entrance to Howe Sounds, and two areas in Portland Canal.
In addition, there are many fjords in British Columbia which contain major populations of hexactinellids that are not in bioherms (Leys et al. 2007). These are discussed in the next section: Amplifying Habitat.
Clearly, the presence of the sponge reefs has a major impact on the types and numbers of species.
