What exactly is a glass sponge?

What first comes to mind when you hear the word sponge? You may think of what you use to clean your kitchen dishes or perhaps the soft and porous creatures found in the warm, sunlit waters of tropical destinations. However, the glass sponges found in Átl’ka7tsem/Howe Sound could not be any more different! Glass sponges are hard, brittle structures that possess a skeleton, composed of siliceous spicules, fused to form a three-dimensional structure (Leys et al., 2007; Dunham et al., 2018). These organisms are as delicate as glass and are intolerant to temperatures outside a narrow range of 9-10⁰C. This makes them them extremely vulnerable to physical damage and environmental changes. Despite this, glass sponges have been around since the Jurassic period, meaning they have survived numerous mass extinctions, an incredible feat considering their delicacy!

Sponge reef vs. garden, what’s the difference?

When learning about glass sponges you will often hear the term reef/bioherm or garden. While both are important, they are not the same thing! So what is the difference?

Some species of glass sponges are capable of forming reefs (similar to coral reefs!) through the attachment of larval sponges to the skeleton of dead sponge. When sponges die, their three-dimensional structure remains intact, allowing these larvae to settle and grow on them. Over time, these skeletons trap sediment carried in water currents, leading to a reef or biohermal structure. If left undisturbed, these reefs can continue to grow, creating structures up to 25 meters high and several kilometers wide ! These reefs are composed mainly of dead sponge skeleton and sediment with the top 1-2m consisting of live sponge (Dunham et al., 2018).

A sponge garden on the other hand, refers to an area that contains a uniquely large biomass of glass sponges but contain no evidence of reef formation (Dunham et al., 2018). These gardens form on rocky substrate and are equally important to sponge reefs in terms of their ecosystem benefits. Despite this, gardens are not awarded the same protections as reefs are, making them particularly susceptible to fishing damage (read about it here).

Importance and biology of glass sponges

Whether a garden or a reef, sponges support a wide diversity of life, everything from small invertebrates and baby fish to adult lingcod over 1m long. These structures not only provide critical habitat but they also act as silicon sinks (used to make their skeletons) and can even help combat climate change, thanks to their filtering capabilities! Each day, sponge reefs filter out bacteria from the entire water column above them and have carbon sequestration rates comparable to that of old growth forests and marine kelp beds.

The glass sponge reefs Átl’ka7tsem/Howe Sound are believed to have begun forming 9000 years ago and each individual sponge is thought to have a life span over 200 years old (Dunham et al., 2018). These sponges are incredibly slow growing, estimated at 1-9cm per year, making recovery (such as physical damage from fishing gear) extremely limited.  While some studies have shown recovery after physical damage, others have shown no recovery after a 3 year period (Kahn et al., 2016). Loss of these organisms from physical or environmental disturbances are devastating to the surrounding ecosystem and result in a significant decrease in species abundance and biodiversity. Because they are extremely slow-growing, it is likely that sponge loss will result in a permanent alteration of the surrounding ecosystem.

These incredibly unique organisms are thus critical to the health of our marine waters and more work is needed to ensure their survival. Read more about the work MLSS is doing to conserve these species by clicking the links below. If you are interested in supporting this work donate or contact us today!

Want to learn more about glass sponges? Additional resources can be found here.