By Rachel Downey (Australia National University & British Antarctic Survey) and Claire Christian (ASOC)
Sponges may historically be one of world’s greatest survivors, but on our planet, we have a number of new human-made challenges that sponges have not come up against before. The deployment of fishing gear that smash seabed habitats, the laying of submarine cables that can destroy tracks of sponge habitat, and oil and gas exploration which can destroy parts or entire sponge habitats. When large numbers of sponges occur together, they are considered indicators of vulnerable marine ecosystems (VMEs), as they likely harbour a rich community of associated animals, like those we often think about in shallow tropical coral reefs, full of brightly coloured fish, worms, crabs and octopus. As the name implies, these areas are sensitive to disturbance and damaging them may have broad ramifications for these ecosystems.
One of the oldest impacts on sponges was sponge fishing, an industry that supplied thousands of tonnes of ‘bath’ sponges for our beauty and industrial needs. Sponge fishing occurred widely in the Mediterranean, but spread globally to encompass the Atlantic and Pacific Oceans. However, overfishing and disease outbreaks destroyed many of these bath sponge grounds in the eighteenth and nineteenth centuries. Bath sponges are now being ‘farmed’ more sustainably, either in the sea by divers and in some areas on ropes, mesh bags, in vitro, or on land-based facilities.
The more recent threat of deep-sea mining for metal-rich nodules has been proposed as a new future hazard to sponges. Vast areas of the deep mid-Pacific Ocean are found to be rich in metal nodules, and on these nodules, completely unique sponges have been found. So far, these new sponges have only been found living on the nodules, rather than on the vast tracts of soft mud that are found throughout deep-sea regions. These nodules are potentially forming ‘biodiversity hot spots’ – localized areas that have a lot of life living on them, like sponges and corals, and their presence means that lots of other animals can use these nodule ‘kingdoms’ to survive, rather than the food-poor mud that surrounds them, a bit like an oasis in a desert.
The scientific community is still unclear about the full impacts of climate change and ocean acidification on sponges. Recent climate change impact studies have given us some idea of how sponges may be ‘winners’ in how they respond to change. In our last post we mentioned that sponges may be able to colonize areas exposed by ice shelf collapse relatively quickly. Research on tropical sponges from the Great Barrier Reef found that some sponges are sensitive to ocean warming, but if oceans simultaneously acidify, then these sponges become less sensitive to temperature increases. Coral bleaching events can also negatively impact sponges directly and indirectly with changes in nutrient cycling, habitat structure, and losses and gains in all the other animals that live among coral reefs. Non-tropical sponges may not be so fortunate either. One recent study showed that more than half of Antarctic species, including sponges, are likely to lose key habitat as sea waters continue to warm in polar regions.
No sponges species are currently threat assessed by the IUCN Red List or protected by the CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora). The IUCN’s Red List, the global authority on the status of threatened species, has begun to recognise the importance in protecting sponges in key environments like the Mediterranean Sea, which will be the first assessment of threats to sponges species in the world.
But it may not be wise to wait for these detailed scientific assessments. Research clearly demonstrates that disturbing seafloor structures has negative ecosystem impacts, and we already know how important sponges are to some of those structures. Bottom-trawl fishing is particularly damaging for the seafloor and can have impacts lasting decades. However, even lower impact methods can disturb sponges and other seafloor dwellers. In Antarctica, for example, longline fishing does result in bycatch. Longline hooks can go as deep as 1000 m and can grab on to sponges, which are often some of the largest parts of the seabed fauna, as the lines are reeled in by fishing vessels. The long-term repercussions of this damage to sponge habitats and associated organisms are not fully known.
To preserve the intricate intra-species relationships and biodiversity that sponges help sustain, we should be proactive and protect VMEs as well as known sponge habitats. Currently, only 3% of the world’s oceans are classified as MPAs (marine protected areas), but only 2% of these are strongly protected. MPAs are located all over the world, not just along countries’ coastlines, like the Great Barrier Reef in Australia and the Channel Islands National Marine Sanctuary in the US, but also in the remote fishing grounds of the high seas, as well as at remote islands, such as the Chagos Marine Protected Area in the Indian Ocean and the Pitcairn Islands in the southern Pacific Ocean.
In the Antarctic, a process is already underway to create a circumpolar network of MPAs around the continent, and the hope is that these MPAs would include seafloor areas representative of Antarctica’s biodiversity. One MPA has already been created in the Ross Sea, one of the largest MPAs in the world, where there are several known large sponge habitats. Another MPA is currently under consideration for the Weddell Sea, which is an area with many vibrant sponge habitats. In the proposed MPA, “ecologically important sponge associations” are singled out for special protection. If the MPA is put in place, no disturbance of these sponge habitats would be allowed, not even for research. Perhaps sponges will get some respect at last.
Earlier this year, Greenpeace sent one of its ships on an Antarctic expedition in hopes of identifying additional areas to be included in other MPAs. So little is known about these areas that the scientist along on the voyage, Dr. Susanne Lockhart, an expert in Antarctic seafloor species, noted that she’d never seen these species in their natural environment. Usually these areas are only seen with the aid of cameras, and specimens are often gathered using the same equipment for bottom trawling and thus are only examined when dead. Greenpeace employed a two-person submarine, however, enabling Dr. Lockhart to see the species she studied for the first time. She told us:
“Leaving the stark grey-scale world above the ocean in Antarctica, I am always struck by the proliferation of colour and life that can be seen when the submarine reaches the seafloor hundreds of meters below. These densely populated invertebrate communities exhibit a surprisingly high biodiversity and structural complexity that could only have been achieved in the absence of disturbance. Sponges play a key role in that structural complexity; providing habitat and protection for a multitude of other organisms. Such communities have a low resilience to disturbances caused by commercial fishing gear, and are therefore extremely vulnerable and in urgent need of the kind of protection afforded by the Marine Protected Areas proposed for the region.”
Most of us won’t be as lucky as Dr. Lockhart, and will have to admire sponges and the vibrant, complex ecosystems they support from afar, on our TV and computer screens. Meanwhile, we hope you will use your new knowledge about sponges to support comprehensive ocean protection. While protecting one species can often have a broader impact, it’s important to make sure we take a more holistic approach. We still have so much to learn about the interconnections between sponges and their associated organisms – relationships that are likely to have been going on for millennia – as well as how sponges positively impact seabed health, and how they and many other organisms are responding to environmental changes, diseases, and new types of disturbance in the marine environment.