Look closely and you’ll see tiny amphipods using this Antarctic sponge as habitat. (Photo credit: Julian Gutt.)

By Rachel Downey (Australia National University & British Antarctic Survey) and Claire Christian (ASOC)

In our last post, we introduced you to one of nature’s underappreciated animals, the sea sponge. Sponges have been around for over 600 million years, by developing some fascinating adaptations that make them one of our greatest global survivors. Long existence has meant that sponges have been able to colonise the deepest trenches, harshest coastal environments, and survive a multitude of mass extinctions. Today, they live in every marine environment in the world, and even in some brackish and freshwater habitats.

In fact, though sponges with their bright colours and unusual shapes may seem ornamental, they are integral to the functioning of many marine ecosystems. By filtering seawater for food, sponges have a significant impact on water quality, with large sponges able to filter 1500 liters of water per day. Sponges alter the flow of water in their environment, which can be beneficial to other filter-feeding animals, such as brittle stars and sea lilies, which use sponges to get a leg up into the higher food-rich currents.

Furthermore, sponges are never an organism alone, as they host bacteria, viruses, fungi and other marine organisms within their bodies. Sponges are therefore seen as an ecosystem, a big community of lots of organisms, with many different types of relationships, rather than just a single animal. Like trees or corals, sponges are ecosystem engineers, creating complex and varied 3D structures in seafloor habitats that influence the rest of the ecosystem. Many animals, such as fish, scale worms, and brittle stars, rely on sponges as places to feed on their favourite prey, rest, lay eggs, keep their young safe, and occasionally munch on a bit of sponge. Unlike coral reefs, which are restricted to shallow, tropical waters, sponge grounds can occur in every marine environment and at great depth, potentially enhancing biodiversity globally.

An Antarctic fish takes shelter in a sponge. (Photo credit: Tomas Lundalv.)

In coral reef environments, a bacteria that lives in sponge tissue is able to capture phosphorus, making this important nutrient available to the rest of the animals living on the reef. Many of these tropical sponge species contain photosynthesising organisms similar to corals, and these can produce up to three times more oxygen and organic matter than they consume. Thus sponges make substantial contributions to nutrients and oxygen for other animals in the surrounding area. Sponges mainly consume dissolved organic carbon and nutrients that most other marine animals can’t consume. Sponge can consume half their weight in this carbon every day, and instead of using this extra carbon to grow, they shed their old cells, producing food for other organisms. Sponges are great recyclers and providers, keeping themselves and other organisms happy and healthy in many different marine environments.

A sponge’s ability to construct amazing skeletons into innumerable shapes, for their own and other animals’ benefit is also being explored by engineers. These industry researchers are convinced that they can build taller, stronger, and more flexible buildings and vehicles in the future, based on what sponges have mastered over millions of years.

But sponges aren’t simply sitting on the seafloor, waiting to be exploited by other species. They can defend themselves quite well. How do they stay safe on the seafloor? Sponges produce noxious chemicals, often with the help of their bacterial associates, to survive attacks and dominate landscapes. In the last few decades, scientists have been exploring sponge species from every part of the globe to see what unique chemicals they produce. Amazingly, they have found sponges that contain substances that are anti-inflammatory, antibiotic, anti-tumor, vital compounds for our future development in modern medicine, and they can even produce chemicals that are alike to fire retardants! One sponge has been found to contain a bacterium that protects it against arsenic poisoning, which may not sound like an exciting revelation, but this new information could be used to help eliminate our global arsenic poisoning issues, which affects millions of people every day.

In Antarctica, our area of expertise, sponges are an unusually important group. We’re going to focus on them for a bit to further illustrate how critical sponges can be in the polar marine environment. Antarctic sponges live in the shallow, rocky coastal regions that are often scoured clean by icebergs, on the hundreds of isolated seamounts dotted throughout this ocean and on the vast expanses of abyssal, muddy seabed bottoms. The Southern Ocean harbours almost 450 different species from all major sponge groups, including the often brightly coloured and wondrously shaped demosponges, giant spiky vase-like glass sponges, and tiny pale calcareous sponges. Close to half of sponge species that live in this ocean are endemic, meaning that they are not found anywhere else in the world. Antarctic sponges are unusual in that they have very broad longitudinal distributions, so one species can be found around the entire, immense Antarctic coastline and the same species can live in a huge depth range, occurring both in shallow waters and in water hundreds of meters deep. In fact, Antarctic glass sponges are found in shallow marine habitats, a rare phenomenon, as elsewhere in the world they are usually found in very deep waters.

Antarctica is a sponge paradise. (Photo credit: Julian Gutt.)

The relative isolation of the Antarctic region by ocean currents is one reason for this diversity of sponges, since isolation often leads to the evolution of new species. Both abundant silica, necessary for constructing most sponge skeletons, and sea water formed in the deep oceans and then washes up to the shallow coast, may have enabled unusual deep-sea sponges to move from the abyssal depths to the icy coast. The furious Southern Ocean currents circling the continent may have helped distribute sponge larvae all around the Antarctic continent. Whatever the reason, the outcome is clear: Antarctica is a sponge paradise. It has been argued that the biomass (the sheer weight of life) on the Antarctic shelves rivals that of tropical reefs, and this is in part due to the dominance of sponge habitats. By creating complex 3D structures on the seabed, they likely enabled the co-evolution of many Antarctic fish, starfish, crustaceans and much more.

So how did sponges manage to triumph in this harsh polar environment? They are long-lived (some species are thought to be able to live for centuries) and can go for a long time without food, enabling them to survive Antarctic winters when little food will be available and resume growth in the summer when nutrients are more plentiful. Yet patience isn’t the only thing they have going for them. Recent research has revealed that despite their sedentary, slow lives, sponges in Antarctica may be able to move quickly when it counts. After the Larsen A ice shelf collapse, glass sponges moved in very quickly and took over areas newly suitable for life. Soon, other species will likely move in to take advantage of the new habitat these sponges have created.

Furthermore, all sponges produce an incredible array of chemicals to defend themselves and compete for space on the seafloor, and our Antarctic sponges are no exception to this. Only a few sponge species have been studied by pharmaceutical scientists hoping to find compounds to synthesise in their laboratories, and these species have not disappointed, with one species that has compounds that eliminate MRSA, a major source of infection in hospital environments. Some sponges have anti-tumor and anti-viral properties, while others have antibacterial, antimicrobial, and antifungal properties, which are all compounds that can be utilised to improve our fights against human disease.

The Antarctic sponge Dendrilla membranosa contains a substance that could one day be used to treat MRSA. (Photo credit: Bill Baker.)

Unfortunately, Antarctic sponges, as well as those elsewhere in the world, aren’t always allowed to do their habitat engineering and chemical defense creating in peace. In the next post, we’ll talk about the threats to sponges, and why protecting their seafloor habitats should be an important consideration for anyone who loves marine life and appreciates the unique beauty found in our oceans.

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