Swimming Amongst Fossils: What Sponges Tell Us About the World

There is something hypnotic about a barrel sponge. The maroon-rust-brown of its ridges and the texture of its hard-but-porous exterior lend it an ethereal quality underwater, forcing your eyes to focus and unfocus all at once. There is no front or back here, no symmetry to orient you. Hovering over it, you are suddenly put to scale, your shadow disappearing inside a cavity often as big as you. But perhaps what the barrel sponge distorts best is your sense of time. What does it mean to have been around – almost unchanged in form – for over 600 million years? What can sponges tell us about the history of coral reefs and our place in this world?

Like corals, it might seem hard at first to believe that sponges are animals. They don’t have eyes or a digestive system, and they are sessile, which means they don’t move. But like animals, they are multicellular and can’t make their own food. Instead, they suck in surrounding seawater and chimney it through their body, allowing specialised cells inside them called choanocytes to absorb bacteria and nutrients as the water passes through. This seems like an absurdly simple process, but its consequences might not be. In pumping large quantities of seawater, sponges are also chemically transforming it.

  

Darwin’s Paradox

When Charles Darwin was travelling around the world on the Beagle, he was struck by something many of us might have wondered about while slipping under the surface of the water and onto the magic of a reef: how, and why, is there so much life flourishing in these shallow habitats, seemingly in the middle of an expanse of a barren blue ocean? In other words, how are highly diverse coral reefs maintained in nutrient-poor aquatic environments? This came to be known as Darwin’s Paradox, and several hypotheses have sought to explain it since. In 2013, an exciting new theory demonstrated how sponges were converting the dissolved organic carbon in the water (released by corals and seaweed) into particulate organic carbon (more simply, sponge poop). This organic carbon is then eaten by other small organisms on the reef. Dubbed the “sponge loop”, this hypothesis helps explain how carbon is cycled on coral reef ecosystems, with sponges playing a central role in returning carbon into the system in a form easily ingested by other reef organisms. This has huge implications for coral reef health and also helps partly explain the mystery of why reefs are hotspots of such concentrated diversity.  

Fish habitat

Apart from their role in nutrient cycling, sponges — barrel sponges in particular — provide a habitat for a range of fish and invertebrates. So far, 16 fish families have been shown to associate with sponge species. Some fish, like gobies are obligate sponge dwellers, never leaving the body of the sponge throughout their life. Others are temporary visitors and may use the sponge to lay their eggs or hide from predators. Many of these species are thought to benefit from the constant flow of water, oxygen, and food particles within the sponge.

Pharmaceutical importance

But sponges aren’t welcoming to every kind of visitor. Many sponges have a suite of chemical defences that make them toxic or unpalatable to many predators. From a human perspective, however, this has led to much interest in their biochemical properties for their potential in drug discovery. A wide range of chemicals with antibacterial and antiretroviral properties have been extracted from barrel sponges. Indeed, sponges have been used in medicine since the medieval period in Europe as components in anaesthetics and to curb various digestive ailments. Today, several species of sponges are being tested for compounds of biological interest. For instance, Avarol, a compound extracted from one sponge species from the Mediterranean, is an important part of a drug that treats psoriasis, a skin disease. However, there are now mounting concerns that many sponge species may be at threat of overexploitation for these purposes.

Oldest animal fossil?

Sponges, in my opinion, also help contextualise human history on our planet. Their basic body structure, simple yet efficient, has enabled them to survive for millions of years — 890 million, according to the latest estimate, drawn from the remains of a fossilised sponge found in Canada’s Northwest Territories. While there is debate on whether this fossil is truly a sponge, if it is, it would predate the earliest animal fossil by over 300 million years. According to this estimate, sponges would have been around when there was barely any oxygen on the planet. Back then, reefs were made of mats of cyanobacteria, and sponges would have been some of the first multicellular organisms to arise. Multicellularity means that cells are specialised to perform different functions, the basis for eventual complexity in the animal and plant kingdoms. Much later, about 160 million years ago, during the Jurassic, sponges would have formed reefs larger than today’s Great Barrier Reef. While they suffered the multiple extinction events on Erath after that, it’s incredible to me that they continue to exist as “living fossils”, forms of life that have witnessed the planet change at timescales we cannot wrap our heads around. 

Success in simplicity

Part of the success of sponges lies in their structural simplicity and adaptability that has enabled them to persist till today. Sponges can regenerate from fragments of a colony, even if you strain them through a fine mesh. Their cells can reorganise and form new colonies. Some sponges can grow several metres tall and live for hundreds of years on a reef. Others are cryptic, growing in small crevices and cracks. And even though they’re essential components of a healthy reef, we are only now beginning to understand the many roles they play here.

So, the next time you swim past a barrel sponge, slow down. Take a good look at it. This is one of your earliest ancestors, and it will likely be around long after you’re gone.