• Emily Higgins

The Invisible Loop: How Sponges Allow Coral Reefs to Flourish in a Marine Desert

Updated: Aug 23, 2021

Since the time of Charles Darwin, the amazing array of biodiversity, productivity, and sheer amount of life on coral reefs has been a mystery. Coral reefs form in what amounts to a marine desert, thriving in nutrient poor (oligotrophic) waters in tropical regions. It was referred to as "Darwin's Paradox" for over 100 years.

School of fish swimming through nutrient-poor waters above a reef in the Solomon Islands.

The question Darwin asked was simple: "How can so many species thrive in such numbers in a nearly nutritionless environment?" Corals have tiny, photosynthetic algae that live in their tissues, and sequester a massive amount of carbon dioxide annually (Crossland et al. 2001). Despite this influx of carbon into the system, only ~25% can be accounted for in additional reef structure and sustained human harvest. For years, scientists posited that there must be a very efficient mechanism for recycling carbon and nutrients on coral reefs - but they couldn't quite prove what it was.

So who is recycling these nutrients? And where do they go? It is estimated that up to 75% of the carbon that is sequestered by coral species on reefs "disappears". So where does it go? Questions like these sparked the formation of the "Sponge Loop" hypothesis in 2000.

Sponges live on rocks and coral skeletons on reefs where they can fasten themselves securely to prevent dislodgement and have lots of access to food. They are completely sessile and do not move for their entire adult lives once they have settled on a hard surface. Sponges are from the phylum Porifera (meaning "having pores"), and in order to eat, they filter huge volumes of seawater through their pores daily to feed. Water enters their tissues through the pores and collects in specialized cells called choanocytes where nutrients from planktonic prey and dissolved organic materials are then transported to the rest of the animal.

Large cryptic and erect sponges on IntelliReefs artificial reef cast from Oceanite in Philipsburg, Sint Maarten.

The Sponge Loop hypothesis hinges on the ability of sponges to recycle and transform the otherwise inaccessible carbon absorbed and excreted by corals into food for a biodiverse array of animals in the lower trophic levels, like crabs, snails, and other bottom feeders (i.e. detritivores). In 2013, a group of researchers from the University of Amsterdam led by Jasper De Goeij set out to empirically test the Sponge-Loop hypothesis.

Desmapsamma anchorata sponge growing in cryptic microhabitats on IntelliReefs after 14 months.

Sponges and other filter- and suspension-feeding animals that do not need sunlight proliferate in the crevices and shaded areas of coral reefs. The animals in these cryptic spaces are estimated to account for up to two-thirds of the volume of reef organisms (Richter et al. 2001) and can readily exceed the biomass of the animals on the open reef (Scheffers et al. 2004).

This means that sponges are the "unsung heroes" on coral reefs (De Goeij et al. 2013). They can pump more than 50,000 times their own volume per day (Fiore et al. 2017), and over large reef regions, it is easy to imagine sponges as the lungs of the ecosystem.

Fish sheltering underneath table coral on a thriving, biodiverse coral reef in Indonesia.

The bottom line is that reefs could not function or thrive without sponges. As reefs around the world are eroding and degrading faster than they can build new structure, they are becoming effectively flattened. One of the massive consequences of losing this 3D relief is that we are also losing the vast expanses of cryptic, shaded networks where animals like sponges grow.

Without sponges, we are breaking the nutrient loop on coral reefs. Consumers like crabs and shrimp that feed higher trophic levels will lose one of their main food sources. The Sponge Loop is the perfect example of the intricate and connected nature of healthy ecosystems. Just because we cannot see a process happening or the value of an animal or plant right away, it does not mean that it is not important. In this case, these hidden animals support one of the most vibrant, biodiverse ecosystems on earth, feeding nearly 1 billion people worldwide.

Examples of the diverse array of sponge species living in the crevices and cryptic spaces created by the complexity of Oceanite substrates.

IntelliReefs recognizes that biodiverse ecosystems are healthy ecosystems. In order to retain ecological integrity and resilience in the face of climate change, we need to maintain all of the species that allow coral reefs to function. IntelliReefs have been shown to enhance biodiversity and cryptic surface area, creating new homes for sponges and other filter feeders on reefs that have lost their 3D structure due to decades of deterioration.

We are providing homes for the ocean's homeless.

One reef, one region at a time.

Photo credits:

  • Solomon Islands, Tracey Jennings / Coral Reef Image Bank

  • Coral Reefs in American Samoa, Shaun Wolfe / Coral Reef Image Bank

  • Large cryptic and erect sponges on IntelliReefs artificial reef cast from Oceanite in Philipsburg, Sint Maarten, Ian Kellet / IntelliReefs

  • Desmapsamma anchorata sponge growing in cryptic microhabitats on IntelliReefs after 14 months, Ian Kellet / IntelliReefs

  • Fish sheltering underneath table coral on a thriving, biodiverse coral reef in Indonesia, Gregory Piper / Coral Reef Image Bank

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