The Sponge Loop: Or How to Power a Coral Reef

Photo © Georgette Douwma / Getty Images.

In 1842, about seventeen years before the publication of On the Origin of Species, Charles Darwin completed a monograph about coral reefs. The work was published under the not-so-short title:

The structure and distribution of coral reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836

The monograph described in great detail Darwin's observations about coral reefs and put forth a theory explaining the formation of coral atolls—ring-shaped reefs that encircle a central lagoon.

It also brings to light a perplexing contradiction. Although coral reefs are among the world's most productive ecosystems, they thrive in marine waters that lack nutrients. This contradiction has since been dubbed Darwin's Paradox.

Since the publication of Darwin's monograph on corals, scientists have been toying with various explanations for Darwin's Paradox. Where do coral reefs find the nutrients needed to sustain such impressive levels of productivity? Now, a team of scientists led by Jasper De Goeij from the University of Amsterdam in the Netherlands, thinks they have one possible explanation.

What Jasper De Goeij and his colleagues have discovered is that sponges play a critical role in the coral reef food chain. Sponges filter vast quantities of dissolved organic matter from the waters around the coral reef. This dissolved organic matter is not readily consumed by other reef animals—it's nutritious, but that nutrition is trapped, inaccessible to most reef animals.

But Jasper De Goeij and his collegues have discovered that sponges can and do consume this dissolved organic matter. And it turns out that it provides sponges with ample fuel to power the rapid turnover of cells (choanocytes) in their body.

As the sponge produces new choanocytes, they shed the old ones into the water, essentially creating a wash of food particles over the reef.

These discarded cells provide a constant and plentiful source of nutrition for a variety of detritivores including crustaceans and polychaetes. The detritivores, in turn, serve as food for the next higher trophic level and in this manner food cycles around the reef.

There are three subgroups of sponges. The glass sponges (Class Hexactinellida) have a skeleton that consists of fragile, glass-like spicules that are made of silica. The demosponges (Class Demospongiae) are often vibrantly colored and can grow to be the largest of all sponges. The demosponges account for more than 90 percent of all living sponge species. The calcarious sponges (Class Calcarea) are the only group of sponges to have spicules that are made of calcium carbonate. Calcarious sponges are often smaller than other sponges.

Sponges are primitive multicelluar animals that have no digestive system, no circulatory system, and no nervous system. They do not have organs and their cells are not organized into well-defined tissues.

This newfound food cycle is referred to as the sponge loop and it explains how hot-spots of biological diversity such as coral reefs might thrive in nutrient poor locations. In the case of the sponge loop, the critical player is the sponge—a simple organism whose importance in its community had previously been underestimated.

References

de Goeij, J. M. (2013-10-04) Surviving in a Marine Desert: The Sponge Loop Retains Resources Within Coral Reefs. Science 342(6154), 108-110. DOI: 10.1126/science.1241981