Sponges feed on coral mucus: Recycling waste on the reef
Corals secrete mucus, which is filtered by sponges. In turn, these sponges turn mucus compounds into sponge tissue, which is next released as shed filter cells (choanocytes). Finally, these old sponge cells are eaten by many reef creatures, possibly corals as well. Image from Rix et al. (2016).
Tropical and deep sea cold water corals constantly release mucus, as a form of protection and as an adhesive to enhance plankton feeding. Water currents constantly dislodge the coral mucus layer, releasing it into the surrounding water as a possible food source in the form of dissolved organic matter (DOM). Marine biologists have recently shown that sponges can feed on DOM, and that they release a part of it as detritus (particulate organic carbon, or POM) in the form of shed filter cells. In doing so, sponges play a key role in transferring energy and nutrients in DOM to other reef creatures, such as detritivores (hermit crabs, polychaetes and snails, De Goeij et al. 2013). This conversion of DOM into POM is known as the sponge loop, and is similar to the recycling of matter by the bacterial loop.
The marine biologists theorized that mucus excreted by tropical and cold water corals could be a major DOM source for the sponge loop, which they had not proven yet. So, they set up a clever experiment to do just that.
By labeling tropical (Fungia, Ctenactis and Herpolitha sp.) and temperate corals (Lophelia pertusa) with so-called stable isotopes of carbon and nitrogen, they gave the mucus produced by the corals a specific chemical signature. By culturing various labelled corals and unlabelled sponges together in aquaria, the scientists were able to determine the fate of the chemically labelled coral mucus. Indeed, both sponge species investigated (Mycale fistulifera and Hymedesmia coriacea) readily took up the dissolved coral mucus, and used its nutrients to grow new tissue. In addition, the sponges released up to 40% of the carbon and nitrogen taken up from coral mucus as detritus (POM), in the form of shed filter cells (choanocytes).
The biologists already knew that sponges were able to convert DOM into POM, which in turn acts as a food source for many reef creatures. Now, they have shown that corals provide sponges with a major source of DOM in the form of dissolved mucus. In short, the corals feed their waste to sponges, and the sponges feed their waste to the reef. This efficient recycling of animal waste by sponges helps explain why coral reefs can thrive in nutrient-poor tropical and cold waters. The next question is whether corals also ingest sponge detritus, just like detritivores, which would complete the circle.
These new insights should be heeded by aquarists, as they show that sponges and corals together create an important food web which can benefit all aquarium life.
- Jasper M. de Goeij, Dick van Oevelen, Mark J. A. Vermeij, Ronald Osinga, Jack J. Middelburg, Anton F. P. M. de Goeij, Wim Admiraal (2013) Surviving in a Marine Desert: The Sponge Loop Retains Resources Within Coral Reefs. Science 342:108-110
- Laura Rix, Jasper M. de Goeij, Christina E. Mueller, Ulrich Struck, Jack J. Middelburg, Fleur C. van Duyl, Fuad A. Al-Horani, Christian Wild, Malik S. Naumann & Dick van Oevelen (2016) Coral mucus fuels the sponge loop in warm- and cold-water coral reef ecosystems. Scientific Reports 6:18715 | DOI: 10.1038/srep18715