Updated: Oct 21, 2019
Coral mucus is utilized by microbes and in turn released as a usable form of carbon (C) and nitrogen (N) for other coral reef-dwelling organisms. However, little work has been done on the relationship between coral mucus and sponges, aka the ‘sponge loop’. Researchers propose that corals produce a mucus, which is consequently used by sponges and turned into a nutritional and energy-rich form of C and N for coral reef ecosystems, instead of the nutrients being lost to the adjacent open ocean.
Rix et al. used SCUBA to collect warm water (WW) corals from the Red Sea at depths between 8-20m and cold water (CW) corals from Tisler Reef at depths from 75-100m. WW corals were transferred to a tank and enriched with stable isotopic tracers 13C and 15N via (13C-NaHCO3 and 15N-NaNO3) for uptake through corals’ symbiotic zooxanthellae. The CW corals were fed 13C and 15N enriched diatoms. Rix et al. then put the sponges in the tanks with corals from their respective environments. The systems were left for 3-5 days and then samples were taken to determine the isotopic ratios within the coral mucus and sponges. Sponge-produced detritus was also collected and analyzed.
Results showed that sponges do indeed take up coral mucus for nutrients and energy. Further results express that the sponges excrete a usable form of C and N that can benefit higher trophic levels (organisms higher up the food chain) in the coral reef ecosystem. CW corals generally have cycles of high and very low productivity, thus this sponge loop could be beneficial to multiple trophic levels in CW coral reef ecosystems. However further research must be conducted to confirm the idea that the sponge loop can positively impact these coral reef ecosystems and provide an alternate trophic pathway via the uptake and assimilation of C and N.