Surviving in a Marine Desert: The Sponge Loop Retains Resources Within Coral Reefs

TLDR

Coral reefs thrive in nutrient‑poor marine deserts, yet the mechanism by which abundant dissolved organic matter is transferred to higher trophic levels remains unclear. The sponge loop, demonstrated in aquarium and in situ experiments using (13)C‑ and (15)N‑enriched DOM, shows that sponges rapidly convert dissolved organic matter into particulate detritus. This process supplies reef fauna with detritus, explaining reef persistence in oligotrophic seas and informing conservation strategies.

Abstract

Ever since Darwin's early descriptions of coral reefs, scientists have debated how one of the world's most productive and diverse ecosystems can thrive in the marine equivalent of a desert. It is an enigma how the flux of dissolved organic matter (DOM), the largest resource produced on reefs, is transferred to higher trophic levels. Here we show that sponges make DOM available to fauna by rapidly expelling filter cells as detritus that is subsequently consumed by reef fauna. This "sponge loop" was confirmed in aquarium and in situ food web experiments, using (13)C- and (15)N-enriched DOM. The DOM-sponge-fauna pathway explains why biological hot spots such as coral reefs persist in oligotrophic seas--the reef's paradox--and has implications for reef ecosystem functioning and conservation strategies.