Selective feeding by the giant barrel sponge enhances foraging efficiency

Foraging theory predicts the evolution of feeding behaviors that increase consumer fitness. Sponges were among the earliest metazoans on earth and developed a unique filter-feeding mechanism that does not rely on a nervous system. Once thought indiscriminate, sponges are now known to selectively consume pico-plankton, but it is unclear whether this confers any benefit. Additionally, sponges consume dissolved organic carbon (DOC) and detritus, but relative preferences for these resources are unknown. We quantified suspension feeding by the giant barrel sponge Xestospongia muta on Conch Reef, Florida, to examine relationships between diet choice, food resource availability, and foraging efficiency. Sponges consistently preferred cyanobacteria over other picoplankton, which were preferred over detritus and DOC; nevertheless, the sponge diet was mostly DOC (similar to 70%) and detritus (similar to 20%). Consistent with foraging theory, less-preferred foods were discriminated against when relatively scarce, but were increasingly accepted as they became relatively more abundant. Food uptake was limited, likely by post-capture constraints, yet selective foraging enabled sponges to increase nutritional gains.