Feeding ecology of coral reef sponges

Sponges are ubiquitous in coral reefs and in terms of biomass they are often second to corals. Inside coral reef crevices sponges are the dominant organisms, providing up to 60% of the living coelobite cover. We categorized them into 3 distinct groups according to their habitat: obligate coelobites (OC), living exclusively in coral reef crevices; facultative coelobites (FC), occurring both inside crevices and on the outer reef surface; and epi-reefal sponges (ER), dwelling only on the exposed reef surface. In incubation experiments, cryptic sponges released 4 times more total inorganic nitrogen (TIN) (0.51±0.41µmol g AFDM-1 h-1) and 2 times more phosphate (0.07±0.05µmol g AFDM-1 h-1) than ER sponges. 72-91% of TIN released was in the form of ammonia, suggesting that coelobite mineralised nutrients are readily assimilable by algae and zooxanthellae in corals. Comparative in situ measurements of ultraplankton uptake showed that retention efficiency differed between plankton groups: larger eukaryotic algae were retained less efficiently (~60%) than the smaller autotrophic prokaryotes Prochlorococcus and Synechococccus (>90%) with no marked differences between sponge groups. Heterotrophic bacteria were retained most efficiently by OC (83±6%, median±MAD), albeit at 8-fold lower pumping rates. Low volume throughput and high retention efficiency appear as adaptations of OC to the limited supply of plankton and low volume flow in framework crevices. Molecular and histological techniques revealed that ER and FC sponges had only very low or moderate numbers of associated bacteria in their tissue whereas OC sponges harboured high densities. OC and FC community uptake amounted to 0.60±0.36 g C d-1 per projected m2 of reef, equivalent to one sixth of the gross productivity of the entire reef. ER community uptake was more than one order of magnitude lower, compounding the importance of coelobite filter feeders in harnessing pelagic material for the reef benthos.