Coral-excavating sponges are the most important bioeroders on Caribbean reefs and increase in abundance throughout the region. This increase is commonly attributed to a concomitant increase in food availability due to eutrophication and pollution. We therefore investigated the uptake of organic matter by the two coral-excavating sponges Siphonodictyon sp. and Cliona delitrix and tested whether they are capable of consuming dissolved organic carbon (DOC) as part of their diet. A device for simultaneous sampling of water inhaled and exhaled by the sponges was used to directly measure the removal of DOC and bacteria in situ. During a single passage through their filtration system 14% and 13% respectively of the total organic carbon (TOC) in the inhaled water was removed by the sponges. 82% (Siphonodictyon sp.; mean±SD; 13±17 μmol L−1) and 76% (C. delitrix; 10±12 μmol L−1) of the carbon removed was taken up in form of DOC, whereas the remainder was taken up in the form of particulate organic carbon (POC; bacteria and phytoplankton) despite high bacteria retention efficiency (72±15% and 87±10%). Siphonodictyon sp. and C. delitrix removed DOC at a rate of 461±773 and 354±562 μmol C h−1 respectively. Bacteria removal was 1.8±0.9×1010 and 1.7±0.6×1010 cells h−1, which equals a carbon uptake of 46.0±21.2 and 42.5±14.0 μmol C h−1 respectively. Therefore, DOC represents 83 and 81% of the TOC taken up by Siphonodictyon sp. and C. delitrix per hour. These findings suggest that similar to various reef sponges coral-excavating sponges also mainly rely on DOC to meet their carbon demand. We hypothesize that excavating sponges may also benefit from an increasing production of more labile algal-derived DOC (as compared to coral-derived DOC) on reefs as a result of the ongoing coral-algal phase shift.

Much recent marine microbial research has focused on sponges, but very little is known about how the sponge microbiome fits in the greater coral reef microbial metacommunity. Here, we present an extensive survey of the prokaryote communities of a wide range of biotopes from Indo-Pacific coral reef environments. We find a large variation in operational taxonomic unit (OTU) richness, with algae, chitons, stony corals and sea cucumbers housing the most diverse prokaryote communities. These biotopes share a higher percentage and number of OTUs with sediment and are particularly enriched in members of the phylum Planctomycetes. Despite having lower OTU richness, sponges share the greatest percentage (>90%) of OTUs with >100 sequences with the environment (sediment and/or seawater) although there is considerable variation among sponge species. Our results, furthermore, highlight that prokaryote microorganisms are shared among multiple coral reef biotopes, and that, although compositionally distinct, the sponge prokaryote community does not appear to be as sponge-specific as previously thought.

The cyanobacteriospongeTerpios hoshinotais expanding its range across the Indo-Pacific. This species can have massive outbreaks on coral reefs, actively kill corals it overgrows and affect the entire benthic community. Although it has received much attention on the post-outbreak follow-up, little is known about its ecology, habitat preferences, and the possible environmental triggers that cause its outbreaks. We present a baseline study in the Spermonde Archipelago, Indonesia, whereT. hoshinotawas first observed in 2012. We surveyed 27 reefs and recorded patches between 14 and 217 cm2, at four reef sites (~15% cover). The sponge was found on both mid-shelf and outer shelf reefs but not close to the coast and the city of Makassar. Differences in benthic community structure, as well as spatial variables relating to the on-to-offshore gradient in the Spermonde archipelago, neither constrained nor promoted its expansion. Patches of the sponge were mostly overgrowing branching corals, belonging to Acroporidae species. Genetic variation withinT. hoshinotawas studied by sequencing partitions of the mitochondrial CO1 and nuclear ribosomal 28S gene. Two haplotypes were found within the Spermonde archipelago, which differed from the CO1 sequence in GenBank. The present study provides an indication of habitat preferences ofT. hoshinotain non-outbreak conditions, although it is still unclear which environmental conditions may lead to the onset of its outbreaks.