Natural diet of coral-excavating sponges consists mainly of dissolved organic carbon (DOC)

Abstract

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 × 10(10) and 1.7 ± 0.6 × 10(10) 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.

Figure 1. VacuSIP system for in situ sampling of DOC and bacteria.

VacuSIP system consisting of two separate samplers (In and Ex) attached to a stand to simultaneously take water samples of the ambient water (IN) and the water exhaled by the sponge (EX). Blue arrows indicate water pumped through the sponge.

Figure 2. Average DOC (A) and bacterial abundance (B) in the inhaled (black) and exhaled (grey) water of Siphonodictyon sp. and C. delitrix.

Error bars indicate SE. P values (Wilcoxon Signed Rank) indicate significance level of the difference in the concentrations between the inhaled and exhaled samples in n pairs of InEx samples.

Figure 3. Removal of DOC (A) and bacterial cells (B) by Siphonodictyon sp. (black) and C. delitrix (grey) plotted against ambient (inhaled) concentrations.

Both species responded linearly to elevated DOC (R² = 0.88; p = 0.004 and R² = 0.84; p = 0.002) and bacterial concentrations (R² = 0.72; p = 0.045 and R² = 0.87; p = 0.001) within the full concentration range encountered. Dashed line represents 100% bacterial removal.