Reef sponges facilitate the transfer of coral-derived organic matter to their associated fauna via the sponge loop

Abstract

The high biodiversity of coral reefs results in complex trophic webs where energy and nutrients are transferred between species through a multitude of pathways. Here, we hypothesize that reef sponges convert the dissolved organic matter released by benthic primary producers (e.g. corals) into particulate detritus that is transferred to sponge-associated detritivores via the sponge loop pathway. To test this hypothesis, we conducted stable isotope (13 C and 15 N) tracer experiments to investigate the uptake and transfer of coral-derived organic matter from the sponges Mycale fistulifera and Negombata magnifica to 2 types of detritivores commonly associated with sponges: ophiuroids (Ophiothrix savignyi and Ophiocoma scolopendrina) and polychaetes (Polydorella smurovi). Findings revealed that the organic matter naturally released by the corals was indeed readily assimilated by both sponges and rapidly released again as sponge detritus. This detritus was subsequently consumed by the detritivores, demonstrating transfer of coralderived organic matter from sponges to their associated fauna and confirming all steps of the sponge loop. Thus, sponges provide a trophic link between corals and higher trophic levels, thereby acting as key players within reef food webs.

Stable isotopes as tracers of trophic interactions in marine mutualistic symbioses

“…Therefore, under low light (i.e., limited autotrophic acquisition) or warm conditions, coral symbionts sequester more resources for their own growth, thus parasitizing their hosts ( Figure 8, pie charts). Finally, recent studies using 13 C-bicarbonate tracer also allowed following the fate of autotrophically acquired carbon by coral symbionts within the coral reef food chain (Rix et al, 2017(Rix et al, , 2018(Rix et al, , 2016. A higher productivity was thus obtained for colonies with high polyp surface area/volume ratio.…”

Climate change alterations to ecosystem dominance: how might sponge‐dominated reefs function?

“…, Rix et al. ), there is clearly a need to understand the importance of the sponge loop for reef ecosystems globally, particularly if the ubiquity of this pathway varies between regions and amongst species (McMurray et al. ).…”

“…), which is released to the surrounding environment, can be consumed by sponges (Rix et al. , ), and is also a carbon source for microbes, upon which sponges can subsequently feed. The strength of these effects will depend on whether DOC and particulate organic carbon (POC) as carbon sources for sponges are limiting on reefs; this is currently unknown.…”

“…Although sponges will be in competition with algae, sponges also have the potential to benefit from increases in macroalgae, either directly through the consumption of greater amounts of DOC produced by the algae, or through the consumption of bacteria, which are thought to be utilizing the DOC on algal-dominated reefs (Haas et al 2016). Similarly, the mucus produced by corals and other benthic organisms (Silveira et al 2017), which is released to the surrounding environment, can be consumed by sponges (Rix et al 2016(Rix et al , 2018, and is also a carbon source for microbes, upon which sponges can subsequently feed. The strength of these effects will depend on whether DOC and particulate organic carbon (POC) as carbon sources for sponges are limiting on reefs; this is currently unknown.…”

“…This data would also strengthen our prediction that sponges could benefit from increased macroalgal derived DOC or from increased bacterioplankton resulting from reef microbialization. While recent research has shown that sponges can take up coral derived DOC (Rix et al 2018), it seems unlikely that they are discriminating between different DOC sources, so declines in coral DOC could potentially be met by increases in macroalgal DOC. Understanding these relationships requires a research focus on carbon assimilation at the sponge assemblage level, rather than at the individual species level (Perea-Blazquez et al 2012).…”