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Abstract Sponges are important ecological and functional components of coral reefs. Recently, a new hypothesis about the functional ecology of sponges in organic matter recycling pathways, the sponge‐loop hypothesis, in which dissolved and particulate organic matter is taken up by sponges and shunted to higher trophic levels as detritus, has been proposed and demonstrated for shallow (< 30 m) cryptic species. However, support for this hypothesis at mesophotic depths (∼ 30–150 m) is lacking. Here, we examined detritus production, a prerequisite of the sponge loop pathway, in a reciprocal transplant experiment, usingHalisarca caeruleafrom water depths of 10 and 50 m. Detritus production was significantly lower in mesophotic sponges compared to shallow samples ofH. caerulea. Additionally, detritus production rates in transplanted sponges moved in the direction of rates observed for resident conspecifics. The microbiome of these sponge populations was also significantly different between shallow and mesophotic depths, and the microbial communities of the transplanted sponges also shifted in the direction of their new depth in 10 d largely driven by changes inOxyphotobacteria,Acidimicrobiia,Nitrososphaeria,Nitrospira,Deltaproteobacteria, andDadabacteriia. This occurred in an environment where the availability of both dissolved and particulate trophic resources changed significantly across the shallow to mesophotic depth gradient where these sponge populations were found. These results suggest that changes in sponge detritus production are primarily driven by differential quality and quantity of trophic resources, as well as their utilization by the sponge host, and its microbiome, along the shallow to mesophotic depth gradient.
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Looking for the sponge loop: analyses of detritus on a Caribbean forereef using stable isotope and eDNA metabarcoding techniques
Coral reefs are biodiverse ecosystems that rely on trophodynamic transfers from primary producers to consumers through the detrital pathway. The sponge loop hypothesis proposes that sponges consume dissolved organic carbon (DOC) and produce large quantities of detritus on coral reefs, with this turn-over approaching the daily gross primary production of the reef ecosystem. In this study, we collected samples of detritus in the epilithic algal matrix (EAM) and samples from potential sources of detritus over two seasons from the forereef at Carrie Bow Cay, Belize. We chose this location to maximize the likelihood of finding support for the sponge loop hypothesis because Caribbean reefs have higher sponge abundances than other tropical reefs worldwide and the Mesoamerican barrier reef is an archetypal coral reef ecosystem. We used stable isotope analyses and eDNA metabarcoding to determine the composition of the detritus. We determined that the EAM detritus was derived from a variety of benthic and pelagic sources, with primary producers (micro- and macroalgae) as major contributors and metazoans (Arthropoda, Porifera, Cnidaria, Mollusca) as minor contributors. None of the sponge species that reportedly produce detritus were present in EAM detritus. The cnidarian signature in EAM detritus was dominated by octocorals, with a scarcity of hard corals. The composition of detritus also varied seasonally. The negligible contribution of sponges to reef detritus contrasts with the detrital pathway originally proposed in the sponge loop hypothesis. The findings indicate a mix of pelagic and benthic sources in the calmer summer and primarily benthic sources in the more turbulent spring.
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- Award ID(s):
- 2218863
- PAR ID:
- 10528556
- Editor(s):
- Hull, M
- Publisher / Repository:
- Taylor and Francis
- Date Published:
- Journal Name:
- PeerJ
- Edition / Version:
- 1
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2167-8359
- Page Range / eLocation ID:
- e16970
- Subject(s) / Keyword(s):
- Detritus Coral reef Trophodynamics Porifera eDNA Stable isotope analyses
- Format(s):
- Medium: X Size: 4.8MB Other: pdf
- Size(s):
- 4.8MB
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.7717/peerj.16970
