Trophic ecology of a freshwater sponge (Spongilla lacustris) revealed by stable isotope analysis
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The vital roles that sponges play in marine habitats are well-known. However, sponges inhabiting freshwaters have been largely ignored despite having widespread distributions and often high local abundances. We used natural abundance stable isotope signatures of carbon and nitrogen (δ 13C and δ 15N) to infer the primary food source of the cosmopolitan freshwater sponge Spongilla lacustris. Our results suggest that S. lacustris feed largely on pelagic resources and may therefore link pelagic and benthic food webs. A facultative association between S. lacustris and endosymbiotic green algae caused S. lacustris to have significantly depleted carbon and nitrogen signatures that may reflect carbon and nitrogen exchange between sponges and their symbiotic algae. Isotopic data from specialist sponge consumers demonstrated that sponges hosting zoochlorellae were the major component of the diet of the spongillafly Climacia areolaris and the sponge-eating caddisfly Ceraclea resurgens suggesting that the symbiosis between freshwater sponges and algae is important to sponge predator trophic ecology. Our results help define the role of sponges in freshwater ecosystems and shed new light on the evolution and ecological consequences of a complex tri-trophic symbiosis involving freshwater sponges, zoochlorellae, and spongivorous insects.
KeywordsFood webs Energy flow Symbiosis Zoochlorellae Sisyridae Ceraclea Sponge predators
Our sincerest thanks to Alec R. Lindsay and Alan J. Rebertus for their advice during the planning and execution of this project, to Kalin Wise and Steve Connolly for help in the field and laboratory and to Thomas D. Getman for providing vital laboratory equipment. Previous drafts of this manuscript were greatly improved by thoughtful comments from Bryan L. Brown, Robert P. Creed, and two anonymous reviewers. All isotope mass spectrometry was conducted by the staff at the Alaska Stable Isotope Facility at the University of Alaska Fairbanks. This study was funded by the Charles C. Spooner Research Fund and the Northern Michigan University Excellence in Education Research Program.
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