ORIGINAL PAPER Coral mucus stable isotope composition and labeling: experimental evidence for mucus uptake by epizoic acoelomorph worms Malik S. Naumann • Christoph Mayr • Ulrich Struck • Christian Wild Received: 31 July 2009 / Accepted: 8 July 2010 / Published online: 22 July 2010 Ó Springer-Verlag 2010 Abstract Mucus released by scleractinian corals can act as an important energy and nutrient carrier in coral reef ecosystems, and a distinct isotopic signature would allow following the fate of this material. This study investigates the natural C and N stable isotopic signatures of mucus released by four scleractinian coral genera (Acropora, Fungia, Pocillopora and Stylophora) in comparison with those of suspended particulate organic matter (POM) in seawater of a Northern Red Sea fringing coral reef near Aqaba, Jordan. The natural d 13 C and d 15 N signatures of coral mucus differed significantly from seawater POM for the majority of seasonal comparisons, but were inappro- priate for explicit tracing of mucus in the coral reef food web. Thus, a labeling technique using stable isotope tracers ( 13 C and 15 N) was developed that produced d 13 C values of up to 122 ± 5% (mean ± SE) and d 15 N of up to 2,100 ± 151% in mucus exuded by Fungia corals. 13 C and 15 N-enriched compounds were rapidly (within 3 h) and light-dependently transferred from the endosymbiotic zooxanthellae to the mucus-producing coral host. The traceability of 15 N-labeled mucus was examined by eval- uating its uptake and potential utilization by epizoic aco- elomorph Waminoa worms naturally occurring on a range of scleractinian coral taxa. This tracer experiment resulted in uptake of coral mucus by the coral-associated aco- elomorphs and further demonstrated the possibility to trace stable isotope-labeled coral mucus by revealing a new trophic pathway in coral reef ecosystems. Introduction Scleractinian corals in coral reef ecosystems continuously synthesize and release organic mucoid exudates (i.e., coral mucus) onto their epidermal tissue surfaces (Ducklow and Mitchell 1979a; Wild et al. 2008). Released mucus principally functions as a protection layer against ambi- ent environmental stressors, like sedimentation, aerial exposure, ultraviolet radiation or microbial diseases (Schuhmacher 1977; Krupp 1984; Drollet et al. 1993; Ritchie 2006). Coral mucus is a transparent exopolymer synthesized by mucus gland cells located in the coral ectoderm (Marshall and Wright 1993) and is principally composed of glycoproteins and lipids (Meikle et al. 1988), which provide a high energy content of signifi- cant nutritious value (Ducklow and Mitchell 1979b; Wild et al. 2004a). Previous studies have shown that the continuous release of scleractinian coral mucus in dis- solved and particulate forms can result in a dominance of coral-derived organic material in reef-surrounding waters (Johannes 1967; Marshall 1968). There, coral mucus can act as an important energy and nutrient Communicated by T. L. Goulet. M. S. Naumann (&)  C. Wild Coral Reef Ecology Work Group (CORE), GeoBio-Center and Department of Earth and Environmental Science, Ludwig-Maximilians Universita ¨t Mu ¨nchen, Richard-Wagner-Strasse 10, 80333 Mu ¨nchen, Germany e-mail: mnaumann@centrescientifique.mc URL: http://www.palmuc.de/core C. Mayr GeoBio-Center and Department of Earth and Environmental Science, Ludwig-Maximilians Universita ¨t Mu ¨nchen, Richard-Wagner-Strasse 10, 80333 Mu ¨nchen, Germany U. Struck Museum fu ¨r Naturkunde, Humboldt-Universita ¨t zu Berlin, Invalidenstrasse 43, 10115 Berlin, Germany 123 Mar Biol (2010) 157:2521–2531 DOI 10.1007/s00227-010-1516-3