Isotopic labeling and antifungal resistance as tracers of gut passage of the sea fan pathogen Aspergillus sydowii

Vectors play a critical role in the ecology of infectious disease by facilitating between-host transmission, emphasizing the multi-species nature of disease. Corals are suffering an onslaught of infectious diseases, yet we know little about the role of vector species in the ecology of these epizootics. The infection of octocorals by the fungus Aspergillus sydowii is a widespread Caribbean coral disease. The snail Cyphoma gibbosum is a likely vector species because it is a specialist predator of octocorals, moves regularly among coral colonies, and aggregates on diseased corals. We used a novel application of stable isotope techniques and antifungal-resistant strains of A. sydowii to identify the potential for this snail to vector disease. The use of both isotopically labeled and hygromycin-resistant fungus was successful in definitively tracing A. sydowii through the guts of C. gibbosum from ingestion to feces. Further, we demonstrated in laboratory experiments that snails fed antifungal-resistant A. sydowii excrete viable spores and hyphae in their feces. Overall, this study demonstrates the usefulness of isotopic labeling and antifungal-resistant fungi to trace the movement of pathogen propagules in microbially diverse environments, and suggests that C. gibbosum may be able to spread aspergillosis between octocoral hosts.