Vermetid gastropods modify physical and chemical conditions above coral–algal interactions

Interaction modifications can arise when a third species alters the physical and chemical environment within which two other species interact. On coral reefs, corals and algae commonly interact amid a suite of other species that may modify their interaction. Massive Porites coral and algal turfs often are covered by mucus nets cast by the vermetid gastropod, Ceraesignum maximum. Previously, vermetid mucus nets have been shown to have deleterious effects on corals. Here, we hypothesized that vermetids not only have direct effects on coral, but they also change the local physical and chemical environment establishing the potential for interaction modifications by intensifying the effects of algae on corals. To test this, we examined the effect of vermetids on physical and chemical aspects of the environments. We quantified light penetration, water flow, diffusive boundary layer (DBL) thickness, and oxygen concentrations in the presence and absence of vermetid nets. Vermetid nets did not affect light levels. Because we observed reduced water flow and increased DBL thickness in the presence of nets, we also expected to observe high oxygen concentration over coral surfaces. Instead, we observed no difference in oxygen concentrations in the presence of mucus nets. To explain the lower than expected oxygen concentrations, we hypothesize that nets decreased photosynthesis and/or increased respiration of corals and algae and their associated microbiota. This is the first study to explore mechanisms underlying the deleterious effects of vermetids on corals, and shows that vermetid mucus nets may modify coral–algal interactions by intensifying physical and chemical conditions.