University of Guam Marine Laboratory, UOG Station, Mangilao, 96923 Guam.
Cyanobacteria are abundant in coral reef habitats around Guam. They sometimes forming thick mats that cover large areas of the reef, which we call cyanobacterial blooms. They produce many different nitrogenous secondary metabolites. We hypothesized that cyanobacteria are unpalatable to a variety of reef herbivores because they are chemically defended. In laboratory experiments, we tested the palatability of 27 different collections of cyanobacteria to three herbivores common to reef habitats on Guam: the parrotfish Scarus schlegeli, the sea urchin Echinometra mathaei and the sea hare Stylocheilus longicauda. We compared the consumption of freeze-dried, powdered cyanobacteria to freeze-dried powdered Enteromorpha clathrata, a palatable green alga. We also compared the consumption of E. clathrata coated with crude organic extracts of cyanobacteria to uncoated E. clathrata. All strains, when freeze-dried and powdered, were deterrent to all three herbivores. While it is clear that powdered specimens are unpalatable, it is unclear whether this deterrence is due to secondary metabolite content, low nutritional value, or other factors. Most organic extracts tested deterred feeding by the parrotfish and sea urchin but stimulated feeding by the sea hare, a more specialized consumer of cyanobacteria. We propose that the production of deterrent secondary metabolites by many benthic cyanobacteria provides protection from grazing by generalist herbivores and can facilitate the formation of cyanobacterial blooms in coral reef habitats.