Fish feeding and chemical defenses of tropical brown algae in Western Australia

Abstract

We tested the susceptibility to herbivory of 7 species of tropical brown algae from Bundegi Reef in Exmouth Gulf in Western Australia, and also analyzed these algae for the presence of polyphenolics and other non-polar secondary metabolites. All algae contained very low levels of polyphenolics, consistent with findings from other tropical regions, and the susceptibility of the algae to herbivory was not correlated with the variation in phenolic levels that did exist. Levels of polyphenolics In all these algae were probably too low to have any effect on the herbivores, and polyphenolics may in general play little role in defending tropical algae against herbivores. Five algal species, including the 2 species least favored in our grazing assays, contained other, more lipophilic metabolites. Organic extracts from these 2 species deterred feeding by herbivorous fishes; extracts from other species did not. Our data and previous work suggest that unpalatable tropical brown alyde will generally contain low levels of polyphenolics but will contain deterrent, non-polar secondary compounds. The consistently low levels of polyphenolics in tropical brown algae (relative to temperate species) remains a paradox, but may in part be caused by a paucity of trace metals which act as cofactors for biosynthetic systems.