Biogeographical variation in brown algal polyphenolics and other secondary metabolites: comparison between temperate Australasia and North America

Polyphloroglucinol phenolics are the best known example of chemical deterrents against herbivores in temperate marine systems. However, most of the research on these compounds has been done in North America, where phenolic levels in algae are often low. I show here that algae in the Orders Fucales and Laminariales in temperate Australia and New Zealand typically contain very high levels of polyphenolics-much higher than species in these orders in North America. The median value for the distribution of mean phenolic levels for 25 North American species is 1.33% total phenolics (dry wt.); for 37 Australasian species, the median is 6.20%. Significant spatial, temporal, and intraplant variation in phenolic content occurs in a number of species in Australasia, but this does not significantly alter my major conclusion. Phenolic levels in drift algae (an important food source for some herbivores) detached for up to two weeks are also not significantly different from living, attached plants. Many species in the Fucales in Australasia also contain non-polyphenolic secondary metabolites that are not found in North American species. Thus herbivores in Australasia face greater amounts, and a greater range,of putative chemical defenses in brown algae than do herbivores in similar systems in North America. Any general theory for the evolution of marine plant/herbivore interactions must take into account such broad-scale biogeographical (and taxonomic) patterns.