Chemical defenses and the susceptibility of tropical marine brown algae to herbivores.

I assayed phenolic and tannin concentrations in a number of species of temperate and tropical brown algae of the genera Sargassum and Turbinaria. Tropical species in both genera contained consistently low levels of phenolics and tannins (species means ranged between 0 and 1.6% [measured as % dry weight of the thallus]). Levels of phenolics in temperate species of Sargassum were variable and consistently much higher than in tropical species (species means ranged between 3 and 12% by dry weight). This pattern of latitudinal variation in phenolic levels in Sargassum conflicts with previous predictions for latitudinal variation in the chemical defenses of marine organisms. The low levels of phenolics present in the tropical species that I analyzed may also explain recent results (Hay 1984; Lewis 1985) demonstrating that tropical Sargassum and Turbinaria are often preferentially consumed by herbivorous fishes and echinoids.