Feeding Preferences of Tegula Funebralis and Chemical Defenses of Marine Brown Algae

Abstract

I examined the effects of phenolic compounds in marine brown algae on the feeding behavior of the intertidal gastropod Tegula funebralis and other marine herbivores. The effects of algal nitrogen content and thallus toughness were also investigated. T. funebralis showed consistent feeding preferences in the laboratory and in the field in pairwise tests of 13 species of marine brown algae (Phaeophyta) from central California. Relative levels of phenolic compounds, which are known to deter feeding by herbivorous snails, differed greatly between preferred and nonpreferred species of algae. The total phenolic content of the six most preferred species of algae averaged 0.83% of dry mass; the phenolic content of the seven least preferred species averaged 4.53%. Differences in relative tanning ability were similarly large. Tannic acid, a terrestrial plant tanin, when experimentally added to one kelp species, deterred feeding by T. funebralis. These results and analyses of studies of additional species of brown algae also showed consistent differences in the phenolic content between species in the order Laminariales (typically low) and those in the order Fucales (uniformly high). Feeding preferences of other gastropod, echinoid, and crustacean algivores from the northeast Pacific Ocean are strikingly similar to those of T. funebralis; this supports the contention that high levels of phenolic compounds in brown algae are a generally effective feeding deterrent against many kinds of marine invertebrate herbivores. Organic nitrogen content of the algae was positively correlated with the feeding preferences of T. funebralis. However, this was most likely an indirect effect of the significant negative correlation between algal nitrogen and phenolic contents. Thallus toughness of the algae did not seem important in determining overall feeding preferences of Tegula, but may have affected choices between algae that were low in phenolics. I suggest that feeding preferences of Tegula funebralis for brown algae are primarily due to avoidance of algal defenses; positive aspects of food quality (such as attractants or nutritional content) probably play a lesser role in determining preference.