Chemical defense of brown algae (Dictyopteris spp.) against the herbivorous amphipod Ampithoe longimana.

Terpenoids, polyphenols, and C metabolites are broadly distributed among brown seaweeds. Terpenoids and polyphenols have often been investigated as chemical defenses against herbivores, while there are only few investigations of the fatty-acid-derived C hydrocarbons and C sulfur compounds as potential defenses. We investigated effects of C sulfur metabolites from the cosmopolitan brown alga Dictyopteris membranacea on feeding and fitness of the herbivorous amphipod Ampithoe longimana. In choice tests between freshly collected thalli of D. hoytii (which lacks C sulfur compounds) and D. membranacea (which contains C sulfur compounds) amphipods consumed about 4 times more of the species lacking the C sulfur compounds. The same feeding preference was observed when these plants were finely ground and embedded in an agar matrix to destroy morphological differences. When a diet made from field-collected thalli of D. membranacea containing C sulfur compounds was tested against a diet made from a laboratory culture of D. membranacea that had lost the ability to produce C sulfur compounds, the same magnitude of preference was observed for the population lacking the sulfur compounds. In addition to the C sulfur compounds, a water-soluble C9-oxo acid that appears to be a by-product in the biosynthesis of the C metabolites also suppressed amphipod feeding to a comparable extent. Both classes of compound may contribute to the effective chemical protection of D. membranacea. When juvenile amphipods were reared for 28 days on artificial diets containing the above compounds, their survivorship (≤10%) closely resembled that of a starved treatment, but differed dramatically from a control treatment (60%) consisting of the same food, but without the metabolites. Most other classes of brown algal secondary metabolites are defensive against a broad spectrum of larger herbivores, but relatively ineffective against the amphipod studied here. In contrast, the fatty-acid-derived sulfur compounds and the C9-oxo acid strongly deter Ampithoe-like mesograzers but appear less effective against other herbivores, suggesting that these metabolites could be ecologically important in defending zygotes and germlings against these small consumers.