MEPS 277:79-93 (2004)  -  doi:10.3354/meps277079

Ambiguous role of phlorotannins as chemical defenses in the brown alga Fucus vesiculosus

Julia Kubanek1,2,*, Sarah E. Lester3, William Fenical4, Mark E. Hay1

1School of Biology, and
2School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0230, USA
3Department of Ecology, Evolution and Marine Biology, University of California at Santa Barbara, Santa Barbara, California 93106, USA
4Center for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0204, USA

ABSTRACT: Brown seaweeds (Fucales) produce phlorotannins that are often considered chemical defenses against herbivores. The many correlative and fewer direct tests conducted have shown effects of phlorotannins on herbivore feeding behavior to be variable. In an attempt to clarify the roles of phlorotannins versus other metabolites in defending brown algae, we conducted bioassay-guided fractionation of herbivore-deterrent extracts from the commonly studied brown alga Fucus vesiculosus. Feeding by the amphipods Ampithoe valida and A. longimana and the sea urchin Arbacia punctulata was suppressed by crude and water-soluble extracts of F. vesiculosus, but this deterrence was lost following storage or fractionation of the active, water-soluble extract. Phlorotannins in these extracts did not decompose in parallel with the loss of feeding deterrence. F. vesiculosus phlorotannins were fed to herbivores at 3 to 12× the isolated yield (or 4.2 to 16.8% of plant dry mass). No herbivore was deterred from feeding by concentrations of 3 or 6×, but A. valida (the only test herbivore that readily consumes F. vesiculosus in the field) was deterred at 12× isolated yield. When juvenile A. valida were raised on an artificial diet containing F. vesiculosus phlorotannins at 3× isolated yield, the phlorotannin-rich diet significantly enhanced, rather than reduced, amphipod survivorship and growth relative to an equivalent diet without phlorotannins. Females ovulated only on the phlorotannin-rich diet. Compounds other than phlorotannins appear to defend the F. vesiculosus populations we investigated, but we were unable to identify these unstable compounds.

KEY WORDS: Phlorotannin · Fucus vesiculosus · Chemical defense · Plant-herbivore interaction · Amphipod · Urchin

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