Oecologia (2003) 136:107–114 DOI 10.1007/s00442-003-1242-3 PLANT ANIMAL INTERACTIONS Michael S. Deal · Mark E. Hay · Dean Wilson · William Fenical Galactolipids rather than phlorotannins as herbivore deterrents in the brown seaweed Fucus vesiculosus Received: 5 August 2002 / Accepted: 19 February 2003 / Published online: 9 April 2003  Springer-Verlag 2003 Abstract The first investigation of seaweed chemical defense against herbivores involved the brown seaweed Fucus vesiculosus and suggested defense via phlorotan- nins. The first demonstration of seaweed induction of secondary metabolites in response to herbivory also involved the genus Fucus and assumed a defensive function for phlorotannins. Many other investigations correlate herbivore feeding preference with changing levels of phlorotannins in this genus and others, but few directly test the effects of phlorotannins. No studies have assessed Fucus chemical defenses using bioassay-guided separation to investigate the complete complement of compounds deterring herbivores. We investigated the deterrence of F. vesiculosus chemical extracts using herbivore bioassays to guide our chemical investigations. Although crude extracts from F. vesiculosus strongly deterred feeding by the sea urchin Arbacia punctulata, phlorotannins from this extract did not deter feeding at 2 or 4 natural concentration by dry mass. Feeding deterrence was due to: (1) a polar galactolipid in the ethyl acetate-soluble extract, and (2) a non-phenolic compound, or compounds, in the water-soluble extract. Although this is the first evidence of galactolipids deterring herbivores, such defenses could be geographi- cally and taxonomically widespread. The galactolipid we discovered in Fucus occurs in marine dinoflagellates, and a related metabolite that deters herbivory has recently been discovered in a tropical green seaweed. We were unable to identify the second deterrent compound, but deterrence occurred in a fraction containing carbohy- drates, including sulfated sugars, but no phlorotannins. Given the polarity of these chemical deterrents, they could co-occur with and confound bioassays of phlorotan- nins if investigators test phlorotannin-containing algal extracts without further purification. Keywords Arbacia punctulata · Glycerolipids · Plant-herbivore interactions · Polyphenolics · Seaweed chemical defenses Introduction Investigations of plant chemical defenses against herbi- vores have shaped our understanding of the ecology and evolution of prey-consumer interactions, coevolution, and the forces structuring populations and communities (Strong et al. 1984; Hay and Fenical 1988; Rosenthal and Berenbaum 1992; Fritz and Simms 1992; Duffy and Hay 2001; Paul et al. 2001). The role of plant tannins as herbivore deterrents has played a central role in virtually all general theories of plant defenses against herbivores (e.g., Feeny 1976; Rhoades and Cates 1976; Bryant et al. 1983; Coley et al. 1985; Herms and Mattson 1992), even though these large molecules are difficult to purify (Ragan and Glombitza 1986) or to accurately quantify (Appel et al. 2001), and have therefore rarely been tested directly for herbivore deterrent effects following careful purification and separation from other potentially con- founding chemical deterrents (but see Steinberg 1988; Clausen et al. 1990; Steinberg and van Altena 1992). Like tannins from terrestrial plants, phlorotannins from brown seaweeds have received considerable attention M. S. Deal Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, NC 28557, USA M. E. Hay ( ) ) School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA e-mail: mark.hay@biology.gatech.edu Tel.: +1-404-8948429 Fax: +1-404-8940519 D. Wilson · W. Fenical Scripps Institution of Oceanography, University of California — San Diego, La Jolla, CA 92093-0204, USA Present address: M. S. Deal, Biopraxis, 10655 Sorento Valley Road, Suite 200, San Diego, CA 92191, USA