Size-specific concentrations of phlorotannins (anti-herbivore compounds) in three species of Fucus

Abstract

Using the Folin-Denis assay, we demonstrated significant (p 0.05) in phlorotannin concentrations among size-classes. Therefore, concentrations of anti-herbivore con~pounds cannot explain different effects of grazers on large and small individuals of F. spiralis. The existence of size-related escapes from herbivory in fucoids is central to the concept of intertidal vegetation structure proposed by Lubchenco (1980). According to this concept, the maintenance of a fucoid vegetation depends on the grazing activities of intertidal herbivores. These grazers are thought to remove competitively dominant ephemeral algae leaving grazer-resistant fucoids to flourish. However, grazer-resistance in Fucus vesiculosus is restricted to larger individuals of more than 3 cm length (Lubchenco 1980). Small specimens were soon eliminated by grazing activities of Littorina littorea, a large periwinkle. In order for a fucoid vegetation to develop, F, vesiculosus must pass through a grazing bottleneck a period of greatest susceptibility when plants are small. In contrast to the results obtained by Lubchenco (1980), Chapman (1989) showed that Fucus spiralis adults were susceptible to grazing by herbivores at natural densities. However, small F. spiralis abundance was not significantly affected by grazing. Geiselman (1980) reported unpublished work by 0. J. McConnell which showed that small ( 10 cm total length) plants, 0.5 g mid-thallus portions, including wing and midrib, were excised with a razor blade and ground in liquid nitrogen. In all cases w e recorded the lengths of individuals used in the analysis. Phlorotannins were extracted in re-distilled acetone from the ground powders. The extracts were evaporated under slight vacuum and the phlorotannins were then re-dissolved in distilled water. After centnfugation, the concentrations of phlorotannins in the supernatant were measured using the Folin-Denis colorimetric procedure (Ragan & Jensen 1977). Purified high 104 Mar Ecol. Prog. Ser. 65: 103-104, 1990 molecular welght polyphenol from F. vesiculosus was used as the reference compound. Concentrations of phlorotannins in the 2 size categories of each Fucus species are shown in Table 1. In F. vesiculosus there were large significant differences in concentrations among large and small plants Table 1 . Fucus spp. Mean -fSD (n = 10 in all cases) concentrat~ons (% of dry weight) of phlorotannins in large and small specimens of 3 species. Lengths of plants (mean + SD) included in the analysis are also given. p-values refer to t-tests for signficant differences among large and small plants within species (see text for details) Species Length Phlorotannin p-value