Geographic variation in halogenated furanones from the red alga Delisea pulchra and associated herbivores and epiphytes

We examine patterns of quantitative variation in halogenated fuaranones, secondary metabolites of Delisea pulchra (Rhodophyta; Bonnemaisoniales), and how this relates to variation in local abundance of herbivores and epiphytes. Fifteen populations of D. pulchra covering a distance of 650 km of temperate southeastern Australia were sampled. Concentrations of the 4 main furanones in D. pulchra (Compounds 1 to 4) showed large variability but there was no latitudinal trend to this variation. We found significant variation in the concentrations of both total and individual furanones among locations, between summer and winter, and between different Life-history stages. Importantly, the range of total furanone concentration among plants within locations was large, often varying by an order of magnitude or more. A total of 5 species of herbivorous fish, 7 species of macroinvertebrate grazer and 6 groups of mesograzer (of varying taxonomic level) were found at the 15 locations. The abundance of both total macroinvertebrate grazers and total mesograzers known to consume D, pulchra varied significantly among locations. However, correlations between furanones and the abundance of macroinvertebrate grazers and mesograzers at the scale of location were all weak and nonsignificant, as was the correlation between furanones and the abundance of epiphytes on D, pulchra. The large variability in concentrations of furanones, and the absence of any positive relationships between furanones, herbivores and epiphytes, suggest that quantitative variation in furanones in D. pulchra is not driven by population-level selection or induction, but is more likely to be a result of small-scale variation in environmental factors such as nutrients and light, and genetic differences among individual plants.