Marine algae produce a variety of secondary metabolites that functionas herbivore deterrents.
Algal metabolites, however, often fail todeter damage by some herbivores such as mesograzers that both live andfeed on their host alga. In addition, the degree to which intraspecificchemical variation in an alga influences a mesograzer's ability to deterherbivores is poorly understood. The red alga Portieria hornemannii contains the secondary metabolite ochtodene, which has been shown to varyfrom site to site on Guam and to act as a significant deterrent to fishfeeding. On Guam, the sea hare Aplysia parvula preferentiallygrazes on and is only found to live on P. hornemannii . Inlaboratory experiments, sea hares preferentially graze on populations of P. hornemannii known to have high levels of ochtodene and preferentiallyfeed on the tips of P. hornemannii thalli compared with its bases.