Chemical defense in the seaweed Dictyopteris delicatula: differential effects against reef fishes and amphipods

ABSTFL4CT: Many seaweeds produce chemicals that deter feeding by fishes and sea urchins. A growing body of evidence suggests that small, relatively immobile herbivores (mesograzers) such as amphipods, polychaetes, and ascoglossan gastropods are often unaffected by these compounds and may preferentially consume seaweeds that are chemically defended from fishes. We tested this hypothesis by examining the responses of reef fishes and amphipods to a mixture of 2 CI1 hydrocarbons, dictyopterenes A and B, produced by the Caribbean brown alga Dictyopteris delicatula. This alga was intermediate in preference for reef fishes, and the dictyopterenes reduced fish grazing by a significant 40 %. In contrast, D. delicatula was highly preferred by a mixed-species group of amphipods and the dictyopterenes had no effect on their feeding. Despite the tendency for mesograzers to selectively consume some seaweeds that are chemically deterrent to fishes, true specialization by these or other marine herbivores appears to be rare in comparison with terrestrial systems. Plant-dwelling amphipods at our study site in the Grenadine Islands were found on, and consumed a variety of, macrophytes; they were not restrictively specialized to D. delicatula. Many terrestrial insects are very specialized feeders, sequester toxins Irom their food plants, and use these as direct defenses against predation. In contrast, sequestering of seaweed toxins by marine mesograzers appears to be relatively rare. However, the indirect advantage of living on seaweeds that are not eaten by fishes may be considerable. We hypothesize that mesograzers living on plants chemically defended from fishes will experience less predation than those living on plants preferred by fishes.