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

Abstract

Many seaweeds produce chemicals that deter feedlng by fishes and sea urchins. A growing body of evidence suggests that small, relatively immobile herbivores (mesograzers) such as amphpods, 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 mutture of 2 C,, hydrocarbons, &ctyopterenes A and B, produced by the Canbbean brown alga D~ctyopteris 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 muted-species group of amphipods and the dlctyopterenes 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 companson with terrestnal systems. Plant-dwelling amphipods at our study site in the Grenadine Islands were found on, and consumed a variety of, macrophytes; they were not restrict~velys pecialized to D. delicatula. Many terrestnal insects are very specialized feeders, sequester toxins from theu food plants, and use these as duect defenses against predation. In contrast, sequestenng of seaweed toxlns by marine mesograzers appears to be relahvely rare. However, the indirect advantage of llving on seaweeds that are not eaten by fishes may be considerable. We hypothesize that mesograzers living on plants chemically defended from fishes wlll experience less predation than those living on plants preferred by fishes.