Lack of avoidance of phenolic-rich brown algae by tropical herbivorous fishes

Abstract

Abstract. High levels of polyphloroglucinol phenolics in marine brown algae are usually interpreted as a defensive response to herbivory. However, tropical brown algae generally contain very low levels of phenolics, even though herbivory in many tropical systems (e.g. coral reefs) is intense. This apparent paradox would be explained ifpolyphenolics did not deter tropical herbivores, in which case selection by herbivores for high levels of phenolics in tropical algae would be weak. To examine this hypothesis, in February 1989 we presented mixed assemblages of herbivorous fishes on the Great Barrier Reef with tropical, phenolic-poor brown algae (primarily Sargassum spp.) and closely related (conspecifics in one instance) phenolic-rich temperate species. Different species of brown algae were eaten at very different rates, but these differences were not correlated with variation in the phenolic levels among the plants. TLC and NMR analyses showed no evidence of other, non-polar, metabolites in these algae, with the exception of the temperate species Homoeostrichus sinclairii. Thus, variation in non-polar metabolites also did not explain the differences in susceptibility to herbivores among these algae. We conclude that the herbivorous fishes studied here were not deterred by phenolic-rich algae, which suggests that levels of phenolics in many tropical algae may generally be low due to their ineffectiveness as defences. However, alternative explanations for the pattern are possible, and these are discussed.

Effects of herbivores and water quality on Sargassum distribution on the central Great Barrier Reef:cross-shelf transplants

“…The heavy epiphyte load and poor condition of plants on open midshelf plots suggest that loss of photosynthetic tissues to grazing may leave the remaining tissue with insufficient resources for growth or defense against epiphytes. Steinberg et al (1991) also found Sargassum to be grazed by fish on mid-shelf reefs in this area, although apparently more rapidly than the current study.…”

“…Thus, unprotected Sargassum may be generally unable to survive herbivory on these reefs. Since Steinberg et al (1991) found several species of Sargassum to be heavily grazed, there seems no a priori reason to assume species-specdic effects. While this study did not consider the survival of juvenile plants, unprotected Sargassum recruits would presumably be grazed at least as severely as the adults.…”

“…However, none of these studies examined possible causes of differences in macroalgae abundance across the continental shelf, or between habitats. Steinberg et al (1991) found Sargassum to be grazed on mid-shelf reefs, but did not test its survival in the absence of herbivory, nor use return transplants.…”

The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts

“…Additionally, as new herbivores invade, they encounter plants that have not been selected to resist these herbivores [53]. In the few experiments where tropical fishes and temperate seaweeds or their tissues have been mixed, the temperate seaweeds have generally been readily consumed [54]. Kelps and fucoids, the main foundation species of temperate rocky reefs, commonly produce phlorotannins, some of which deter herbivory [55].…”

Biogeography of sponge chemical ecology: comparisons of tropical and temperate defenses

“…Herbivorous fishes are more abundant on tropical reefs than on temperate reefs, creating greater herbivore pressure in the tropics (Meekan and Choat 1997), which may have selected for increased defenses in tropical seaweeds (Hay 1991;Bolser and Hay 1996). In addition, tropical grazers may be more tolerant of chemical defenses than temperate grazers (Steinberg et al 1991;Cronin et al 1997).…”