Macroalgal chemical defenses and their roles in structuring tropical marine communities

Abstract

The impacts of herbivores on macroalgae have been relatively well studied in the last decades (e.g., Carpenter 1986; Choat 1991; Hixon and Brostoff 1996; Harborne et al. 2006; Vinueza et al. 2006), probably because of their profound effects in both temperate and tropical communities and their importance as major conduits of energy between autotrophs and the rest of the food web. Nonetheless, very little is understood about how macroalgal chemical defenses - against herbivores or against competitors, epibionts, pathogens, etc. - function and what their exact role is in structuring marine communities. Tropical marine communities are well known to possess a plethora of macroalgal species (see Lüning 1990; Kerswell 2006), as well as an equally high diversity of herbivores (Floeter et al. 2005), which exert an intense, constant, and unparalleled pressure on the former, presumably selecting, over evolutionary timescales, for the presence of diverse and effective chemical defenses. In this scenario of bottom-up control, a number of counteradaptations are supposed to have occurred in herbivores, including feeding specialization and sequestration of defenses, which represent potential steps toward a top-down control of macroalgal communities. However, experimental evidence to support these assumptions remains largely elusive. This chapter attempts to review, although not exhaustively, our current knowledge of different aspects of chemical defenses from tropical macroalgae and their potential effects on tropical marine benthic community structure. The focus will be on known examples that support or rebut theoretical assumptions about tropical communities in an effort to point the reader toward new trends or research priorities that emerge from our current knowledge about marine tropical systems. This chapter primarily considers aspects of chemical defenses in tropical macroalgae belonging to the divisions Chlorophyta, Phaeophyta, and Rhodophyta, but also includes macroscopic and conspicuous tropical Cyanobacteria that can be important in structuring tropical marine benthic communities.