becomes less valuable as a food for herbivores, and chemical 

 defenses decrease. Additionally, new growth of Halimeda remains 

 unpigmented until just before sunrise. Thus, the valuable, 

 nitrogen-containing molecules associated with photosynthesis are 

 not placed in the new, and more herbivore susceptible, growth 

 until light is available and they can start producing income for 

 the plant. 



Experiments in a coral-reef microcosm, where diel patterns 

 of light and water chemistry could be altered, indicated that 

 Halimeda ' s growth pattern is cued by the timing of light-dark 

 cycles rather than by co-occurring diel changes in water 

 chemistry. 



Although the growth patterns of Halimeda seem unusual, 

 similar patterns appear to occur in numerous other seaweeds and 

 in microalgae such as diatoms and dinof lagellates. 



INTRODUCTION 



Herbivory plays a major role in determining the distribution 

 and abundance of primary producers in many marine communities 

 ( Lubchenco and Gaines, 1981). This is especially true on coral 

 reefs where herbivores are abundant, diverse, and directly affect 

 the species composition and biomass accumulation of benthic algae 

 and seagrasses (Hay, 1985; Carpenter, 1986; and Lewis, 1986). 

 Even though it is clear that herbivory is one of the major 

 factors affecting algal biomass on tropical reefs, ecologists 

 have only recently begun to investigate seaweed characteristics 

 that may deter herbivores. Spatial escapes (Hay, 1984b, 1985; 

 Lewis, 1986; and Taylor et. al., 1986), seasonal temporal escapes 

 (Lubchenco and Cubit, 1980), chemical deterrents (Paul and 

 Fenical, 1986; Paul and Hay, 1986; Hay et al . , in press a, b), 

 morphological deterrents (Littler and Littler, 1980; Lubchenco 

 and Cubit, 1980; Hay, 1981b; Steneck and Watling, 1982; Steneck, 

 1986; and Lewis et al . , 1987), and potential deterrents 

 associated with nutritional inadequacy (Lubchenco and Gaines, 

 1981) have all been discussed as important components of seaweed- 

 herbivore interactions. The potential importance of diel 

 escapes, when associated with rapidly mobilizable defenses, has 

 not been investigated. 



When herbivore activity is temporally predictable, plants 

 may reduce losses to consumers by producing vulnerable portions 

 only during periods of minimal herbivore activity. Examples of 

 this phenomenon include: (1) plants that produce seeds in mass 

 at intervals of several years, thus exceeding the ability of 

 local granivores to consume all seeds (Janzen, 1976); (2) plants 

 that produce new, and unprotected, growth early in the season 

 before insect herbivores reach high densities; as herbivores 

 become more abundant these plants may defend their foliage by an 

 increased production of chemical deterrents (Feeney, 1970); and 



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