DIEL PATTERNS OF GROWTH, NITROGEN CONTENT, HERBIVORY, 



AND CHEMICAL VERSUS MORPHOLOGICAL DEFENSES: 



CAN TROPICAL SEAWEEDS REDUCE HERBIVORY BY GROWING AT NIGHT? 



Mark E. Hay 1 , Valerie J. Paul 2 , 5 , Sara M. Lewis 3 , 

 Kirk Gustafson 2 , Jane Tucker 4 , and Robbin N. Trindell 1 



-'-University of North Carolina at Chapel Hill, 

 Institute of Marine Sciences, 

 Morehead City, NC 28557 



^Scripps Institution of Oceanography, 

 La Jolla, CA 92093 



3 Aiken Laboratory, Harvard University 

 Cambridge, MA 02138 



^Ecosystems Center, 

 Marine Biological Laboratory, 

 Woods Hole, MA 02543 



5 Present address: University of Guam Marine Laboratory, 

 UOG Station, Mangilao, Guam 96923 



ABSTRACT 



Tropical seaweeds in the genus Halimeda reduce losses to 

 grazing by capitalizing on diel patterns of herbivore activity. 

 These seaweeds produce new, more herbivore-susceptible growth at 

 night when herbivorous reef fishes are inactive. Plant portions 

 more than 48 hr old are low in food value, well defended 

 morphologically (calcified and high in ash content), and 

 relatively resistant to herbivory. Younger plant portions 

 represent 3-4.5 times the food valued (nitrogen or organic 

 content) of older portions but are only moderately more 

 susceptible to herbivores due to their high concentrations of the 

 terpenoid feeding deterrents halimedatrial and 



halimedatetraacetate. Halimedatrial significantly deters grazing 

 by both parrotfishes ( Scaridae ) and surgeonf ishes ( Acanthuridae ) 

 and occurs in high concentrations (2-4.5% of plant ash-free dry 

 mass) in plant portions that are 4-12 hr old, intermediate 

 concentrations (0.3-2.3%) in portions that are 16-26 hr old, and 

 low concentrations (0.3%) in older plant portions. The related 

 compound halimedatetraacetate is absent from the youngest plant 

 portions, shows a rapid increase in concentration (from to 1%) 

 in plant material that is approximately 16 hr old, and then 

 rapidly declines to low levels (0.1 to 0.2%) in older plant 

 portions. Thus, newly produced tissues are nutritionally 

 valuable but contain high concentrations of defensive chemicals. 

 As these tissues age, morphological defenses increase, the tissue 



® 1983 Springer Verlag. Oecologia , Vol. 75, p. 233. 



365 



