Ultimate factors selecting for nocturnal segment production 



Herbivore avoidance may have played an important role in 

 selecting for the nocturnal timing of Halimeda growth shown in 

 Fig. 3. All of our data suggest that nocturnal production of new 

 segments results in lowering losses to day-active herbivorous 

 fishes which have been shown to be the herbivores of primary 

 importance on many coral reefs (Randall, 1961; Hay, 1981a, 1984a, 

 1985; Hay et al., 1983; Lewis, 1986). 



Mature portions of most Halimeda species appear to be well 

 protected from herbivory. Of more than 100 algal species that 

 have been exposed to guilds of herbivores (primarily fishes) on 

 16 different reefs in the Caribbean Sea and Indian Ocean, species 

 of Halimeda are consistently among the species least susceptible 

 to removal (Ogden, 1976; Hay, 1981a, 1984b; Lobel and Ogden, 

 1981; Littler et al . , 1983; Lewis, 1985, 1986; Paul and Hay, 

 1986). Decreased vulnerability appears to result from multiple 

 defensive characteristics that include: (1) adult tissues 

 composed of 22% (H^ cuneata ) to 85% (H^ opuntia ) calcium 

 carbonate by dry weight (Hillis-Colinvaux, 1980); (2) production 

 in Halimeda of toxic diterpenoid trialdehydes (Paul and Fenical, 

 1983; Paul, 1985); and (3) an aggregated growth form in some 

 species (H^ opuntia ) that decreases herbivore access and may 

 increase the probability of herbivores being attacked by their 

 predators while feeding on the plant (Hay, 1981b; Hay et al . , 

 1983). Although formidable, these defenses are not absolute. 

 Some parrot fish commonly eat Halimeda , often biting the tips, 

 (Lobel and Ogden, 1981; Wolf, 1985; N. Wolf, personal 

 communication; D. Morrison, personal communication). We 

 occasionally observed parrotfish feeding on the tips of Halimeda 

 at our study sites in St. Croix and we often observed the effects 

 of what appeared to be parrotfish grazing. Young segments were 

 preferred to older segments by fishes on the reef slope (Fig. 8) 

 and by fishes in the shallow lagoon. At the lagoon site, we 

 often saw small bite marks in young segments; in addition, mature 

 apical segments occasionally had small grazing scars that 

 coincided with locations where new buds or tips would have 

 occurred (see Fig. 1). Grazing scars were rare in other 

 locations on apical segments or on mature segments on any portion 

 of the plant. On the deep sand plain, we seldom saw evidence of 

 grazing on mature Halimeda segments; however, during the 2 days 

 that we repeatedly measured the width of newly forming segments 

 (Fig. 4), some of these appeared to have been grazed by small 

 fishes. 



Several factors may explain why herbivorous fishes consume 

 young segments more frequently than older segments. When first 

 initiated, tips of Halimeda incrassata are not calcified and 

 they have nitrogen and organic contents that are over 300% higher 

 than those found in mature segments (Fig. 5). By 1200 hr when 

 the branches with young versus old apical segments were 



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