REEF PROCESSES 



165 



attention but can now be rejected. They found filamentous 

 boring algae in almost all calcareous substrata they exam- 

 ined and concluded that this was a major contributor to 

 total primary productivity of the system as a whole. This 

 has been fairly conclusively disproved by the work of 

 Kanwisher and Wainwright (1967), Franzisket (1968), and 

 Halldal (1968). They presented evidence that very little 

 light penetrates through the outer layers of coral tissue to 

 the skeletal boring algae, which in turn saturate at low 

 light levels and have low total photosynthetic output. It is 

 thus unlikely that these boring algae play a significant role 

 in either the nutrition of individual corals or the total pro- 

 ductivity of the system. 



All these individual population studies are preliminary, 

 but they are the only attempts so far to make a statement 

 about the contributions of particular populations to total 

 productivity of the Enewetak system. Thus there continues 

 to be a gap between whole-ecosystem studies and 

 individual-population studies. 



Coral Nutrition, Metabolism, and Growth 



Several studies conducted at Enewetak have been con- 

 cerned with one of the longest-running debates about any 

 aspect of reef biology: how corals obtain their nutrition 

 and the role of symbiotic zooxanthellae in meeting their 

 energy requirements. Studies in this area started with the 

 work of Yonge et al. on the Great Barrier Reef Exp>edition 

 of 1929, and the ensuing debate of whether corals are 

 "autotrophic" (i.e., obtaining all their energy requirements 

 from their symbiotic algae) or "heterotrophic" (i.e., depen- 

 dent on the capture of zooplankton for at least part of 

 their energy requirements) continues to this day. 



A significant contribution was made by Muscatine 

 (1967), who demonstrated that zooxanthellae isolated from 

 reef corals and giant clams incorporated radioactively 

 labeled CO2 during photosynthesis. In the presence of 

 some component of host tissue, up to 40% of the labeled 

 algal photosynthate was liberated from the algal cells, pri- 

 marily as glycerol. Muscatine was unable to evaluate the 

 rates at which this occurred in situ but suggested that 

 excretion of glycerol by the algae and its control and utili- 

 zation by the host may represent a mechanism whereby 

 the zooxanthellae contribute to the productivity of reefs. 

 The work of Muscatine has subsequently been widely cited 

 and has been influential in shaping the way that research- 

 ers think about coral nutrition and its role in reef function, 

 although quantitative determinations of transfer rates are 

 needed. 



Roffman (1968) worked with several species of intact 

 corals (with their enclosed zooxanthellae) removed from 

 the reef and placed in respirometers exposed to ambient 

 sunlight, to reduced light levels resulting from various 

 layers of screening, and to complete darkness. He calcu- 

 lated P:R ratios and concluded that some species have at 

 least the capability of obtaining all their nutritional require- 

 ments from their symbiotic algae. This is representative of 

 similar conclusions drawn from a variety of studies con- 



ducted at Enewetak and elsewhere; in fact, the research 

 itself is representative of a commonly used approach in 

 dealing with the question of coral nutrition. 



Wethey and Porter (1976a, b) likewise used a 

 respirometer approach in obtaining evidence for sun and 

 shade differences in corals, as reflected in variable rates of 

 net photosynthesis of individual colonies exposed to differ- 

 ences in the radiant-energy flux. Working with the folia- 

 ceous species Pauona praetorta, Wethey and Porter found 

 that colonies from a depth of 25 m had a lower 9^^ (max- 

 imum rate of net photosynthesis) and a lower K^, (light 

 level at which the intact coral-algal association photosyn- 

 thesized at half its maximum rate) than colonies from 10 

 m. In this case, net P was expressed as mg O2 mg chloro- 

 phyll a ' h ^ however, they did not report the amount of 

 chlorophyll in the colonies from the two depths. Wethey 

 and Porter estimated that the ratios of gross P:R for shal- 

 low and deep corals were 1.79 and 1.81, respectively, for 

 sunny days and 1.44 and 1.50, respectively, for overcast 

 days. A shallow-growing individual placed at 25 m was cal- 

 culated to have a ratio of 1.08 on overcast days. Hence, 

 shallow and deep colonies were considered to fare equally 

 well under parallel weather conditions. 



Wethey and Porter also estimated the percentage of 

 gross photosynthesis needed to sustain the coral-algal 

 association for 24 hours and calculated this to be 31% and 

 30% for shallow and deep corals, respectively, on sunny 

 days and 45% and 42%, respectively, on overcast days. A 

 shallow colony, if placed at 25 m, was calculated to 

 require 68% of its gross photosynthesis on overcast days. 

 According to Wethey and Porter, the acclimation of deep 

 corals compensates completely for low available light. They 

 stated that the species studied is morphologically special- 

 ized for autotrophy and is capable of a purely autotrophic 

 existence down to 25 m, even under overcast conditions. 

 They suggested that this species has acclimated to the 

 worst conditions that it is frequently exposed to rather 

 than to the worst conditions ever encountered on an infre- 

 quent basis. Their work appears to have important implica- 

 tions for coral nutrition and is beginning to be followed up. 



There has been much interest in the physiology of 

 skeletal formation in corals since the late fifties; important 

 work in this field was done at Enewetak. Goreau (1959), 

 in a paper which has been widely cited and which contains 

 a widely reproduced schematic figure of the chemical path- 

 ways in calcification, employed the then-new technique of 

 measuring Ca uptake to examine the calcification process 

 and factors influencing it. He reported that the rate of 

 uptake of radioactive calcium was significantly lowered for 

 corals incubated in the dark versus those incubated in the 

 light. Furthermore, the calcification rate of corals held in 

 darkness for long enough periods to cause expulsion of 

 their zooxanthellae was considerably reduced but was 

 apparently independent of light intensity. The existence of 

 growth gradients for different parts of coral colonies was 

 shown in a number of species. Calcium uptake was greatly 

 reduced by a specific carbon anhydrase inhibitor; but there 

 was still some uptake with complete inhibition, even in the 



