ECOLOGY AND TAXONOMY OF Holimeda 273 



scanty. There are no field data beyond the general observation that 

 white moribund segments are observed on wild plants, that plants do 

 die as entire thalli and that death of the entire thallus always follows 

 the release of gametes. None of these observations of wild plants 

 allows estimates of either the frequency of death or the frequency at 

 which single segments are shed. For the best approximations it is 

 necessary to use data from our laboratory cultures. 



In the aquaria cultures individual plants of incrassata have lived 

 for as long as two years, though the latter part of such a life seems to 

 be spent covered in epiphytes, or under attack from epiphytes which 

 have to be removed at frequent intervals. Plants in that condition 

 have been seen in the field, but they are not usual and it is hard to 

 believe that they can persist. On the other hand, there is no evidence 

 for rapid turnover of populations as occurs in culture with Penicillus of 

 which there may be four or six laboratory generations per year. For 

 want of better data it is, perhaps, conservative and best to assume that 

 a typical wild incrassata would live out the more vigorous part of its 

 laboratory term of months, which allows us to assume that it contri- 

 butes its total stock of segments in the white senile condition in a year. 



The mean number of segments on 15 incrassata thalli sacrificed for 

 other purposes in the laboratory was 131. These were field-grown 

 incrassata that had been in the laboratory only a few weeks, though 

 some were in one sense "mature" since they became fertile. However, 

 the field collections were always of smaller plants, because these were 

 best suited to the aquaria, and the figure of 131 segments per plant is 

 biased towards the small side. It seems reasonable to assume that the 

 typical incrassata thallus in the wild has 200 segments when full grown. 



The flux of carbonate to reef sediments from an incrassata popula- 

 tion, therefore, is 200 x 4 mg =0-8g per thallus per year. This and 

 other conversion factors for computing productivities from Halim.eda 

 census data are given in Table XXXII. 



(c) Carbonate flux from Halimedae at Glory Be reef. The Halimeda 

 census data for Glory Be reef that were used when calculating the flux 

 of reduced carbon can be used for calculating the carbonate flux also, 

 if we accept some simplifying assumptions. All Halimedae of the reef 

 are considered to be equivalent to incrassata thalli, and the rate of loss 

 of senile segments is considered to be constant at all parts of the reef. 

 The more dangerous assumption is that all of the Halimedae are 

 equivalent to incrassata, though the errors introduced may not be 

 great in the light of the other uncertainties. Table XXX shows that of 

 the two common rock-growing Halimedae of cushion life-form in the 



