ECOLOGY AND TAXONOMY OF Holimeda 223 



3. Halimeda strategy as a variant of the strawberry-coral theme 



The correspondence of the Halimeda breeding strategy with the 

 requirements of the strawberry-coral model is obvious and close. There 

 are large populations established and maintained by cloning, and sex is 

 an intermittent and unusual procedure. It becomes profitable to look at 

 the life-history of Halimeda with the predictions of the model in mind. 



(a) Fluctuations in the habitat are small compared with the generation 

 time. Halimedae, in most environments, conform to this prediction in 

 the same way that the corals discussed by Williams do. The habitat is 

 unchanging on time-scales long enough to permit competitive exclusions 

 between clones whose reproductive cycle is of the order of a calendar 

 year. There is, however, the possibility of devastating grazing on clones. 

 A number of browsing animals are known to eat Halimeda, raising the 

 possibility that the almost pure stands produced by a clone could be 

 systematically wiped out by a population or migration event among the 

 herbivores. If Halimeda were to be regularly and drastically cropped, 

 then it would be expected that competitive advantage would go to 

 strains which are cropped least. The clonal habit would then be a dis- 

 advantage, the chances of fitness would be improved by variety, and the 

 vegetative production of a clone should yield to sexual reproduction. 

 There is at least reason to argue, therefore, that the unchanging en- 

 vironment demanded by the strawberry-coral model may not always 

 exist for tropical Halimedae, even when the physical habitat remains 

 constant. 



Our data are not yet good enough to resolve this question. One way 

 of looking at it is to note that it is a prediction of the strawberry-coral 

 model that the environment be essentially constant, and to extrapolate 

 from this the prediction that cropping of Halimeda must be slight. 

 There are no data for a satisfactory test of this prediction, though 

 there are lines of argument which suggest that the prediction may not 

 be met. Opuntioid forms of Halimeda are grazed by urchins, and 

 numerous taxa have been shown to eat the sand-dwelling forms 

 (Randall, 1964, 1967; Mathieson et al, 1971 ; Earle, 1972). Also there is 

 much patchiness in the distribution of Halimeda clones which might be 

 easy to understand if the patches represented a pattern of grazing. 



(b) Resting stages are produced by sexual reproduction. The scant data 

 on zygote development are in keeping with this prediction, in that the 

 zygotes develop very slowly. It is also true, however, that the adult 

 thallus can persist for a very long time in an apparently moribund 



