ECOLOGY AND TAXONOMY OF Halimeda 225 



Rios in Jamaica. The collections were planted in 44 aquaria, at densities 

 of a dozen per tank on 2 July, 1969. In 11 tanks a single thallus became 

 fertile and released gametes between 12 August and 6 November. In 

 none of these 1 1 tanks did another plant become fertile, although all 

 were bathed in an opaque green cloud of gametes for a number of hours. 

 In five tanks more than one thallus developed gametangia and gametes, 

 but individual plants waited up to a month after their neighbour's 

 sexual episode before they too developed gametangia. These data 

 suggest rather strongly that Halimeda thalli do not take sexual cues 

 from one another. In addition, in 20 years of field work in three oceans, 

 I have never seen any incidence of gametangia that would suggest that 

 a synchronous reproductive effort was being made. This observation 

 appears to be in keeping with the observations of other workers (Beth, 

 1962; Merten, 1971), and it is probably a safe, if tentative, conclusion 

 that Halimeda populations do not produce gametangia synchronously. 

 It may be that Halimedae represent an extreme version of the 

 strawberry-coral model which is an adaptation to some of the world's 

 most constant physical environments. What changes there may be in 

 the local physical circumstances of life in the tropical range of its 

 growth are too small to be worth the expense of sexual reproduction, 

 involving, as it does, the destruction of an entire thallus. Asexual cloning 

 therefore, spans the seasons and persists from year to year. The only 

 circumstance then left by the model, when sex will pay, is that of 

 individuals in the centres of clones, for whom making carbon-copies of 

 themselves is pointless. The sporadic sexuality oi Halimeda may, in fact, 

 represent the incidence with which individuals are surrounded by their 

 own clonal descendants and thus forced into sex. 



VIII. BlOGEOGRAPHY AND PhYLOGENY 



When identifying specimens of Halimeda, geographic distribution 

 is often a helpful character with which to supplement morphological 

 criteria. In spite of being an ancient pantropical genus, therefore, the 

 evolutionary history of Halimeda is still recorded in its geography. 

 There seem to be few species of very wide range but rather patterns of 

 local parallel evolution. However constant is the distribution of warm 

 equatorial waters the Halimedae of different parts of it are effectively 

 isolated. They cross ocean gaps but poorly, as would be expected from 

 our knowledge of their dispersal systems (Section VII). We may approach 

 the study of biogeography and phylogeny of this genus, therefore, with 

 the expectation that there is preserved the print of ancient episodes of 



