ECOLOGY AND TAXONOMY OF Halimeda 231 



Mediterranean region (one) and the Red Sea (three to five) have been 

 rather carefully worked, at least in the shallower parts of their reefs. It 

 therefore seems to be established that these sites do indeed support very 

 few Halimeda species. And there can be very little doubt that the 

 Galapagos Islands are without Halimeda, even though the related 

 siphonaceous genus Caulerpa is present. It is necessary to ask why the 

 Halimeda flora of these places is so poor. 



These sites combine two characteristics that might be expected to 

 restrict the opportunities for Halimeda : 



the sites are remote, insular or isolated; 



the sites have either patchy or variable environments which 

 may frequently be outside the physical tolerance of the genus. 



The remoteness of the sites should be expected to restrict the rate of 

 immigration or recruitment. This must work particularly strongly for an 

 organism which seems to have very poor powers of dispersal. The 

 variable environments might be expected not only to restrict the rate 

 of establishment, but to increase the rate of local extinction. The 

 paucity of Halimedae in these places, therefore, may be explained in 

 the classic manner of MacArthur and Wilson (1967): we have equili- 

 brium numbers of species set low by a high extinction rate and a low 

 recruitment rate. 



What we know of the reproductive strategy of Halimedae (Section 

 VII) suggests that dispersal must indeed be slow. To reach a site as 

 remote as Easter Island or the Galapagos with a piece of broken thallus 

 drifting the seas seems an unlikely process, and we have no reason to 

 believe that zygotes could survive such a voyage. And establishment 

 of propagules of these kinds is probably very unlikely. The evidence 

 of our cultures supports this. Other Halimeda relatives such as 

 Batophora and Acetahularia frequently appear in culture tanks, even 

 though pains have been taken to clean sand, water and cultivars. But 

 no Halimeda has appeared which cannot be shown to be a vegetative 

 outgrowth of another plant. The Halimeda reliance on vegetative 

 cloning is an excellent tactic for spreading from a place of establish- 

 ment, but it is not accompanied by an effective mechanism for making 

 the first invasion. 



It may be that the variable environments of these sites with few 

 species operate more as further obstacles to immigration than as agents 

 of extinction. All the sites have something unusual about their oceanic 

 regimen : the periodic extremes of El Nirio at the Galapagos (Wyrtki, 

 1973); the winter in Bermuda and the Mediterranean. These events 

 must narrow the window of opportunities for colonizations. In short, the 



