230 L. HILLIS-COLINVAUX 



relatively recent scuba collections. In reality, it may be longer, and 

 some increase in numbers of species for neighbouring Central American 

 countries is also likely, although the eventual total from these areas 

 may never be as high. In the south-western Pacific, New Caledonia and 

 other island groups probably have a more diverse Halimeda flora than 

 is now recorded. 



2. Halimeda and coral distributions compared in outline 



Wells (1969) suggests that only two reef coral provinces are clearly 

 recognizable, the Indo-Pacific and the Atlantic, and that subprovinces 

 can be identified only "with misgivings". The preliminary data for 

 Halimeda (Table XIX) suggest greater local differentiation than this,- 

 though the same major separation between Atlantic and Pacific is 

 evident. It may be that when the Halimeda and other algal populations 

 of reefs have been studied with the taxonomic intensity of the corals, 

 their separation into provinces will be no more precise. Yet other 

 explanations for the more local distributions are possible. 



It seems likely that corals disperse much better than do Halimedae, 

 since a dispersal phase is a normal part of the life-cycle of coral species 

 whereas in most Halimeda reproduction is vegetative. Halimeda 

 populations of a remote shoreline, therefore, are more insular than coral 

 populations and likely to exhibit more local endemism. It may also be 

 that plant habitats are less restricted than those of reef corals, because 

 Halimeda grows not only on reefs, but also in sand, on dead reefs and at 

 depths well below the lower limits for reef-building corals. There may 

 thus be more opportunities for local speciation for Halimeda, perhaps 

 particularly in sites which might not be reached so easily by propagules 

 transported across oceans. 



When the distribution of Halimeda species is better known it will be 

 an interesting exercise to compare species distributions between corals 

 and Halimeda species in some detail. 



3. Halimeda distributions in species -poor areas 



Since relatively few areas have been extensively worked for 

 Halimeda, we cannot, at present, realistically ask some of the more 

 interesting questions pertaining to the geographical distribution of the 

 genus. Within this framework of caution some additional comments, 

 nonetheless, are possible. A few areas with a low number of Halimeda 

 species such as the Hawaiian Islands (four), Costa Rica (one), Easter 

 Island (one), Galapagos Islands (none), Bermudan Islands (four), 



