234 L. HILLIS-COLINVAUX 



low diversity, so much so that it is not necessary to resort to processes 

 of extinction required by an equihbrium model. If recruitment is not 

 only difficult and slow, but is actually confined to brief geologic periods, 

 when current patterns are favourable, then the slow -recruitment model 

 is amplified. It is argued above that the Mediterranean may have 

 collected its ancestral tuna population during a brief favourable period 

 after the Messinian crisis. A similar argument can be applied to the 

 flora of the Canary and Cape Verde Islands, which may been have 

 acquired during the same limited opportunity. 



Other historical events may explain the present low species diversity 

 of Halimeda elsewhere. The eastern Pacific, for example, would have 

 become isolated from any eastern centre of distribution after the 

 uplift of the Isthmus of Panama during the Neogene. In addition, its 

 biota then as now was separated from western Pacific centres of 

 distribution by a distance which Halimeda propagules appear unable to 

 bridge. 



For the southern Atlantic, a change in the pattern of surface 

 currents, possibly accompanied by increasing separation from the 

 Tethyan centre of distribution and succeeded by a slow rate of specia- 

 tion, could explain much of the present-day low Halimeda species 

 diversity. Berggren and Hollister (1974a) postulated current systems 

 which could have introduced Tethyan elements along the entire 

 Brazilian coast as late as the early Palaeogene. Present-day current 

 patterns isolate both northern and south-eastern coasts of Brazil from 

 the modern centre of distribution of Halimeda in the Atlantic which is 

 the Caribbean. Additional collecting in the southern Atlantic may 

 increase the species list somewhat, but the Brazilian flora has been 

 worked extensively by Joly and his students. 



These available studies of the fossil record of Halimeda seem to 

 hold the clear inference that the genus tends towards conservative 

 species, that dispersal is indeed slow, and that present distributions 

 may well reflect past geological events as well as contemporary 

 ecological processes. 



C. Rates of speciation within the genus 



Once evolved, Halimeda relatively quickly seems to have replaced 

 Arahicodium and Bouenia which are not known after the Cretaceous 

 (Elliott, 1965). Elliott suggests, too, that the genus diversified rather 

 rapidly during its early history into a number of species, some fossil, 

 such as H. nana (Pia, 1932), H. praeopuntia (Morellet and Morellet, 

 1922), H. praemonilis and H. eocaenica (Morellet and Morellet, 1941), 



