ECOLOGY AND TAXONOMY OF HaUmeda 235 



and some modern. There was also an increase in the size of segments and 

 possibly in the size of the entire thallus. Three of the species, praeopuntia, 

 eocaenica and praemonilis, are described by their authors as resembling 

 the modern species opuntia, opuntia f. minor { = goreauii) and incrassata 

 f. monilis { = monile) respectively (Morellet and Morellet, 1941), with 

 similarities such that Morellet and Morellet questioned the assignment 

 of separate names to them. 



For the remaining taxa, H. nana and the undescribed species, the 

 literature data are too sparse to associate them definitely with Recent 

 species. Even so, two of the five types of nodal filament pattern 

 delimited in Section III, those present in the Opuntia and Rhipsalis 

 sections, and representing the sprawling and sand-binding habitats 

 respectively, are represented in the species praeopuntia, eocaenica and 

 praemonilis. It seems likely that one or more tunoid species (section 

 Halimeda) also had differentiated before the Messinian crisis. If so, 

 then three of the five types of nodal filament structure would have 

 existed. These would correspond to the only patterns of nodal filaments 

 present in modern species of pantropical distribution (Table XIX), 

 plants which also represent the major habitat types of sprawling, 

 sand-binding and rock-growing. And interestingly, only pantropical 

 species are present in modern reef communities of the Hawaiian Islands, 

 the American shores and nearby islands of the eastern Pacific (with the 

 exception of Easter Island), the Mediterranean, the Canary and Cape 

 Verde Islands and Brazil (Table XIX, bottom line), while three of the 

 four species growing in the Bermudan Islands are pantropical also. 



Halimeda has been an important sediment-producer throughout its 

 history (Wray, 1977), which implies that overall conditions for its 

 growth have been favom-able, and that there have been substantial 

 populations during much of this time regardless of alterations in 

 oceans and currents. Some of these changes would have led to extinc- 

 tion, others to isolation sometimes followed by speciation. With the 

 development of various barriers such as the Isthmus of Panama, or the 

 land mass at the eastern end of Tethys, separate Indo-Pacific, western 

 Pacific and Atlantic groups would have been created out of the pan- 

 tropical stock. Subsequent speciation within them would lead to the 

 development of taxa unique to these particular regions. Figure 76 

 illustrates possible lineages among Recent Halimeda species, and the 

 directions in which development from pantropical stock may have 

 occurred in different ocean regions as suggested by known distribution 

 and similarities of habit and morphology. Some pantropical species, 

 particularly tuna and incrassata, occur infrequently in many of the 

 collections from the Pacific Ocean, and it is possible that satellite 



