ECOLOGY AND TAXONOMY OF Halimeda 233 



region than is shown by the taxon today for the Mediterranean, where 

 only two forms of H. tuna are known. 



Events in the history of the area during the intervening mi Dions of 

 years, as presently interpreted by plate tectonics and continental drift, 

 may provide insights into the cause of this dramatic decrease in 

 diversity. They include the closing of the eastern end, then the western 

 end of the Tethys Sea during the first half of the Cenozoic. The effect 

 was to isolate the Tethys from the marine biota of both east and west. 

 These events were followed by extreme evaporation in the region, with 

 a sea-level drop of several thousand feet in the western Tethys during 

 the Miocene (Berggren and Hollister, 1974a, b; Hsii et al., 1977). 

 This Messinian crisis was accompanied by massive extinction of marine 

 life. It seems likely that the Halimeda flora did not survive, or at most 

 existed in a relict flora to the immediate west. 



Following this mass extinction of the Tethyan flora, the present 

 Halimeda of the Mediterranean must have been the result of a re- 

 colonization event, probably from the Atlantic Ocean. This would be a 

 plausible event following the Messinian crisis because there was then a 

 more northerly extension of warm water in the Atlantic Ocean than 

 now (Berggren and Hollister, 1974a, b). This warm water allowed 

 penetration of the new Mediterranean basin by the panoceanic species, 

 tuna. 



This reconstruction of the history of the Mediterranean Halimeda 

 flora is of special interest because it gives us a clue to the slow pace of 

 evolutionary events within the genus. The events of the Pliocene and 

 Pleistocene combined have left the Mediterranean with but a single 

 species in circumstances which suggest that the initial colonization was 

 at the beginning of this period. If this is true, then the panoceanic 

 species H. tuna is ancient and conservative. 



It is true, of course, that the above conclusions on longevity of 

 species drawn from the Mediterranean reconstruction are vulnerable to 

 arguments that tuna is a recent immigrant, but with the Suez Isthmus 

 having been closed and the cool waters now prevailing in the adjacent 

 Atlantic this seems unlikely. It may also be that a more complicated 

 history of speciation in the Mediterranean is hidden from us by extinc- 

 tions in the Pleistocene. It would be very useful indeed to recover a 

 fossil history of the genus from the Mediterranean covering the last 

 five million years. 



This view of the paucity of species in the Mediterranean adds an 

 extra dimension to the discussion of places with impoverished Halimeda 

 fioras that is given above. It is concluded there that the very low 

 recruitment rate of Halimedae may be the principal factor in producing 



