ECOLOGY AND TAXONOMY OF Halimeda 71 



JRhipocephalus, Tydemania and Udotea to the Udoteae, and Avrain- 

 villea, Boodleopsis, Callipsygma, Chlorodesmis, Cladocephalus, Flabellaria, 

 Rhipidodesmis, Rhipilia and Ehipiliopsis to the Flabellarieae. 



The Gepps considered the calcified and uncalcified groups to have 

 developed from calcified and uncalcified ancestors, and that they were 

 "fundamentally and physiologically distinct", with their separation 

 occurring far back in their developmental history. 



This interpretation of evolution within the group has, as one of its 

 weaknesses, the re-establishment of the two species of the type genus 

 Flabellaria in the genus Udotea from whence the Gepps had removed 

 them. In addition, MacRaild and Womersley (1974) have demonstrated 

 the alternation of a calcified (aragonitic) with an uncalcified phase in 

 the life-cycle of a single alga, the former Derhesia clavaeformis (J. Ag.) 

 De Toni, which they transferred to the new genus Pedobesia MacRaild 

 and Womersley of the order Derbesiales. Such an alternation of calcified 

 and uncalcified phases, which may prove to be more widespread, indi- 

 cates that the Gepps' scheme is neither as useful nor as basic as the 

 proposed separation along cytological and reproductive lines of the two 

 orders Caulerpales and Derbesiales. 



D. Summary: the identification and classification of Halimeda 



Many of the characters which separate the different species of 

 Halimeda require the use of a compound microscope (magnification 

 usually X 100 and x 400), and some prior sectioning, followed by 

 decalcification of the material. Nonetheless, a few macroscopic charac- 

 ters are separated which, within limits, are helpful in field determina- 

 tions of groups or taxonomic sections of species, and occasionally of 

 species themselves. These are : the appearance of the plant as a whole, of 

 the holdfast system, and the of majority of the segments. Finer deter- 

 mination of species, or of species groups, in the field is helped by a 

 knowledge of their geographic range. 



Microscopic taxonomic characters which are most used are : pattern 

 of medullary filaments at the node (Table III), size and appearance of 

 the different series of utricles (Figs 17, 20), and pattern and extent of 

 cortex (Fig. 20). All these characters vary somewhat with age and the 

 position of the segment on the thallus. The specialized reproductive 

 structures, the grape-like clusters of gametangia, do not occur on most 

 material, and have not proved particularly useful in taxonomic work. 



The different patterns of nodal filaments found in different groups 

 of Halimeda species are useful in delimiting groups of species and are 

 considered to reflect fundamental phyletic divergence. Five taxonomic 



