ECOLOGY AND TAXONOMY OF Halimeda 67 



of the thallus. In my laboratory aquaria I have occasionally observed 

 Halimeda to produce the conspicuous grape -like clusters of gametangia 

 on only one to two branches of the thallus, or a relatively small portion 

 of the entire plant. Subsequently, it was only this part of the plant that 

 died. 



Although the entire thallus (or branch) is involved in this produc- 

 tion of gametes, specialized gametangia are formed in the process. 

 Feldmann (1951) had described them on a Mediterranean Halimeda. 

 Hence his key character "holocarpie sans gametocystes individualises" 

 is surprising, and is misleading for Halimeda, although appropriate for 

 Caulerpa as currently known. For the Codiales he indicates "gametes . . . 

 se formant dans des gametocystes individualises". The difference 

 between the Codiales and Caulerpales, based on the available sexual 

 thalli, is more appropriately described as the presence of a cross wall 

 at the base of the gametangium in the Codiales, its absence in the 

 Caulerpales. 



(ii) Wall chemistry. Parker (1970) provided a very useful review of 

 wall chemistry which included this group of algae and presented the 

 problems of using this character for taxonomy. Data available on 

 Caulerpalean algae have shown [3-1,3-xylan to be a component of 

 Caulerpa, Chlorodesmis, Halimeda and Udotea (Miwa et al., 1961; 

 Parker, 1970). It also occurs in Penicillus, and in Dichotomosiphon, the 

 only member of the Dichotomosiphonales of Feldmann's classification 

 (Frei and Preston, 1964), as well as in algae that would not be assigned 

 to either the Caulerpales or Dichotomosiphonales, such as Bryopsis sp. 

 (Miwa et al, 1961; Frei and Preston, 1964; Maeda et al., 1966). The 

 complexity of wall chemistry and the difficulties of applying it broadly 

 in the siphonaceous algae have recently been illustrated by the identi- 

 fication of different wall polysaccharides as predominating in alternating 

 stages of the life-cycle of some taxa (Huizing and Rietema, 1975). Their 

 evidence showed a mannan (probably p-l,4-mannan) as the principal 

 filament-wall polysaccharide of the sporophytic stages of Bryopsis 

 plumosa (Huds.) C. Ag. and Derbesia tenuissima (De Not. in Mor. et De 

 Not.)Crouan frat. (Banyuls material), and a xylan (probably p-l,3-xylan) 

 and cellulose as predominant wall polysaccharides of the gametophytic 

 stages of both genera. 



The work of Huizing and Rietema (1975), as they point out, does not 

 support the division of the siphonaceous green algae into a "xylan 

 group" (including Bryopsis) and a "mannan group" (including Derbesia 

 and Codium) as proposed by Miwa et al. (1961) and adopted by Maeda 

 and Nisizawa (1972). However, the evidence so far available on wall 

 chemistry in Caulerpalean genera {sensu Feldmann, 1954) indicates that 



