176 L. HILLIS-COLINVAUX 



supposedly good to optimal conditions of growth. The comparison may 

 not be meanmgful, however. These are two widely distinct species of 

 Halimeda and different growth rates can be expected. In addition, a 

 linear measure is perhaps not the best for either comparing or measuring 

 their growth, although for these two species it is considerably more 

 meaningful than the number of segments produced. 



This growth activity of relatively rapid segment formation has, in 

 the laboratory, generally been followed by a quiescent period of a week 

 or two to a month or more, in which no new segments form. Goreau 

 (1963), observing a number of representative species of calcareous algae 

 over a period of 15 months, also noted this pattern of growth in spurts 

 in cultured Halimedae. A quiescent phase may be a laboratory pheno- 

 menon, although many of the thalli I have observed in the reef have not 

 given the impression of active growth at that particular time. It also 

 may be a species-related phenomenon. Merten (1971) calculated the 

 life-span for the shallow-water macroloba populations she worked with 

 to be four months. The pattern for them appeared to be one of growth, 

 sexual reproduction and death. 



Growth of many of the species has another component as well, that 

 of "negative growth". Some of the mature segments turn white or 

 yellowish, and eventually fall off, to be added to the calcareous sedi- 

 ments (Figs 52, 53). In culture their death and final dehiscence may take 

 a month or so, depending upon the presence and extent of grazing 

 activity, intensity of water flow around the thallus and weight of 

 epiphytes. The first losses of segments are generally restricted to the apical 

 portion of the thallus, and vigorous new branches generally develop to 

 replace fallen ones (Figs 52, 53). Occasionally there has been a massive 

 whitening in a laboratory aquarium, with the segments of the upper 

 third or half of the thalli of an aquarium population falling off a week 

 or so after transplanting. New branches routinely develop at these 

 sites. 



In some taxa this decay of segments or "negative growth" may 

 occur just as commonly, or even more so, in basal portions. The species 

 opuntia, and probably others with sprawling or prostrate habit such as 

 macrophysa, micronesica, distorta, gracilis and possibly copiosa, at times 

 show signs of decay in the lower regions of their thalli. The younger and 

 more vigorous segments and branches above them have mitiated 

 rhizoidal development and attachment and continue as separate plants 

 when the older segments fall away. This pattern of growth thereby also 

 functions as a method of vegetative reproduction. In some species or 

 environments, thalli being buried in shifting sand behave similarly. The 

 buried basal portions whiten and loosen, while a new holdfast system is 



