ECOLOGY AND TAXONOMY OF Halimeda 175 



In very old segments the peripheral utricles may separate more readily 

 or may be somewhat thickened. 



New segments may develop daily or every other day on each branch 

 so that many new segments can be added in a relatively short time. 

 This growth pattern has been observed also with cylindracea, opuntia 

 and gigas, growing in running seawater aquaria mamtained for three 

 weeks in the roofed open-air wet laboratory of the Mid-Pacific Marine 

 Laboratory at Enewetak. 



The growth which occurred on two thalli of incrassata over a time 

 span of 72 and 61 days is shown in Figs 52 and 53, respectively. Thirteen 

 days after the photographic series was started for the plant of Fig. 52 

 it had added 35 segments or 114 mm. Seventy -two days after the begin- 

 ning of the series it has added 96 segments or 265 mm. These figures 

 represent a rate of growth of 6-1 cm wk~i for the first two weeks, and 

 2-6 cm wk~^ at the end of the tenth week. For the plant of Fig. 53, 16 

 segments or 48-3 mm were added in 18 days yielding a growth rate of 

 l-9cmwk~i, after 29 days 31 segments or 87-3 mm had been added 

 representing a rate of 2-1 cm wk~^, and by 61 days 43 segments or 

 116-5 mm had been added making the growth rate 1-3 cm wk~i. 

 Twenty-two days later a new plant as well as new segments had been 

 produced, and some segments had been shed. These rates and other 

 similar ones from my cultures indicate some of the growth potential of 

 incrassata at low light intensities, as does the growth analysis by Chen 

 and Jacobs (1966) on Caulerpa prolifera (Forsskal) Lamouroux. They 

 cultured Caulerpa on supplemented seawater at 24 + 1 °C at 100 ft-c on 

 a 12-hour light : 12-hour dark cycle, and found that rhizomes, rhizoid 

 clusters and "leaves" elongated at the rate of approximately 4-4 mm d~^ 

 or 3-1 cm wk-^. It seems likely that the incrassatae of the reef, when 

 actively growing, may have a growth rate which is at least comparable 

 to the best of those reported, that is, to 6 cm wk~^. 



Merten (1971), in Guam, worked with 200 plants of macroloba in 

 both field and laboratory environments. The average growth rates for 

 her four populations followed the same pattern, and all four were of the 

 same general order of magnitude. For the 200 plants she gives an average 

 growth rate of 5-8 cm for the first month, and 1-5-3-0 cm for the other 

 months, with the laboratory populations performing as well or slightly 

 better than the three field populations in overall height, although the 

 plants were less branched and the segments irregular. The higher rate 

 obtained is of the same order of magnitude as the opportunistic growth 

 measure of Finckh (1904) on an unknown species of Halimeda. 



Marten's rates are lower than those given for the incrassata plants 

 although three-quarters of her measurements were of reef plants, under 



