ECOLOGY AND TAXONOMY OF Halimeda 



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Fig. 103b. Many of the H. cylindracea populations at Enewetak develop in the vicinity 

 of rock although it sometimes is buried, rather than in the more open patches of 

 loose substrate. In the photograph, thalli of cylindracea have developed at the 

 edge of rock substrate. 



extensive. Halimeda macrophysa populations were prominent on rock 

 surfaces or in crevices, sometimes accompanied with lesser amounts of 

 gigas. Halimeda opuntia was much less extensive than in the island 

 channels, though it sometimes festooned branching Acropora. All these 

 populations were greatest in shallow water (as shallow as — 1 m), but 

 nowhere were there dense populations like those of opuntia in the 

 rapidly moving waters of channels between islets. There also appears 

 to be some geographic separation of populations of rock-attached 

 forms within the atoll because in the northern parts the commonest 

 species was none of the above three, but lacunalis f. lata. At depths of 

 1-2 m at low tide it was common near the undercut bases of coral 

 heads and hidden in Acropora thickets. It is possible that it is left in 

 these protected places by grazing pressure, for the segments showed 

 clear signs of being nibbled, perhaps by the common parrot fish. 



Two Rhipsalian species, cylindracea (Figs 102, 103) and stuposa 

 (Fig. 72, bottom), were abundant on unconsolidated substrates of the 



