110 L. HILLIS-C0LINVAT7X 



associated with mangroves, and is associated with sea grasses and 

 species of Avrainvillea, Udotea and sometimes with H. opuntia. 



This is a well-known species of which many collections have been 

 made. Hence, although it may grow somewhat more deeply than 12 m, 

 it seems reasonable to consider this a species of very shallow to shallow 

 waters, and one often achieving prominence in quiet mangrove waters. 



Geographic distribution. Western and eastern Indian Ocean 

 including the Red Sea; western Pacific, both north and south. 



This species can usually be distinguished macroscopically from other 

 Rhipsalian Halimedae by its broad, flat segments with distal margins 

 usually very little lobed. These segments are relatively large, to 

 29 mm long and 40 mm broad, although in certain plants they do not 

 exceed 1 1 mm in length and 20 mm in breadth. Histologically, the 

 diameters of the utricles decrease towards the outer surface, those of the 

 peripheral utricles being relatively small. 



This species is at times confused with discoidea and taenicola, two 

 members of section Halimeda. Although doubtful specimens can be 

 readily identified on histological grounds, macroscopic criteria are 

 usually adequate. These include the extent of development of the 

 holdfast region, the general appearance of the basal part of the plant, 

 and the size and appearance of the segments. In discoidea and taenicola, 

 large segments, although often present, usually do not account for 

 50% or more of the segments as they often do in macroloba. 



Of the Rhipsalian species, macroloba is most likely to be confused 

 with borneensis because they both have relatively large, broad segments. 

 In general, those of macroloba has a less-lobed distal margin and are 

 larger. The mature thallus of macroloba is also larger than that of 

 borneensis, and it lacks the distinctive fan-shaped basal or suprabasal 

 segment which is characteristic of most thalli of borneensis. 



Microscopically, the diameters of the peripheral utricles oi borneensis 

 tend to be somewhat larger than those of macroloba, and these utricles 

 adhere, at least in some material, somewhat more firmly to each other. 

 The secondary utricles of borneensis also tend to be longer than those 

 of macroloba. 



2. Section Opuntia J. Ag. ex De Toni 



Halimeda opuntia (Linnaeus) Lamouroux 

 Figures 19, 51, 92. 

 Corallina opuntia Lmnaeus (1758), p. 805 p.p. 



