ECOLOGY AND TAXONOMY OF HaUmeda 106 



Habitat. Grows in sand, mud or other unconsolidated substrate, 

 from about — 0'3 to —75 m, but is comparatively uncommon at 

 the extreme depths. HaUmeda simulans may be associated with 

 monile in the Caribbean but is more commonly found in patches of 

 incrassata. In the laboratory it sometimes can be initially mistaken 

 for incrassata (see below), but in the reef it often seems fairly 

 distinct, and sometimes can be readily spotted from above, by noting 

 the rounder thalli in patches of Rhipsalian Halimedae. These 

 individuals are generally simulans, since thalli of incrassata tend 

 to have a more linear appearance. Relatively large populations of 

 simulans may occur, or it may be present as a few thalli at the edge 

 of a grove of incrassata. 



Geographic distribution. Eastern Indian Ocean, north-western 

 Pacific, north and south in the western Atlantic including Bermuda. 

 It is best known from the Caribbean. 



This species often may be distinguished from incrassata macroscopic- 

 ally by its substipitate or occasionally stipitate habit with basal 

 segments of the lowermost branches commonly imbricated. Other 

 distinguishing characteristics are the predominantly greenish-cream, 

 reniform or slightly trilobed segments, and the smaller diameters of the 

 peripheral utricles which average 45 (xm in simulans as compared to 

 73 [xm in incrassata. In incrassata the peripheral utricles adhere more 

 firmly, whereas in simulans the corners often appear slightly rounded 

 in surface view, the utricles nevertheless usually remaining attached. 

 The pores between adjacent nodal medullary filaments in simulans at 

 times may be relatively small and consequently not particularly 

 conspicuous in longitudinal sections. Their presence then is more easily 

 demonstrated by transverse section through the node. 



The reniform segments of this species sometimes lead to confusion 

 with tuna from which it is entirely distinct microscopically. Character- 

 istics of habit, segment colour, the tendency for segments to be lobed in 

 simulans rather than entire as in tuna, and particularly the presence of a 

 more extensive holdfast with adhermg particles of fine substrate 

 distinguish these two species macroscopically. 



HaUmeda borneensis W. R. Taylor 



Figure 27. 



HaUmeda borneensis Taylor (1975), p. 81, Figs 1, 2. 



Plants erect, compact and rather small, to 12 cm broad and 7 cm tall 

 excluding the holdfast region which may extend to 3 cm in length; basal or 



