ECOLOGY AND TAXONOMY OF Halimeda 49 



(i) Identification without a microscope. In scabra the surface of each 

 peripheral utricle is prolonged into a spine (Fig. 20, No. 4), a feature 

 which occurs in no other Halimeda. Consequently, thalli of this species 

 without a surface growth of epiphytes feel slightly rough. This character 

 may be used, with extreme caution, to distinguish scabra from tuna in 

 the field. Halimeda scabra may be often bypassed in Caribbean reefs 

 where it grows because it is considered to be tuna. Field identification 

 should be checked with a handlens or (preferably) a microscope to 

 verify the presence of spines. A lateral view (as in a longitudinal 

 section) is preferred. 



Halimeda macrophysa can be tentatively identified in the field by 

 the very finely stippled appearance of the segment surface (Fig. 40). 

 This pattern results from exceedingly large surface utricles which are 

 round rather than hexagonal in appearance (Fig. 17, No. 16) and are 

 separated by calcium carbonate partitions. However, if a first-time 

 field identification for a given site is made this way, it should be verified 

 microscopically . 



Halimeda favulosa may give somewhat the same impression as 

 macrophysa. The diameters of its surface utricles are larger, but the 

 calcium carbonate partitions are not as complete. The two species 

 cannot be confused because they look different macroscopically, and 

 there is no overlap of distribution. 



An awareness of this characteristic for favulosa may ease its being 

 mistaken, and therefore bypassed in the reef, for incrassata which is 

 considerably commoner. 



(ii) Surface diameter of peripheral utricles. The surface diameters 

 of the peripheral (primary) utricles together with their surface 

 appearance are, perhaps, the most useful characters for separating 

 Halimeda species at the microscopic level, and surface preparations are 

 the most easily made. With x 100 magnification these utricles generally 

 appear as a "sheet of polygons" or like a honeycomb. Sometimes the 

 units are roundish, and then may be disconnected rather than united 

 into a "sheet". High-power magnification ( x 400) is usually required 

 for accurate measurement. 



The range in diameters is shown in Fig. 17. The smallest occur in 

 species of section Opuntia ; the largest are in the species favulosa. 



(iii) Surface appearance of peripheral utricles. The predominant 

 surface pattern of the peripheral utricles is hexagonal (Fig. 17), a result 

 of the pressing together of the many branch tips of the cortical filament 

 system. In some species such as macrophysa, the separateness of the 

 utricles is maintained by relatively thick deposits of calcium carbonate 

 between utricles, and the utricles retain their roundness. Intermediate 



