36 L. HILLIS-COLnsrVATJX 



I describe macroscopic and microscopic characters in the sub- 

 sections that follow, but reserve distributional and ecological data for 

 Sections VIII and X respectively. All may be useful in delimiting 

 groups of species, or individual ones. Specialized reproductive structures 

 or gametangia are described in Section VII. They are not present on 

 most of the material collected and so far have not contributed signifi- 

 cantly to taxonomy. 



1. Macroscopic characters 



Three characters are commonly used: 



appearance of segment ; 

 appearance of holdfast ; 

 appearance of thallus. 



The first of these, segment appearance, is the original character used 

 to separate and establish the early species of Halimeda (Section II). 

 The other two were developed as key characters by Hillis (1959), so that 

 some specimens could be identified in the field. All three, together with 

 data on geographic and ecological distribution, are essentially the only 

 characters available for the field determinations essential to any critical 

 ecological study involving the genus. 



(a) Segment pattern. The usefulness of the appearance of segments to 

 species identification may rightly be questioned when one recalls the 

 early taxonomic confusion created by relying on this characteristic. 

 From direct observation of extensive collections of dried material one 

 is well aware that the segments, even of a single thallus, may be highly 

 variable in shape and size, and species such as incrassata and opuntia 

 are good examples. 



Such doubts are further strengthened by evidence that some 

 variability is environmentally induced. Techet (1908), for example, 

 attributed the changed shape of the segments of Mediterranean tuna, 

 when kept in laboratory tanks, to reduced salinity. In some of their 

 culture studies, Colin vaux et al. (1965) observed a marked change in the 

 segments ofdiscoidea which may be a response to reduced light intensity. 

 I have also noted a considerable reduction in the size, as well as change 

 in shape of the segments oi gigas growing in aquaria (Fig. 16). My field 

 observations have also indicated changes of shape in response to 

 environmental factors. The dwarf opuntia growing shallowly in a fast- 

 flowing inter-island channel of Enewetak Atoll in the Marshall Islands 

 is a good example. 



