ECOLOGY AND TAXONOMY OF Halimeda 



Fig. 1. Halimeda tuna, as Opuntia marina in Parkinson's "Theatrum Botanicum", 

 1640. (Photograph by the British Museum (Natural History).) 



The discovery of other species, in the intervening years, has shown 

 that the genus is not always dainty and the descriptive terminology 

 has grown accordingly. Nevertheless, the overall appearance of the 

 genus is characteristic, and whether one first encounters it while 

 swimming in a coral reef, or examining an array of dried herbarium 

 specimens, it can be identified almost at once, for it looks like some 

 form of underwater cactus, with photosynthetic portions consisting 

 of series of calcified segments or joints, the strung leaves of Parkinson, 

 which may be arranged in some branching pattern. The plant body, 

 or thallus, also possesses a holdfast which provides attachment to or in 

 the substrate (Fig. 2). 



All species of Halimeda deposit calcium carbonate in the form of 

 aragonite. Hence, the green colour of the segments, which are the 

 photosynthetic portions of the thallus, is muted. Calcification begins 

 when the segment is about 36 hours old (Wilbur et ah, 1969) ; therefore, 

 even those small young thalli that appear flatulent and green are 

 calcified. The only uncalcified portions are nodes (to be described 

 below), apical segments 3'ounger than about 36 hours old, and rhizoidal 

 filaments. -^ 



