170 L. HILLIS-COLINVAUX 



Elysia, a lone specimen, appeared to feed mostly on one individual 

 Halimeda although there were others of the same species in the aqua- 

 rium. This it did for a year, when it was preserved and sent off for 

 identification. During the year it was alive, new segments started to 

 form many times on the host Halimeda plant, but they generally did 

 not complete their development. 



These observations show that it is possible to maintain simple 

 marine grazing systems in the laboratory. 



D. Summary of Halimeda culturing 



Eleven species of Halimeda have been grown in the laboratory, 

 some through one or more vegetative generations, while individuals 

 have been maintained for more than two years. 



It is not yet possible to start cultures with clean zygotes since the 

 segmented Halimeda has not been grown from them in the laboratory. 

 Cultures, therefore, must be started with direct transplants from the 

 sea, a technique that introduces troublesome contaminants. 



The basic procedure is to collect good field material, to clean it by 

 hand, to plant it in simple aquaria, to light it, to bubble air into the 

 water and to combat epiphytes. 



Other Caulerpacean and various Dasycladalean genera also grow 

 satisfactorily in this system, as well as a number of inadvertently 

 introduced animals, showing that it is possible to maintain simple 

 marine grazing systems in the laboratory. 



Although large numbers of epiphytes in laboratory culture usually 

 lead to the swamping of Halimeda thalli, on the reef this may be a 

 hazard to which Halimeda populations are adapted, and the reproduc- 

 tive and persistence strategies of Halimedae may be influenced by this 

 recurrent population event. 



VI. Growth and Calcification 



Patterns of growth in Halimedae must result in fronds which may 

 be likened to strings of beads because each frond is a linear array of 

 segments fastened at the nodes by a string-like structure of filaments. 

 This string-of-beads structure is constructed from a non-cellular 

 syncitium. The segmental "beads" of each species tend to have charac- 

 teristic shape, suggesting a rather refined system for controlling the 

 web of growdng filaments ; and this system operates as nuclei are spread 

 along the filament, without obvious physical separation of individual 



