224 L. HILLIS- COLIN VAUX 



condition. Plants survive for at least 2 years in culture when covered 

 with epiphytes. It may be that Halimeda has a dual system of providing 

 resting stages, j ust as auxospores of diatoms may be sexually or asexually 

 produced. 



(c) Dispersal stages are expected to be produced by sexual reproduction. 

 The evidence for how Halimeda disperses is extremely slender and, 

 indeed, essentially no more than speculation. It is possible that the 

 limited amount of transoceanic dispersal accomplished by members of 

 the genus (Section VIII) may be accounted for entirely by the chance 

 drifting of bits of broken thalli, and so do not require a sexual event. On 

 the other hand, the zygotes are made after fusion of motile gametes and 

 may result in producing something that would pass as part of the phyto- 

 plankton. It could also be that the filamentous mat which Meinesz 

 (1972b) describes can float in the plankton and thus travel for months 

 before settling. But there are no data. It would be worthwhile to test 

 this prediction of the strawberry-coral model : that there may be in the 

 tropical plankton zygotes or filamentous mats of Halimeda. 



(d) Sexual episodes will be triggered by environmental cues for changing 

 conditions or will occur in groups of individuals at the centres of clones. 

 There are no convincing data that the production of gametangia is ever 

 synchronous across a Halimeda clone or population. There have been 

 some suggestions that chemical cues may be needed to bring on a sexual 

 episode, a postulate which involves the prior postulate that sexual 

 reproduction be synchronous. But observations of my cultures argue 

 against this. Table XVIII describes the appearance of fertile individuals 

 in my cultures over a six-month period following collection from Ocho 



Table XVIII. Incidence, over a Six-month Period, or Fertile incrassata 

 FROM Jamaica, Established in 44 Aquaria in Columbus, Ohio, on 2 July, 



1969 



Aquarium identification No. of thalli fertile Approximate dates thalli fertile 



Niimbers in parentheses represent number in aquaria. 



