ECOLOGY AND TAXONOMY OF Holimeda 221 



These few data from other Caulerpales suggest that similar breeding 

 strategies prevail for much of the group. The principal method appears 

 to be cloning by sending out "runners" of filaments. Sexual reproduc- 

 tion is infrequent and always leads to the death of the thallus. Develop- 

 ment of zygotes seems to require many months, at least five, although 

 the complete process of development has not been followed for any 

 taxon of the group. 



D. Reproductive strategy and the strawberry-coral model 



Holimeda populations appear to be clones which maintain them- 

 selves by vegetative means for numbers of generations that seem to be 

 unlimited. Yet occasionally individuals, or, more rarely, parts of 

 individuals, devote their entire resources to a sexual episode. Although 

 the full course of the development of the zygote is still not known, there 

 is reason to suggest that sexual reproduction is followed by either a 

 resting stage or dispersal. The value of both vegetative cloning and sex 

 to plants and animals with life-cycles like this has been explained by 

 Williams (1975) with his strawberry-coral model. 



1. The strawberry-coral model 



For an organism living in a habitat which remains unchanged for a 

 time which is long in terms of the generation time, Williams (1975) 

 argues that it is a better reproductive strategy to produce individuals 

 identical to successful parents than it is to leave the next generation 

 open to the risk of a great variety of competing genotypes. Individuals 

 of a parental strawberry or coral clone will already have been selected 

 during a series of competitive exclusions with neighbouring clones, and 

 continued success requires that the winning formula be repeated. A 

 sexual effort in such clones should only follow when the very high cost 

 of producing many disastrously unsuitable experiments becomes less 

 than the cost of making more copies of the parent. This cost of a sexual 

 endeavour may well only be met when the cost of more vegetative 

 reproduction is local extinction. 



Sex, in Williams' view, should only be required when it is necessary 

 to make individuals for competitive struggles in new and untried 

 circumstances. Sex, therefore, should precede the making of resting 

 stages that will survive a hostile environmental episode, since the 

 circumstances in which the clones of the future will compete are likely 

 to be different to those familiar to the parent clone. In a like manner, 

 sex will also precede episodes of dispersion, since the propagules must 



