THE BURROWING BARNACLES (CIRRIPEDIA: ACROTHORACICA ) 29 



while eliminating direct competition between them. This touches on 

 the general problem of Gauss' Competitive Exclusion Principle (of. 

 Hardin, 1960). 



Conjectures on Sexuality 



An important aspect of the study of the Cirripedia is the spectrum 

 of sexual types displayed. The Thoracica are hermaphroditic with 

 few exceptions, while the Acrothoracica are gonochoristic (dioecious). 

 A mathematical analysis of the advantages of hermaphroditism and 

 parthenogenesis was conducted as a concommitant to research on 

 acrothoracicans (Tomlinson, 1966). 



It is generally agreed that sexuaUty is an advantage because it 

 results in recombination. Because the gametes have to get together, 

 it is obvious that one or both must be motile, or at least mobile. It is 

 also obvious that a reserve food supply must be available to one or 

 both gametes to support the zygote until it can get its own food. 

 These latter two features are to a degree incompatible, inasmuch as 

 stored food reduces motiUty. It follows logically, therefore, that the 

 optimum adaptation would be to have all of the reserve food not 

 needed for the sustenance of the gamete itself in one gamete, and 

 have the other gamete motile, i.e., an egg and sperm. 



In the production of eggs and sperm, the efficiency of specializa- 

 tion and division of labor would evolutionarily tend toward organisms 

 with a separation of gonads. If hormonal control is used in gamete 

 production, efficiency of production would favor a single sex per 

 organism, or at least a single sex at a time (e.g., protandric hermaph- 

 roditism). It is of interest to observe that plants, with their local 

 hormones (auxins, etc.), tend to be hermaphroditic (monoecious), 

 while animals, with their systemic hormones, tend to be gonochoristic 

 (dioecious) . 



The presence of males and females in a population poses potential 

 problems. If there are any differences in their morphology or physi- 

 ology, the sexes will tend to compete with one another, following the 

 Competitive Exclusion Principle mentioned above. Of course, species 

 usually cannot tolerate the complete exclusion of a sex, and adaptive 

 mechanisms exist to correct this in at least two ways : the sexes may 

 converge back to nearly identical configurations, or they may change 

 their morphology and/or physiology drastically and also change 

 their environmental niche. This latter requires a compensating adapta- 

 tion to enable the two sexes to assure contact at the breeding season. 

 The reader can provide many examples in the vertebrate groups, but 

 since we are discussing acrothoracicans I will simply state that they 



