CHAPTER XVII 



INFLUENCE OF CROWDING UPON SEX 

 DETERMINATION 



We have just seen that when gametes are shed free into the sur- 

 rounding sea-water, their period of hfe is distinctly hmited. If two 

 gametes of opposite sex are unable to meet during this fertilizable 

 period, death results for the spermatozoa, probably from starva- 

 tion, and for the egg, perhaps from suffocation (Child, 1915). This 

 means that animals of different sexes must be relatively close to- 

 gether in order that there may be a successful union of the shed 

 gametes. Grave and Downing (1928) give us a chance to estimate 

 some of the space requirements for the successful operation of this 

 system in sea-water. They report that the most vigorous sperm of 

 the sea urchin, Arhacia, and of the mollusk, Cumingia, can travel 

 30 cm. by their own effort in still water. Hydroides sperm are less 

 active. Spermatozoa of the first two animals in suspensions of 

 1/2,000 to 1/10,000 per cent survive from 3 to 12 hours. Sperma- 

 tozoa under natural conditions in sea-water may survive and may 

 fertilize eggs for this period, but many die after 3 hours and the 

 majority succumb after 7 hours. Where currents are present, these 

 survival periods would allow of much greater distribution than 30 

 cm. ; but the fact remains that even in the most favorable sea-water, 

 animals must be relatively closely aggregated for fertilization to be 

 successful. In fresh water the life of the shed gametes is quite short. 

 After ID minutes (Reighard, 1893) the eggs of the wall-eyed pike 

 lose the power to be fertilized. The same observer has stated in a 

 lecture at Woods Hole that the sperm of certain fishes loses fertiliz- 

 ing power within a half-minute after shedding. With animals re- 

 quiring internal impregnation, the necessity for close co-operation 

 of at least two individuals is obvious. These considerations must 

 be fundamental for the long-recognized breeding aggregations of 

 animals, particularly of animals shedding their gametes into the sur- 

 rounding water during the breeding season. 



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