Physiological Changes with Age in Fish 187 



male is much smaller than the female. His growth stops soon 

 after sexual maturity is reached, but the female continues to 

 grow until death. Fertilization is internal and the sperm 

 transfer is accomplished by a greatly modified anal fin, the 

 gonopodium, which acts as the male copulatory organ. The 

 sperm penetrate the ovarian wall and fertilize the eggs while 

 they are within the ovarian follicle. Development proceeds, 

 and the young are born in various stages of development 

 according to the species. 



Reproduction and senescence have been studied only in the 

 western mosquitofish, Gambusia a. affinis. Krumholz (1948) 

 has provided an unusually good series of observations on the 

 number of young in successive broods of females from the 

 onset of sexual maturity until death. Females were collected 

 from ponds in southern Michigan and in the vicinity of 

 Chicago during the summers of 1939 to 1944, and the number 

 of embryos in the ovary was counted. The number of young 

 produced by a single female depends on: (1) the number of 

 broods liberated during a season, (2) the length of the mother 

 fish, (3) the time of season at which the individual broods are 

 cast, and (4) the locality in which the mother fish lives. The 

 first two factors are pertinent to the present discussion. 



The age and size at first maturity are correlated with the 

 time of year when the female offspring are born and their 

 rate of growth. Faster-growing individuals generally reach 

 maturity at an earlier date than slower-growing individuals. 

 Mosquitofish born late in the summer do not reach maturity 

 until the following spring while those born early in the 

 summer may reproduce within a month or two. The number 

 of young in a brood increase^ as the length of the female 

 increases, but at large sizes the rate of increase is drastically 

 reduced. The rate of increase in fecundity in relation to length 

 of the female is greater in the second brood than in the first or 

 subsequent ones. In one case the number of embryos in the 

 first brood increased as the 1 • 3 power of the length ; the second 

 brood as the 1 • 4 power, and the third brood as the • 8 power. 



