The Distribution of Senescence 



dence supports Bidder in that growth in the male plaice ceases 

 relatively early, and there is evidence that it has a shorter life- 

 span than the female (Wimpenny, 1953). On the other hand, 

 in many small teleosts reproductive senescence is known to 

 occur, and both the sexes appear to exhibit specific age, in spite 

 of the fact that growth in the female may continue throughout 

 life. The reproductive failure of many teleosts with increasing 

 age is familiar to aquarists. So is the tendency of particular 

 species to have a limiting age, although there have previously 

 been no published life-tables for any teleost in captivity by 

 which this impression could be confirmed. The growth of 

 some small teleosts has been studied (Felin, 1951; Wellensieck, 

 1953). 



There is a good deal of evidence that small teleosts, and 

 perhaps teleost species generally, undergo both reproductive 

 and actuarial senescence comparable to that of mammals. The 

 most dogmatic assertions on this score are those of pathological 

 anatomists. On the basis of concretions occurring in the testis 

 of a single teleost species (Astyanax americanus), Rasquin and 

 Hafter (1951) hold that the 'appearance of senility changes 

 shows that the teleosts conform to the common vertebrate pat- 

 tern of ageing despite a widespread misconception to the con- 

 trary'. The decline of fertility in some aquarium species pro- 

 vides more solid evidence in support of this view. Many species 

 of fish are in any case exposed to a specialized series of fluctua- 

 tions in mortality associated with reproduction — the difficulties 

 of treating these fluctuations as a form of senescence in those 

 species which always die after breeding, such as the male of 

 Callionymus (Chang, 1951) and the lamprey, are indicated by 

 observations upon other fish in which there are a limited num- 

 ber of survivors from each breeding season, and these thereafter 

 acquire a new lease of life. It is doubtful if any cyclical or 

 potentially cyclical change in mortality can properly be called 

 senile. In Callionymus lyra in the wild, the male appears to live 

 5 and the female up to 7 years. The males disappear, probably 

 through death, but possibly by migration to deeper water, after 

 breeding once. Females may first breed in their third, fourth, 

 or fifth year of life, depending on their rate of growth, and 

 probably breed more than once (Chang, 1951). In such a case, 



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