The Biology of Senescence 



colonies was also found by Hamai (1937) in Patelloida grata. The 

 most suggestive evidence of a determinate life-span in limpets 

 comes from Fischer- Piette's (1939) observations which showed 

 a definite inverse relationship between longevity, judged by 

 growth rings, and rate of growth in different stations (Fig. 29). 

 This strongly suggests that a process of morphogenetic ageing is 

 occurring at different rates depending on the rate of growth. 

 In some pelecypods, there might be ground for regarding 

 growth as genuinely indeterminate. It continues in the oldest 



</> 40 



LU 



cL 



\- 



LLI 

 Z 



z 



I 



20 



10 



UJ 



YEARS 



I 2 3 4 5 6 7 8 9 10 I! 12 13 14 15 16 17 



Fig. 29. — Growth and longevity of Patella vulgata in various stations, showing 

 the short life of rapidly-growing populations (from Fischer-Piette, 1939). 



recorded specimens, and the occurrence of abnormally large 

 individuals of species whose age can be estimated by growth 

 rings makes it possible also to estimate the rate of decline of 

 growth rate by measurement of the intervals between the rings. 

 Growth continuing actively to the maximum recorded age has 

 been found in many forms (Cardium, 16 years — Weymouth 

 and Thompson, 1930; Venus mercenaria, 40 years — Hopkins, 

 1930; Siliqua, 14-16 years — Weymouth, 1931; Pecten jessoensis, 

 8 years — Bazykalova, 1934; Mya, 7-8 years — Newcombe, 1935, 

 1936). The larger fresh-water pelecypods, which have fewer 

 enemies and are not subject to tidal disturbances, reach even 

 greater ages. Huge specimens of the washboard mussel, Mega- 



106 



