The Distribution of Senescence 



lonaias gigantea (Unionidae) showing 54 and 36 annual rings are 

 recorded (Chamberlain, 1933) — these molluscs were still grow- 

 ing, and even larger examples exist. Longevity of this order is 

 not, however, confined to large species — marking experiments 

 have enabled some Unionidae (Quadrula) already over 20 years 

 old to be followed for a further 15 years, and the correspond- 

 ence of adult growth rings to years confirmed (Isely, 1931). 

 (See also Coker, Shira, Clark and Howard, 1919-20.) In the 

 small Margaritana margaritifera no important decline in the 

 growth rate has occurred at 13 years (Saldau, 1939.) Geyer 

 (1909) gave this species a life-span of at least 60 years. The 

 estimate of 100 years quoted by Korschelt from Israel (1914) 

 is unlikely but not impossible. 



The supposed longevity of Tridacna has already been men- 

 tioned, together with the fact that nothing whatever is known 

 about its real life-span. The same applies to many large marine 

 pelecypods, whose probable age can only be discussed when we 

 know something of their growth rate. 



The pelecypods also illustrate the risks of purely ideal and 

 mathematical representations of growth-pattern. Pseudo-specific 

 size from environmental causes is common. In Siliqua (Wey- 

 mouth, 1931), some populations reach an apparent limiting 

 size, cease altogether to grow thereafter, and die early: this, 

 like Fischer- Piette's observation on limpets (1939) might sug- 

 gest that a senile process is at work. Wild limpets apparently 

 die while in active growth, but those which grow fastest die 

 earliest (Fig. 29). Other molluscan populations have growth 

 records which, though fitted for practical purposes to an 

 asymptotic curve, actually give observed readings in the highest 

 age groups which lie well above such a curve, and indicate that 

 in these groups growth is continuing (Weymouth, 1931). There 

 is an obvious objection to the use of growth rings to measure 

 age, however, if conclusions are then to be drawn about con- 

 tinuing growth — arrest of growth lasting for years would leave 

 no record of itself in this system of notation. The results ob- 

 tained by the use of the ring method in pelecypods have so far 

 been reasonably consistent (see Newcombe, 1936). The validity 

 of growth rings as annual markers requires careful confirma- 

 tion in each population examined, however (Haskings, 1954). 



107 



