FRITZ and HAVEN: HARD CLAM SHELL GROWTH 



and early fall and were associated with narrower mi- 

 crogrowth increments than opaque zones, or light 

 bands. Mercenaria mercenaria in Georgia, however, 

 apparently grow throughout winter since no distinct 

 winter growth cessation marks were observed (Clark 

 1979). Consequently, two aspects of seasonal shell 

 microstructure appear to vary with latitude: 1) For- 

 mation of dark bands or translucent zones in summer 

 and early fall is more common at lower latitudes, and 

 2) formation of distinct winter growth cessation 

 marks is more common at higher latitudes. These 

 trends are similar, at least in concept, to changes in 

 sublayer crystal structure in the inner shell layer of 

 Geukensia demissa which have been observed with 

 latitude (Lutz 1977; Lutz and Rhoads 1978 (see foot- 

 note 5); Lutz and Castagna 1980). Latitudinal varia- 

 tion in the ultra- or microstructure of annual shell 

 increments may preclude application of defined 

 increments to all populations along its range. 



Latitudinal variation in seasonal water temperature 

 range may be the most important factor regulating 

 seasonal microstructural growth patterns inM. mer- 

 cenaria (Rhoads and Pannella 1970). In this study, it 

 was found that dark band formation tended to occur 

 when water temperatures exceeded 25°C, or the 

 upper limit of the optimum range for shell growth 

 (Ansell 1968). There is little evidence to support the 

 contention that the optimum temperature range, 

 15°-25°C, changes with latitude in populations of M. 

 mercenaria (Ansell 1968). Consequently, growth pat- 

 terns within shell microstructure may reflect ambient 

 seasonal cycles of water temperature (Lutz and 

 Rhoads 1980). 



The relationship between decreased microgrowth 

 increment width (growth rate) as well as location of 

 growth cessation marks with respect to elevated 

 water temperatures has been well documented (Ken- 

 nish and Olsson 1975; Kennish 1977). Furthermore, 

 circadian formation of microgrowth increments by 

 M. mercenaria has also been reported (Pannella and 

 MacClintock 1968; Thompson 1975). However, Pan- 

 nella and MacClintock (1968), Kennish and Olsson 

 (1975), and Kennish (1980) stated that one incre- 

 ment was formed during each solar day regardless of 

 season or age (up to 8 yr). Each annual shell incre- 

 ment would thus contain about 365 microgrowth 

 increments, and age estimates (in years) could be 

 obtained by dividing counts of all microgrowth 

 increments formed by 365 (Kennish 1980). The 

 results of this study, and that of Crabtree et al. ( 1 979/ 

 1980) on daily increment formation by Chione flucti- 

 fraga, shed doubt on this method of age 

 determination, since the percent agreement between 

 increments and days in annual shell increments 



decreased with increasing age. Thus, dividing total 

 microgrowth increment counts by 365 could 

 underestimate age in years. 



Decreasing number of days of growth each year with 

 age, as well as individual variability in the number of 

 days of growth in each age group, must be accounted 

 for when shell microstructure of bivalves is used to 

 monitor environmental change. Studies by Kennish 

 and Olsson (1975), Pannella (1976), Kennish (1977), 

 and Jones (1980) are testimony to the quality of 

 information on environmental change stored in 

 bivalve shell microstructure. However, individual 

 variability among bivalves of the same age may 

 require the use of large sample sizes to safely con- 

 clude that patterns observed in microstructure of 

 recent or fossil shells were due to changes in environ- 

 ment and not artifacts of individual differences in 

 shell growth. 



ACKNOWLEDGMENTS 



We thank J. P. Whitcomb and the VIMS Depart- 

 ment of Applied Biology for the yearly clam 

 measurements; A. C. Stubbs for word processing; J. 

 N. Kraeuter for the clams from the Eastern Shore; R 

 A. Lutz, G. R. Clark II, R. Morales- Alamo, and two 

 anonymous reviewers for their critiques of early 

 drafts of this manuscript; and M. A. Foell for drafting 

 the figures. Special thanks to A. T Fritz. 



LITERATURE CITED 



Ansell, A. D. 



1968. The rate of growth of the hard clam Mercenaria mer- 

 cenaria (L) throughout the geographical range. J. Cons. 

 Perm. Int. Explor. Mer 31:364-409. 

 Barker, R. M. 



1964. Microtextural variation in pelecypod shells. Malacol- 

 ogia 2:69-86. 

 Castagna, M., and J. N. Krael ter. 



1977. Mercenaria culture using stone aggregate for predator 

 selection. Proc. Natl. Shellfish. Assoc. 67:1-6. 

 Clark, G. R. 



1979. Seasonal growth variations in the shells of Recent and 

 prehistoric specimens of Mercenaria mercenaria from St. 

 Catherines Islands, Georgia. Anthropol. Pap. Am. Mils. 

 Nat. Hist. 56:161-179. 



Crabtree, D. M, C. D. Clausen, and A. A. Roth. 



1979/1980. Consistency in growth line counts in bivalve 



specimens. Palaeogeogr., Palaeoclimatol., Palaeoecol. 



29:323-340. 

 Franz, D. R., and A. S. Merrill. 



1980. Molluscan distribution patterns on the continental 

 shelf of the Middle Atlantic Bight (Northwest Atlan- 

 tic). Malacologia 19:209-225. 



Greene, G. T. 



1975. Incremental shell growth patterns as affected by en- 

 vironment in Mercenaria mercenaria. B.A Thesis, Prince- 

 ton University, Princeton, 77 p. 



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