FISHERY BULLETIN: VOL 81. NO. 4 



100 



75 



50 - 



25 



UJ 75- 



LU 



0T 50 

 O 



^ 25 - 



e 



75 - 



50 - 



25 



FIGURE 6. — Distribution of average microgrowth increment width 

 (/xm/inc) in light and dark bands formed from 1969 to 1971 by lot XI 

 hard clams, Mercenaria mercenaria. Figures are drawn as in Figure 5. 

 Increments were counted only between annual measurement distur- 

 bance marks in the number of hard clams listed with each spring light 

 or summer dark band. The number of these which had formed light 

 bands prior to measurement each fall in 1969 and 1970 are also 

 shown 



(at 1 m depth) near the location of T and TI hard 

 clams remained above 25°C from 22 June to 26 Sep- 

 tember 1980, which was approximately the period of 

 dark band formation by all short-term experimental 

 hard clams. For instance, microgrowth increment 

 counts in hard clams collected on 8 August and 1 

 November 1980 (Fig. 2 A, B) date the time of dark 

 band initiation and completion as 29 June and 26 

 September 1980, respectively. 



DISCUSSION 



Dark bands in the middle homogenous layer of 

 polished sections and on acetate peels of radial sec- 

 tions of Mercenaria mercenaria shells were formed 

 each summer and early fall and were associated with 

 slower growth rates (narrower microgrowth in- 

 crements) than light bands. Consequently, hard 

 clams from Chesapeake Bay may be aged on the basis 



of dark band counts in shell microstructure. Distinct 

 winter growth cessation marks were not formed each 

 year by each individual primarily because of a lack of 

 light band formation in fall, especially by hard clams 

 older than 8 yr. However, hard clams younger than 8 

 yr also did not form light bands consistently in fall 

 which would separate winter growth cessation marks 

 from summer dark bands. Consequently, dark bands 

 were the only seasonal growth pattern in microstruc- 

 ture which was formed annually by each hard clam 

 analyzed. 



The relationships between bands in polished shell 

 section and middle homogenous layer ultrastructure 

 are unknown. According to a theory of growth line for- 

 mation proposed by Lutz and Rhoads (1977), the 

 ratio of organic matrix to shell carbonates could 

 increase during extended periods of slow shell 

 growth due to dissolution of carbonates in anaerobic 

 (inactive) periods. This may cause shell deposited in 

 summer to appear dark because of higher pro- 

 portions of organic matrix. However, differences in 

 average microgrowth increment width between light 

 and dark bands in this study were only between 10 

 and 20 jum, which may be too small to cause such fun- 

 damental changes in shell appearance. Alternatively, 

 differences in crystal size and/or orientation may 

 also account for bands in the middle homogenous 

 layer. Clark 7 hypothesized that translucent zones in 

 thin section may result from slightly larger, and more 

 uniformly oriented, crystals. This may result in light 

 transmittance by translucent zones in thin section, 

 and absorption by dark bands in polished shell sec- 

 tion. However, it is not known why these areas also 

 appear dark on acetate peels. 



Latitudinal variation in seasonal microstructure of 

 Mercenaria mercenaria shells is apparent from other 

 studies (Pannella and MacClintock 1968; Rhoads 

 and Pannella 1970; Greene 1975; Kennish and 

 Olsson 1975; Clark 1979). North of Chesapeake Bay 

 (Massachusetts, Connecticut, New York, and New 

 Jersey), distinct winter growth cessation marks were 

 formed each year and the fastest growth during the 

 year occurred most often in summer (Pannella and 

 MacClintock 1968; Rhoads and Pannella 1970; 

 Greene 1975; Kennish and Olsson 1975). However, 

 there was no discussion of association of wide or 

 narrow microgrowth increments with bands in the 

 middle homogenous layer. Seasonal shell micro- 

 structure of M. mercenaria in Georgia is very similar 

 to that in lower Chesapeake Bay (Clark 1979). Translu- 

 cent zones, or dark bands, were formed each summer 



'G. R. Clark II, Department of Geology, Kansas State University, 

 Manhattan, KS 66506, pers. commun. March 1982. 



706 



