FRITZ and HAVEN: HARD CLAM SHELL GROWTH 



surface with acetone and air- dried for at least 1 h, 

 after which they were stored between glass micro- 

 scope slides. 



Analysis of Shell Microstructure — 

 Terminology and Methods 



Clark (1979) described a series of translucent and 

 opaque zones in the middle homogenous layer of thin 

 sections of M. mercenaria shells. These zones were 

 associated with narrow and wide prismatic micro- 

 growth increments, respectively (Table 2). In this 

 study, acetate peels of etched radial sections, rather 

 than thin sections, were used. Regions of low and high 

 light transmittance through acetate peels correspond- 

 ed exactly with "dark" and "light bands," respective- 

 ly, in the middle homogenous layer of polished shell 

 sections (Table 2; see Figure 2). For conve- 

 nience, we refer to regions of low and high light 

 transmittance through acetate peels as dark and light 

 bands. Clark's description of translucent zones in 

 thin sections could also be correctly applied to dark 

 bands in acetate peels, since both were associated 

 with narrow microgrowth increments. However, 

 regions of the middle homogenous layer associated 

 with narrow increments appear translucent in thin 

 sections, but optically dense or "dark" in acetate 

 peels. Conversely, opaque zones in thin sections 

 appear transparent or "light" on acetate peels (Table 

 2). Since Clark's terminology from analyses of thin 

 sections does not strictly apply to middle homogenous 

 layer growth patterns observed on acetate peels, we 

 have used the new terms "dark" and"light bands,"as 

 outlined in Table 2. However, translucent zones and 

 dark bands, and opaque zones and light bands describe 

 the same growth pattern in shell microstructure. 



Acetate peels were analyzed on a compound mi- 

 croscope at 100X magnification with nonpolarized 

 light. Known annual shell increments formed be- 

 tween 1967 and 1972 by experimental hard clams in 

 lots I, II, XI, and XIV were analyzed for annually pro- 

 duced patterns. Similarly, total shell increments 



deposited between 1972 and the date of final collec- 

 tion by each experimental hard clam in the four lots 

 should contain the same number of annual in- 

 crements as there were years in the period. Annual 

 increments were defined primarily in the middle 

 homogenous layer due to the simplicity of its growth 

 pattern (dark and light bands) compared with the 

 outer prismatic layer (microgrowth increments). 

 Band color at the shell margin was catalogued by 

 season of collection in both experimental and wild 

 hard clams to determine time of year of dark and light 

 band formation. To increase the number of fall obser- 

 vations, band color was also observed dorsal (toward 

 the umbo) to each disturbance mark in shell micro- 

 structure of long-term experimental hard clams 

 caused by measurements in 1967-72. Observations 

 of band color in each season were catalogued by three 

 age groups defined by Kennish (1980): Young — 

 under 3 yr of age; mature — 3 to 8 yr; old — over 8 yr. 

 Microgrowth increments in the prismatic layer 

 (their average width and number) were used to de- 

 scribe bands in the middle homogenous layer. 

 Individual increments were traced through the pris- 

 matic to the middle layer to identify single 

 increments corresponding to the dorsal (toward the 

 umbo) and ventral (toward the shell margin) surfaces 

 of each band. Band width was measured along the 

 surface of maximum growth (SMG) in the prismatic 

 layer (Pannella and MacClintock 1968) using an 

 ocular reticle with an estimated accuracy of ± 1 reti- 

 cle unit (10.8 ju.m at 100X). Microgrowth increment 

 counts were made only in shell regions bracketed by 

 growth disturbance marks of known formation time 

 or one growth disturbance mark and the shell margin 

 (collection date). This allowed determination of the 

 periodicity of increment formation. All microgrowth 

 increment counts in bands or annual shell increments 

 were averages of three trials. Guidelines suggested 

 by Crabtree et al. (1979/1980) were used to dis- 

 tinguish and count microgrowth increments. Least 

 squares linear regressions (Sokal and Rohlf 1969) of 

 increment counts on days in monitored growth 



Table 2.— Terminology used to describe growth patterns in the middle homogenous layer 



of Mercenaria mercenaria. 



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