FRITZ and HAVEN: HARD CLAM SHELL GROWTH 



(after the summer dark band was completed) in a 

 greater percentage of mature than old hard clams. 

 This could also have been due to a lack of growth by 

 old hard clams in fall and winter, leaving the summer 

 dark band at the shell margin. 



Enlargements of acetate peels in Figure 2 further 

 illustrate the time of dark band formation each year. 

 Figure 2A-C form a representative summer-to- 

 winter series of shell margin bands formed by mature 

 (T and TI) hard clams. Dark bands were at the shell 

 margin in hard clams collected in summer (Fig. 2A), 

 while in hard clams collected in fall and winter, dark 

 bands became separated from the margin by light 



TABLE 5. — Summary of seasonal shell margin growth bands in long- 

 term experimental and wildMercenaria mercenaria (in summer, win- 

 ter, and spring collections) and those dorsal to each fall measure- 

 ment disturbance mark in 1968-72. Group collected in each season 

 was subdivided by age according to Kennish (1980; see legend to 

 Figure 1). 



SEASON OF COLLECTION 







tit* 



> y 



YOUNG 



5 18 139 6 3 34 17 39 



MATURE OLD 



AGE GROUPS 



8 55 156 45 

 ALL AGES 



Figure 1. — Percent of young, mature, old, and all ages of long-term 

 experimental and wild stock Mercenaria mercenaria with light (un- 

 shaded) or dark (shaded) bands at the shell margin in each 

 season. Age groups: Young- under 3 yr, Mature - 3 to 8 yr. Old- 

 over 8 yr (Kennish 1980). 



bands with increasing numbers of microgrowth 

 increments (Fig. 2B, C). Light bands continued to be 

 formed through spring and early summer but appear 

 differently in the two hard clams pictured (Fig. 2D, 

 E). Eighty-five percent (23/27) of the hard clams 

 collected during or after winter 1980 (from Decem- 

 ber 1980 to June 1981) had a growth cessation mark 

 within the shell margin light band formed during win- 

 ter (Fig. 2D). This mark, termed a distinct winter 

 growth cessation mark, was a thick microgrowth 

 increment boundary in the light band with 

 narrow microgrowth increments dorsal and ventral to 

 it. It was also separated from the dark band by a light 

 band representing growth in fall. Thus, one annual 

 shell increment in these hard clams consisted of a 

 dark band formed in summer and a light band formed 

 in fall through spring which was bisected by a distinct 

 winter growth cessation mark This was the typical 

 seasonal growth pattern of mature hard clams (Fig. 

 1). The remaining 15% (4/27) of the hard clams 

 collected during this period did not have distinct win- 

 ter growth cessation marks within the light band (Fig. 

 2E). This does not mean, however, that these hard 

 clams grew throughout winter. In order for a winter 

 growth cessation mark to be distinct, a light band 

 formed in fall must separate it from the summer dark 

 band. Consequently, lack of a distinct winter mark 

 was more likely caused by lack of light band forma- 

 tion in fall. One annual increment in these hard clams 

 consisted of a dark band formed in summer and fall 

 and a light band formed in the following spring. This 

 seasonal growth pattern was similar to that described 

 for old hard clams (Fig. 1). Thus, there can be signifi- 

 cant variation even among individuals in a single year 

 class in seasonal shell growth patterns. Dark bands 

 formed in summer and fall, however, were the only 

 annually produced and universal component of the 

 shell growth pattern of M. mercenaria in lower 

 Chesapeake Bay. 



Periodicity of 

 Microgrowth Increment Formation 



Experimental hard clams formed one microgrowth 

 prismatic increment during each solar day of activity. 

 Inactive periods, represented by growth cessation 

 marks or thick organic lines in the prismatic layer, 

 became longer and/or more frequent with increasing 

 age and length of monitored growth periods. Thus, 

 both factors (increasing age and length of monitored 

 growth periods) tended to decrease the increment- 

 to-day ratio. Three sets of increment counts were 

 used to formulate these conclusions: 1) The number 

 of increments from the growth disturbance of 30 May 



701 



