FISHERY BULLETIN: VOL. 81, NO. 4 



periods were used to support conclusions on the 

 periodicity of increment formation. Comparisons of 

 regression coefficients were done using either a Mest 

 (0 if the comparison was between a calculated coeffi- 

 cient and its expected value, or an F-test (F s ) if the 

 comparison was between two calculated coefficients 

 ( Sokal and Rohlf 1 969) . To ascertain effects of age on 

 number of increments formed in annual shell 

 increments, microgrowth increments were counted 

 between each pair of fall measurement disturbance 

 marks (MDM) in long-term experimental hard clams 

 (lot XI) formed 1 yr apart. Each count was divided by 

 the number of solar days between measurements, 

 yielding the percent agreement (Richardson et aL 

 1979) between increments and days. Data were 

 pooled by absolute hard clam age. 



RESULTS 



Annual Shell Increments — 

 Light and Dark Bands 



The series of fall MDM divided the shell micro- 

 structure of 89 long-term experimental hard clams 

 into 177 known years of shell growth formed between 

 1967 and 1972 (Table 3). A single dark band in the 



TABLE 3. — Total numbers of known years of shell growth be- 

 tween 1967 and 1972 and dark bands in the middle homogenous 

 layer observed in the known shell increments of long-term ex- 

 perimental Mercenaria mercenaria. 



1 Many of the hard clams from each lot were added to it after the lot had been 

 established. Because of this, the number of known years of shell growth for all 

 hard clams in each lot is always less than the number of hard clams analyzed 

 multiplied by the maximum number of known years of shell growth in each hard 

 clam from 1 967 to 1 972 (5 yr) 



middle homogenous layer had been formed within 

 each known annual shell increment (see Figure 2). 

 Furthermore, dark bands were located dorsal to the 

 MDM, suggesting that they had been formed each 

 summer. To confirm this observation, the number of 

 complete summers from fall 1972 to the date of final 

 collection in each hard clam was compared with the 

 number of completed dark bands observed in shell 

 microstructure formed in these periods (Table 4). 

 Complete dark bands were defined as those which 

 were not at the shell margin. Ninety- four percent (84/ 

 89) of the hard clams examined contained the same 

 number of completed dark bands as there were com- 

 plete summers, while the remaining 67c (5/89) 

 formed one fewer dark band than years. This is re- 

 garded as the error estimate (6%) of this procedure 

 for determining age of M. mercenaria. 



Analysis of shell margin growth bands of all 

 experimental and wild hard clams collected 

 seasonally also revealed that dark band formation 

 occurred during summer (Table 5; Fig. 1). The per- 

 centage of all ages of hard clams collected in summer 

 which had a dark band at the shell margin (91%) was 

 over twice that of hard clams collected in winter 

 (40%). However, a significant proportion of hard 

 clams collected in fall had a dark band at the shell 

 margin (78%), indicating that the period of dark band 

 formation also extended into fall. 



Further examination of Figure 1 reveals differences 

 among age groups in color of shell margin bands in 

 fall and winter. The percentage of hard clams in all 

 age groups with a dark band at the shell margin in 

 summer ranged between 88 and 100% (Table 5; Fig. 

 1). However, in fall and winter, differences between 

 age groups began to appear. In fall, 100% of old and 

 75% of mature hard clams had dark bands at the shell 

 margin (Table 5; Fig. 1). The percentage with dark 

 bands in winter declined in both age groups, but was 

 still larger in old (447c) than mature (17%) hard 

 clams. Thus, light band formation began sooner 



Table 4. — Number of long-term experimental Mercenaria mercenaria with 

 expected number of complete dark bands in the middle homogneous layer in 

 known years of shell growth between 1972 and dates of final collectioa 



700 



