FRITZ and HAVEN: HARD CLAM SHELL GROWTH 



fidence limits on the regression coefficient from the 

 winter 1980 to summer 1981 counts were almost 

 twice as wide as from the spring to fall 1980 counts 

 (Table 6). This was also reflected in the reduced, but 

 significant correlation coefficient from the winter 

 1980 to summer 1981 counts compared with those 

 from hard clams collected from spring to fall 1980 

 (Table 6). Despite the lack of significant statistical 

 results, these data suggest that the ratio of 

 increments to days was lower in hard clams collected 

 during or after winter 1980 than in those collected 

 from spring to fall 1 980. This could have been due to 

 growth cessations of varying durations in winter. 

 However, growth cessations could also have occurred 

 at anytime during the monitored growth period, and 

 thus obscured the effects of winter on the number of 

 increments in hard clams collected during or after it. 

 Individual variability in numbers of days of growth 

 was evident in the increasing range in increment 

 counts from single collections with time. Chances of 

 disturbances (such as storms, predation attempts, 

 etc.) occurring in any season which could cause 

 growth to cease in some hard clams would also 

 increase with the length of monitored growth periods. 

 Consequently, a one-to-one increment- to- day relation- 

 ship only applied to short periods of monitored 

 growth during favorable seasons, such as the TI hard 

 clams discussed previously (Fig. 3). Prismatic mi- 

 crogrowth increments, however, each represented a 

 solar day, despite the lack of one-to-one correspon- 

 dence for long periods of monitored growth. 



Lot XI, From 1969 to 1971 



Percent agreement between increment counts and 

 days between annually formed MDM decreased with 

 increasing age of long-term experimental hard clams. 

 Results of counts from lot XI hard clams age 3 to 10 in 

 annual shell increments formed between 1969 and 

 1971 are shown in Figure 5. Results from other long- 

 term experimental lots were similar. Consequently, 

 experimental hard clams formed increments (were 

 active) for fewer days each year with increasing age, 

 indicating that growth cessations became more fre- 

 quent, longer, or both. 



Microgrowth Increment Widths, 

 Seasonal Growth Rates 



Average microgrowth increment widths associated 

 with dark bands were generally smaller than those 

 associated with light bands in all long-term and short- 

 term experimental hard clams. The distribution of 

 average increment widths formed between 



Ld 



100 



90 



80 



H 



Ld 



cr 70 



o 



< 



^ 60 

 Ld 



o 



£ 50|- 



40 



RANGE { 



75th 

 PERCENTILE 



MEDIAN 



25 th 

 PERCENTILE 



-U 



10 II 



AGE (YEARS) 



FIGURE 5. — Distribution of percent agreement (number of mi- 

 crogrowth increments divided by number of days between annual 

 measurements (Richardson et al. 1979)) for each age of lot XI hard 

 clams, Mercenana mercenaria. Data from annual shell increments 

 deposited from 1969 to 1971. Number of annual shell increments 

 analyzed at each age is shown. 



annually induced MDM in 1969-71 in lot XI hard 

 clams (ages 3-10) are shown in Figure 6. Results from 

 other experimental hard clams were similar. Since 

 microgrowth increments were formed daily, these 

 data indicate that growth rates tended to be slower in 

 summer than in spring or fall of the same year. 

 Median average summer growth rates (dark band) 

 ranged between 21 and 33 jum/d, while those in 

 spring and fall (light bands) ranged between 3 1 and 48 

 /tm/d, respectively between 1969 and 1971 (Fig. 6). 

 However, these figures represent only growth rates 

 for days of growth and activity; there was a consider- 

 able number of inactive days in each annual shell 

 increment (Fig. 5) which would make the actual 

 seasonal average daily growth rate lower. There was 

 also a large range in average increment width in any 

 single band, and individuals in certain annual in- 

 crements had average increment widths associated 

 with dark bands which were greater than with either 

 or both light bands. This occurred in only 17 of 181 

 bands analyzed in hard clams from both lots XI and 

 II, or with a frequency of 9' < . 



Decreased growth rates associated with dark bands 

 were probably due to summer water temperatures 

 above the optimum for growth of hard clams (15°- 

 25°C; Ansell 1968). York River water temperatures 



'05 



