FISHERY BULLETIN: VOL. 82, NO. 1 



interruption in shell deposition as did a 66.4 mm 

 shell-length specimen collected during a 1980 winter 

 clam survey (Fig. 7 a, b). A depression outlined the 

 entire shape of the clam at the time of its formation 

 and at 25.9 mm shell length. The shell formed before 

 the depression was raised laterally above that formed 

 afterwards in a shinglelike fashion. Both valves of the 

 clam showed the interruption. The smaller shell 



shape was only slightly atypical for ocean quahogs, 

 and no irregularity was found as an indication that an 

 injury had occurred. The ratio between the greatest 

 shell height (21.4 mm) and shell length (25.9 mm) of 

 the smaller shell was 0.826, and for the entire shell 

 (50.2 mm shell height; 66.4 mm shell length) was 

 0.756. These values suggest that growth after the 

 depression departed from the more typical, isomet- 



10mm 



0.1 mm 



FIGURE 7.— (a) Right valve of an ocean quahog, Arctica islandica, 23 yroldand 66.4 mm in shell length collected 

 near the marking site during 1980. An obvious interruption of growth (arrow) and radiating lines in the anterior 

 half of the valve are shown, (b) Optical photomicrograph (compound microscope) of an acetate peel of the sec- 

 tioned valve at the site of growth interruption. Scale bars of magnification are included. 



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