FISHERY BULLETIN: VOL. 82, NO. 1 



Local ICM 



Outer 

 Shell 

 Layer 



Inner 

 Shell 

 Layer 



Periostracum 



Pallial 

 Myostracum 



FIGURE 10.— Idealized, partial, radial cross section through the shell of a typical ocean quahog, Arctica islandica, 

 showing the distribution of shell microstructures. Ventral margin is toward the left. Section is located inside the 

 pallial line. Legend of acronyms: ICM= isolated crystal morphotypes; FP = fibrous prisms; SphP = spherulitic 

 prisms; CompP = composite prisms; ISP = irregular simple prisms; HOM = granular homogeneous; CA = 

 crossed acicular; CL = crossed lamellar; FCCL = fine complex crossed lamellar. 



growth line of ISP continued parallel to the earlier 

 growth lines. 



The typical outer shell layer structure formed by the 

 time of marking is noted in Figures 5b and 6b. These 

 figures clearly show the alternation of the growth line 

 and growth increment sublayers. However, im- 

 mediately following the marking event all specimens 

 showed a disruption in microstructural development, 

 especially out near the shell surface. This coincided 

 with the presence of loosely organized SphP 

 immediately following the marking event line (see 

 particularly Fig. 6c, d). The disruption zone con- 

 sisted of cavernous, poorly organized, microstruc- 

 ture (Fig. 5d). 



In all six shells examined, the growth line associated 

 with the marking event continued inward toward the 

 shell interior, even beyond the zone of epoxy penetra- 

 tion (Figs. 5b, 6b). When this line was traced inward, 

 it was indistinguishable from the many earlier formed 

 growth lines. Such a view is seen in Figure 5c located 

 well off the field of view Figure 5b, to the bottom left. 

 Here the growth line consistedof a diagonal ISP band 

 bounded on both sides by transitional CA-CL 

 microstructure. 



DISCUSSION 



The layering and separation between growth lines 

 and growth increments of small ocean quahogs (< ca 



60 mm shell length) are often visible macroscopically 

 on the external surfaces of whole valves and in the cut 

 surfaces of radial sections. However, macro- or mi- 

 croscopic examinations of large ocean quahog 

 valves are consistently frustrated by a lack of clear 

 differentiation of the same growth phenomena. Pre- 

 paration of acetate peels of shell cross sections, as 

 has been described and photographically docu- 

 mented, greatly enhances discrimination of the lines 

 and increments of growth throughout the range of 

 shell sizes. 



Past investigators of the microstructure of ocean 

 quahog shells described some of the basic com- 

 ponents, but did not clearly elucidate differences 

 between the lines and increments of growth. Sorby 

 (1879: 62) appeal's to have given the first description 

 of the structure of the Arctica {= Cyprina) islandica 

 shell: "In C'yprina islandica we have another extreme 

 case, in which the fibres perpendicular to the plane of 

 growth are so short as to appear like granules, though 

 the optic axes are still definitely oriented in the nor- 

 mal manner." B<2>ggild (1930:286) reported that he 

 was unable to confirm Sorby's observations. Instead 

 he stated that Arctica islandica belongs to a group of 

 species within the Arcticidae (= Cyprinidae) having 

 the least visible structure among all the bivalves. He 

 terms this structure homogeneous but suggests there 

 are small traces of other structures in the shell. 

 Boggild (1930) goes on to point out that the lower 



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