ROPES ET AL.: GROWTH LINES OF OCEAN OIAHOGS 



part of the shell (inner layer) is perhaps more 

 "... representative of the common, complex struc- 

 ture . . . and . . . there are alternating layers of more 

 transparent layers and finely grained ones." More 

 recently Taylor etal. (1969, 1973) examined the shell 

 microstructure ofArctica islandica, which they adopt- 

 ed as their "type species" to illustrate homogeneous 

 shell microstructure. Basically, the general picture 

 by Btfggild (1930) agrees with that of Taylor et al. 

 (1969), who used electron microscopy in their inves- 

 tigation. However, they disagreed sharply with 

 B^ggild that the inner shell layer was "representative 

 of the common complex structure." After examining 

 unetched fractured sections and polished and etched 

 sections of both shell layers, Taylor et al. (1969, 

 1973) concluded that both shell layers in Arctica 

 islandica are built of minute, irregular rounded 

 granules, quite variable in size (1.5-3 fim across), 

 having highly irregular contacts with their neighbors 

 and being poorly stacked. Taylor et al. (1969:51) 

 further reported: "In peels and sections of the inner 

 layer, within the pallial line there is a marked colour 

 banding, in greys and browns. The only fine structure 

 that can be resolved is a suggestion of minute grains, 

 which are most conspicuous in the translucent, 

 grey-colourless parts of the shell. These grains are 

 arranged in sheets parallel to the shell interior. In the 

 outer layer grains can also be resolved, but are 

 arranged in sheets parallel to the margin of the shell 

 and growth lines." They also noted that these 

 features are more clearly seen in the umbonal region 

 where the orientation of grains normal to layering is 

 very conspicuous. Taylor et al. (1969) suggested that 

 the layering is a reflection of repeated (?diurnal) 

 deposition of carbonate (a prospect deemed very 

 unlikely by Thompson et al. 1980a). Also in the 

 umbonal region are thin (2-3 ju.m) prismatic bands 

 which parallel the layering. Outside the pallial line, 

 Taylor et al. (1969) reported the outer shell layer to 

 be very dense and opaque, with the most obvious 

 structural features being fine grains arranged in 

 sheets giving the layer a finely banded appearance. 

 Analyses under SEM of oriented fractured, and 

 polished and etched sections of ocean quahog shells 

 revealed that microstructural variation is more com- 

 plex than had been proposed by Btfggild (1930) or 

 Taylor et al. (1969, 1973). Thin sections of isolated 

 periostracal fragments examined under crossed 

 nicols confirmed the presence of embedded 

 aragonite granules in the periostracum of ocean 

 quahogs reported for other recent bivalves (Carter 

 and Aller 1975). These granules probably form a 

 layer like that described for the blue mussel, Mytilus 

 edulis, by Carriker (1979). After special treatment of 



the valves for examination by SEM, he found "a thin 

 discrete calcareous layer continuous over the outer 

 surface of the valves between the periostracum and 

 the outermost shell layer." The layer is called mosaio- 

 stracum. The shell microstructure in the growth incre- 

 ment sublayer beneath the periostracum is HOM, as 

 B0ggild (1930) and Taylor et al. (1969, 1973) re- 

 ported. The "... minute, irregular, rounded gran- 

 ules . . . have highly irregular contacts ..." (Taylor 

 et al. 1969:51) that are particularly well exposed in 

 fracture sections. An abundant transitional CA- 

 CL microstructure was found in the middle portion 

 of the outer shell layer and growth increment sub- 

 layer. This study confirmed its presence in ocean 

 quahogs as reported by Carter (1980). The growth 

 line sublayer of the outer shell layer had four 

 grades of prismatic structure (FP, SphP, ComP, and 

 ISP). Lutz and Rhoads (1977) examined the inner 

 shell layer near the umbo of ocean quahogs and found 

 bands of simple aragonitic prisms alternating with 

 complex-crossed lamellar and homogeneous struc- 

 tures. We found similar microstructures in the inner 

 shell layer of the valve of ocean quahogs. Our 

 analyses identified distinct microstructures, not 

 unlike those found in the valve for the growth line and 

 growth increment layers in the hinge plate. 



Growth line deposition more nearly approximates 

 an annual event than any shorter or longer interval. 

 Marked clams recovered in late August 1979 had 

 formed only one growth line other than the mark- 

 induced check soon after the notching operation in 

 1978. They had been free about 22 d longer than a 

 calendar year. Those recovered in early September 

 1980 all had formed the growth line soon after the 

 notching operation, like those recovered in 1979, and 

 a second line appeared midway to the ventral valve 

 edge, which in all probability had been formed after 

 the late August 1979 recovery effort. These clams 

 were free about 33 d more than 2 calendaryears since 

 the notching operation. A feature of the specimens 

 recovered in 1980 was that about half had formed a 

 third line very near the ventral valve edge and along 

 the inner margin. All of the narrow growth lines were 

 separated by relatively even, broad areas of growth 

 increment deposits suggestive of no more or less than 

 an annual interval for the deposition of growth lines, 

 even though the time of formation of such lines may 

 not correspond to an exact number of calendar days. 

 These observations confirm similar conclusions of an 

 annual periodicity of growth line formation by 

 Thompson and Jones (1977), Thompson et al. 

 (1980a, b), and Jones (1980). 



Radiometric techniques for aging bivalve shells 

 have recently been applied to ocean quahogs. 



17 



