ROPES KT AL.: GROWTH LINES OF OCEAN Ql'AHOC.S 



were not usually oriented in the above plane of radial 

 sections, so additional radial cuts were made to 

 expose growth lines in the notch area. Subsequently, 

 shells were embedded in an epoxy resin, and the cut 

 shell edge ground on wetable carborundum paper 

 and polished. Acetate peels were produced after 

 etching the shell cut surfaces in a 1 c /\ HC1 solution for 

 1 min, followed by microscopic examination of the 

 peels that were sandwiched between slides. 



Preparation of polished and etched radial sections 

 of marked and unmarked ocean quahog valves were 

 further examined by scanning electron microscopy 

 (SEM) 8 . These examinations included vertical tran- 

 sects from the periostracum to the shell's interior and 

 specific sites affected by the marking operation. 

 Shell microstructure was diagnosed by using the 

 classification scheme of Carter (1980), wherein shell 

 microstructures are elucidated on the basis of their 

 major (i.e., first-order) structural arrangement, inde- 

 pendent of genetic or optical crystallographic criteria. 



RESULTS 

 Whole Shells 



Notch marks showed clearly on wet shells but 

 periostracum obscured the ventralmost ends extend- 

 ing well beyond the ventral valve margin on all 

 specimens (Figs. 1-4). Cuts made in the shell-free 

 periostracum beyond the ventral margin of some 

 large quahogs had not been repaired after 2 yr (Fig. 

 4a); small individuals, however, had completely 

 formed yellowish-brown periostracum (grayish white 

 in the photographs) over new shell growth, which con- 

 trasted sharply with darker, earlier deposition (Fig. 

 3a). 



In some specimens, the mark formed U-shaped 

 notches at the marginal edge of the old shell. New 

 shell deposition was obviously disrupted for quahogs 

 with deep U-notches, since the marginal shell be- 

 tween the notches was outlined in relief over new 

 shell (Fig. 3b, c). Faint paired bulges were also found 

 on the ventral inner surface of the notched valve of a 

 few shells and occasionally the notches extended 

 part way onto the opposite valve. An occasional live 

 quahog was found with a cracked valve caused from 

 handling during the marking operation. The black- 

 ened margins of the cracks, suggestive of reducing 

 conditions, indicated that the cracks were old. There 



*SEM work was performed on an ETEC Autoscan instrument at 

 the Dental Research Center, University of North Carolina. Chapel 

 Hill. N.C.: on the JEOLJSM-35 of the Biology Department, Princeton 

 University, Princeton, N.J.; and on the ISI 1 200 of the Department 

 of Geology, University of Florida, Gainesville, Fla. 



was no evidence of repair by shell covering the cracks 

 in quahogs recovered 2 yr after marking. 



Sectioned Shells 



An interruption of shell deposition from notching in 

 some sectioned shells was visible without magnifica- 

 tion in the cut surfaces. Microscopic examination 

 revealed a depression that curved dorsally back into 

 the shell from the external surface and became 

 increasingly attenuated until it was unrecognizable 

 from the usual shell features along the inner margin. 

 This type of interruption was greatest in shells of 

 small quahogs, probably due to some mantle tissue 

 incision. Periostracum penetrated into the interrup- 

 tion to a depth of about 1 mm. The thicker and 

 tougher periostracum of large clams was less easily 

 incised during marking and probably served to 

 minimize incision of mantle tissue. 



Acetate Peels of Sectioned Shells 



Acetate peels enhanced detection of interruptions 

 in shell deposition due to notching (Figs. Id, 2d, 3d, 

 4d). In small clams (<80 mm shell length), the 

 interruption was immediately followed by a line 

 similar to a succession of lines formed throughout the 

 valve before marking. No additional lines were evi- 

 dent thereafter to the marginal tip in shells examined 

 from the late August 1979 recovery, but in 34 shells 

 of small clams recovered in early September 1980, a 

 second line occurred about midway to the tip in all 

 shells and a third line had formed very near the 

 marginal valve tip and along the inner margin in 47% 

 (Fig. 3d). These were all considered to be annual 

 growth lines for reasons dicussed later. Shell deposi- 

 tion between growth lines in the outer layer had a 

 granular appearance, which was sometimes broken 

 by a faint line of uncertain origin (Figs. Id, 3d). The 

 interruption of shell deposition from marking was 

 also evident in large ocean quahogs (Figs. 2d, 4d), 

 although an infiltration of the depression with perios- 

 tracum was not clearly evident. The separation of 

 shell deposits was more definite and extended 

 deeper into the shell of large clams, sometimes to a 

 depth of 2 mm (Figs. 2d, 4d). Growth lines were very 

 closely spaced (ca. 100 jura) and the shell 

 depositional texture in between lines appeared 

 similar to that seen in smaller clams. In large ocean 

 quahogs, new shell was formed laterally beyond the 

 notch mark and was an indication that the notching 

 operation had little effect on shell deposition and 

 growth (Figs. 5a, 6a). 



None of the marked quahogs had as severe an 



