a 







_L 



0.5 

 Centimeters 



1.0 



Figure 32. — Two small C. virginica growing attached to tar paper. Ma.ximum dimension of shell: a — n.S5 cm. 



b — 1.0 cm. At b the principal axis curves to the left. 



1.0, as, for instance, in C. rimilaris (fig. 8) and 

 0. (Alectryonia) megodon Hanley (fig. 3) (Olsson, 

 1961). Oysters of the latter species from the 

 Pacific Coast of Central and South America grow 

 singly, in vertical position, cemented to the rocks 

 by their left valves. The specimens 1 collected 

 on Pearl Islands, Gulf of Panama, measured 17 

 to IS cm. in height and 16 to 17 cm. in length. 

 The European flat oyster, 0. edulis (fig. 9) usually 

 forms rounded shells in which the length exceeds 

 the iieight. Small, noncommercial species, O. 

 sandwicliensifi of the Hawaiian Islands and 0. 

 mexicana from the Gulf of Panama, are almost 

 circular witli the tendency to extend in lengtli 

 rather than in height. Crowded conditions under 

 which these species thrive attached to rocks in a 

 narrow tidal zone greatly obscure and distort tlie 

 shape of their shells. 



Small C. virginica growing singly on fiat surfaces 

 without touching each other are usually round 

 (fig. 32). In a random sample consisting of 100 

 single small oysters (spat about 6 weeks old) 

 varying from 5 to 15 nun. in height and growing 

 on tar paper, the height/length ratio varied from 

 0.6 to 1.2. Xearly half of them (49 percent) were 

 perfectly numd (H/L ratio=l); in .30 percent the 

 ratio was less than 1 ; and in 21 percent the length 

 exceeded the height. 



In snuill single oysters less than 10 mm. in 

 height the principal (normal) axis of growth, is 

 clearly marked. All other radii synunetrically 

 oriented on both sides of the principal axis are 

 indicated by the pigmented bands on the surface 

 of the shell. The newly deposited shell, dis- 

 cernible at the periphery of the oyster, forms a 

 band which is wider at the ventral edge of the 

 shell and slightly narrows anteriorly and pos- 

 teriorly (fig. 32a). With the growth of the oyster 

 its principal axis is shifted to the side, curves, and 

 is no longer confined to one plane. The curvature 

 of the valve becomes a turbmate-spiral. Grad- 

 ually the oyster becomes slightly oval-shaped 

 and asymmetrical. 



The change in the direction of tlie principal 

 axis of growth is not associated with the environ- 

 ment since it takes place only in some of the 

 oysters growing under identical conditions. Oc- 

 casionally oysters are formed in which the pig- 

 mentation along the principal axis is so pronounced 

 that the dark band which marks its position may 

 be mistaken for an artifact (fig. 33) while the 

 secondary axes are not visible. The shells of 

 adult C. nnjinica usually curve slightly to the 

 left (if tiie f)yster is placed on its left valve and 

 viewed from above). Frequently, however, in- 

 verted specimens are found in whicli the growth 



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FISH AND WILDLIFE SERVICE 



