Centimeters 



Figure 36. — Shell of an adult C. virginica showing periodic 

 changes in the direction of the principal axis of growth. 

 Note the zigzag line of pigmented bands in the middle of 

 the valve. Actual dimensions: S.5 by 6 cm. (3.25 by 2.5 

 inches). 



terminations of tlie effect of local conditions on 

 growth and shape of shells. 



CHALKY DEPOSITS 



The glossy, porcelainlike inner surlacc of an 

 oyster shell is frequently marred hv irregularly 

 shaped white spots wliicli consist of soft and 

 porous material of different appearance and text- 

 ture than the surrounding shell suhstance. These 

 areas are called "chalky deposits". They are 

 very common in C. nrginica and 0. edulis. Since 

 the first record of their presence in edible oysters 

 made by Gray (1833) they have been mentioned 

 frequently by many biologists. Recent review 

 of the literature on the sul)ject is given l)v K<ir- 

 ringa (1951). 



Tlie e.xact location of chalky deposits is <if 

 interest since some si)eculations regarding their 

 role and origin are basefl on the position they occu- 

 py on the shell. Orton and Amirtlialini;aiii 

 (1927) assumed that chalky material is foiined in 

 the i)laces where the mantle loses contact uilli 

 the shell. No experimental evidence in support 



I 



UJ 



m 



WWsA/v- 



05 0.6 0,7 0.8 0,9 tO I I \.2 \ i 

 INDEX OF SHAPE OF C, virginica 



Figure 37. — Histogram of the distribution of the index of 

 shape (height + width) of shells of C. virginica from the 



length 

 Atlantic Coast. Frequency distribution of the index of 

 Nortli Atlantic oysters (open circles) and South Atlantic 

 oysters (points) are shown by two separate curves. 



of this explanation was presented by the authors 

 or by Ranson (1939-41), who fully accepted tlie 

 theory without making additional studies and 

 stated positively that chalky deposits are formed 

 wherever there is a local detachment of the mantle 

 from the valve. 



( 'onsidering the possibility that the mantle may 

 be more easily detached from the valve if the 

 oyster is placed with its lower (cuplike) side upper- 

 most, K<irringa (19.")!) made a simple field ex- 

 periment. In one tray he placed 25 metlium 

 sized (>. t'l/iilis in their normal position, with their 

 cupped valves undermost; the other tray contained 

 an equal number of oysters resting on their flat 

 valves. At the end of the growth season he oli- 

 served no significant differences in the deposition 

 of shell material in the oysters of the two groups. 



To determine whether chalky deposits are 

 foi-mcd in |)laces of partial detachment of the 

 mantle, 1 performed the following e\])eriment: 

 Small pieces of thin plastic about 1 cm.'' were bent 

 as shallow cups and introduced between tlie 

 mantle and the shell of ('. rircjirnca. In 10 

 oysters the cups were inserted with the concave 

 side facing the mantle, in anotiier 10 oysters the 

 position of the cup was reversed, i.e., the concave 

 side faced the valve. The oysters were kept for 



32 



FISH AND WILDLIFE SERVICE 



