File:Bulletin (1971-) (20235316758).jpg

Title: Bulletin
Identifier: bulletin7073sout (find matches)
Year: 1971- (1970s)
Authors: Southern California Academy of Sciences
Subjects: Science; Natural history; Natural history
Publisher: Los Angeles, Calif. : The Academy
Contributing Library: New York Botanical Garden, LuEsther T. Mertz Library
Digitizing Sponsor: The LuEsther T Mertz Library, the New York Botanical Garden

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IV 7 3 MAdNI'.SIIIM AND MKONTIIIM IN Mil I I \ (>l flVEl 45 a. a. 350 - ^\MA 250 - I\^0A 150 - I0A °~^\Z^' \\k<r y^ ^ 1 1 B o-t^—o- I0B <r \. II B I0B 50 - Sr = 2400t 300 Mg=270 t20 Sr= 1900*300 Mg=220120 POSITION ON SHELL Figure 3. Results of magnesium deposition experi- ment after three months in circulating sea water. A and B represent the halves of the shells put in sea water for prolonged exposure and those that were not, respectively. of shell calcification. She concludes that the epithelial regions of the viscera are responsible for secretion of the shell layers including the periostracum and the two calcareous layers in gastropods. An extensive literature exists on depo- sition of calcium in various genera and species of molluscs. The prismatic layer thickens from the beak radi- ally, and the nacreous layer thickens in the reverse direction (Fig. 4). Dark colored growth rings commonly present on the shell can be seen dis- tinctly in the Torrance population, but they are lacking in the Pismo population. Their origin is due to a decreased rate of growth in August of each year when the temperature is highest and the seas invariably most calm (Fitch, 1950). The clams from Pismo Beach display a slight deviation from the Torrance Beach clams in that their mantles overlap the pallial line and extend almost to the shell margin. The exterior layer (Fig. 4) is the prismatic layer, and the interior layer is the nacreous layer. The contact on the interior plane of the valve where the nacreous layer and prismatic layer abut, pinpoints the outermost position where the mantle was attached to the valve. It has been established by Pilkey and Hower (1960), that MgCO:! content is directly related to water temperature and salinity, and that dif- ferent species may secrete substantially different amounts of MgCO:» under similar environmental conditions. Harriss (1965), however, proposed that in the case of magnesium, mineralogy is the
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Mg = 170120 Figure 4. Cross section ol clam X depicting the relationship of concentration values in ppm at two key positions. dominant controlling factor in its distribution. He also staled that metabolic processes and crystal growth considerations are of secondary impor- tance with respect to magnesium, but of major importance in strontium distribution. This latter point was initially determined by Odum (1957). He stated that the main factor controlling the Sr C'a ratio was due to biological differences be- tween groups of organisms. Thus, the main dis- crepancy in the literature exists between Pilkey and Hower (1960) and Harriss (1965) as water temperature coupled with salinity and mineralogy are the respective arguments for the dominant factor in MgCO:! deposition. In order to discuss the results of the analyses presented in this paper, the following point must be analyzed. Cox (1969) has shown that alternat- ing layers of conchiolin (a scleroprotein) and aragonite are present in the nacreous layer. The fact that organic compounds may have strontium and magnesium as constituents has been outlined by Dodd (1967). These considerations present a problem which must be solved in order to validate the results presented in this paper. If the conchio- lin contains magnesium or strontium which has entered the solution phase in the analyses, then the results of this study will be altered by that factor. It is possible, however, that a variation of mag- nesium or strontium may be substantiated if the problem is approached indirectly. The nacreous layer contains much less magnesium than does the prismatic layer, therefore, it would seem that in the outer margin of the shell where the nacreous layer does not exist (Fig. 41. there would be an increase in the magnesium present. Instead, there is a significant decrease. Therefore, a variation in the carbonate material is still a strong bility. Figure 4 presents a cross-section of clam

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