weight bands of the soluble fraction of Mercenaria meraenaria on electro- 

 phoresis on 10% SDS polyacrylamide gels were eluted from the gel with 0.01% 

 SDS . The protein content, as estimated from an amino-acid analysis, of the 

 most prominent bands accounted for about 10% of the total protein loaded on 

 the gel. The remaining protein fraction of M. meraenaria is composed of 

 very high molecular weight components which do not enter the 5% polyacryla- 

 mide at the top, and lower molecular weight components which enter the gel 

 and form a dense background of nondiscrete molecular weight material. Since 

 the organic matrix of most species examined resembles M. mercenaria in this 

 respect, it is thought that the protein content of the discrete molecular 

 weight components constitutes only a small part of the soluble fraction pro- 

 tein, so that the degree of heterogeneity of these samples is probably much 

 greater than the sum of the observed components. The discrete molecular 

 weight constituents from the organic matrices of species of the same genus 

 are most similar. The presence of these components shows that organic ma- 

 trices are heterogeneous and presumably capable of performing diverse 

 functions. - J.L.M. 



2228 



Wilbur, Karl M. 1972. 



Shell formation in molluscs. Irj Chemical Zoology. VII. Mollusca. Marcel 

 Florkin and Bradley T. Scheer (eds.). Academic Press, New York: 103-145. 



Crenshaw (1971) found that mucopolysaccharide accounts for about 1/5 of the 

 dry weight of the non-dialyzable material in extrapallial fluid of Mercen- 

 aria, the remainder being protein. Extrapallial fluids of Meraenaria were 

 slightly alkaline, mean pH range being 7.33 to 7.41. When valves were 

 closed pH decreased slightly (Wilbur 1964) . Extrapallial fluid is probably 

 saturated with respect to calcite and aragonite (Crenshaw 1971a) . Ionic 

 concentrations of extrapallial fluid and blood in Mercenaria are not identi- 

 cal with Anodonta (Crenshaw 1971c) . Exposure to air gives some precipita- 

 tion of calcium carbonate in extrapallial fluid of Meraenaria (Crenshaw 

 1971a) . If residues in matrix of Meraenaria occur as glutamine and aspara- 

 gine, Ca ++ could not be bound to them (Crenshaw 1971b) . In the second 

 mechanism Ca ++ would be selectively bound to a highly sulfated glycoprotein 

 as in Mercenaria (Crenshaw 1971b) . One of the striking features of Mercen- 

 aria and other molluscan shells is its growth by increments (Pannella and 

 McClintock 1968; Pannella et al. 1968) . Shell growth in Meraenaria is a 

 two-way process, 'adding material when the shell is open, turning it over 

 when the shell is closed (Crenshaw and Neff 1969) . Changes in extrapallial 

 fluid have been described during closure of the valves. Succinic acid makes 

 up most of the organic acids in Meraenaria, lactic acid only about 2%. Equi- 

 librium was not reached in Mercenaria for several hours, as shown by sampling 

 extrapallial fluid with a catheter inserted through a hole in the shell 

 (Goddard 1966; Crenshaw and Neff 1969) . Mercenaria is normally aragonitic 

 and deposits only aragonite on regeneration (Wilbur 1964) . In Mercenaria 

 the aragonitic prismatic and nacreous layers have a similar Sr/Ca ratio 

 (Odum 1957) . - J.L.M. 



2229 



Wilbur, Karl M. 1976. 



Recent studies of invertebrate mineralization. In The Mechanisms of 

 Mineralization in the Invertebrates and Plants. Norimitsu Watabe and Karl 

 M. Wilbur (eds.). The Belle W. Baruch Library in Marine Science Number 5, 

 Univ. South Carolina Press, Columbia, S.C.: 79-108. 



From the soluble fraction of shell protein in Meraenaria mercenaria Cren- 

 shaw (1972) isolated a glycoprotein with high aspartic and glycine content 

 which specifically binds Ca. A possible method of binding may be chelation 

 of Ca by ester sulfate groups from two adjacent polysaccharide chains. The 

 glycoprotein is thought to initiate crystal formation by acting as a nucle- 

 ating factor by releasing Ca, thus bringing about a local increase in con- 

 centration and the precipitation of mineral. Our understanding of the 

 manner in which crystals of CaC03, together with organic material form a 

 molluscan shell, has been advanced considerably recently. The literature 

 has been reviewed by Taylor et aL 1969; Kennedy et al. 1970; Gregoire 

 1972; and Wilbur 1972. - J.L.M. 



618 



