SHELL INORGANIC COMPOSITION AND ONSET OF SHELL MINERALIZATION DURING BIVALVE AND GASTROPOD EMBRYOGENESIS

Abstract

Embryos of Spisula solidissima (Bivalvia); Crepidula fornicata, C. convexa, Ilyanassa obsoleta (Gastropoda: Prosobranchia); and Dendronotus frondosus, Aeolidia papillosa, and Hermissenda crassicornis (Gastropoda: Opisthobranchia) were reared in the laboratory and examined periodically to determine when shell mineralization began, as detected by birefringence under polarized light. Shell birefringence was detected at preveliger stages in all species. At onset of shell birefringence, samples were prepared for transmission electron microscopy to determine ifthe transitory shell field invagination (SFI) was still present. Contrary to previous reports for other molluscs, the present ultrastructural evidence indicates that at least for three species studied (S. solidissima, A. papillosa, and H. crassicornis), the SFI is still present during initial shell mineralization; thus for these three species the cells of the SFI may be involved in initial shell calcification. Electron cytochemical staining with pyroantimonate followed by electron probe microanalysis of thin sections indicated the presence of calcium within the lumen of the SFI in one species (H. crassicornis) fixed at onset of shell birefringence. These data suggest that the SFI may play an active role in initial molluscan shell formation. Analysis of veliger shell composition with scanning electron microprobe analysis indicated the variable presence of minor and trace amounts of Na, Mg, Al, P, S, Cl, and K in addition to large amounts of calcium. In two species (C. fornicata and A. papillosa) shell calcium carbonate was determined by X-ray diffraction analysis to be aragonitic.