Early shell formation during molluscan embryogenesis, with new studies on the surf clam, Spisula solidissima

Abstract

Shell formation in molluscs begins early in embryogenesis during some stage of archenteron formation. Ultrastructural information on early formation of external shells is available from only a few bivalves and gastropods. Secretion of the very first shell material by shell field epithelial cells is preceded by an invagination of the dorsal ectoderm in the region of the shell field. A century ago, this invagination was termed the "shell gland." As a secretory function for this invagination has not yet been demonstrated and as the term "shell gland" has taken on various meanings in the literature, the invagination will be referred to as the shell field invagination. The opening into the shell field invagination seems to be circular in gastropods and elongate in bivalves. Accordingly initial organic shell material seems to form a ring in gastropods and a saddle in bivalves. As in adult molluscs, shells of pre-metamorphic molluscs are composed of both organic and inorganiccomponents. Ultrastructural data from bivalves and gastropods indicate that the initial organic shell material is secreted just outside the shell field invagination (across the pore). Initial inorganic shell materials have not been localized nor their pathway traced into or through any pre-metamorphic molluscs. New SEM and TEM data show that the invagination in the bivalve Spisula solidissima is composed of a wide outer region and very narrow inner region with the first shell material forming at the junction between the two. This is unlike ultrastructural data available for other species. Many sections give the false impressions that: 1) the shell field invagination is closed to the outside and, 2) that the first organic shell material lines the innermost region of the invagination. It is not clear whether the cells of the outer invagination in this species are shell field cells. It is suggested that they are not. © 1984 by the American Society of Zoologists.