As the Conus shell grows by spiraling of the outer lip around the axis, profound internal shell dissolution thins the walls of the protected penultimate whorl from several millimeters to <50&m. Shell material is added to the inside of the spire and the anterior part of the columella. The resulting shell has a uniformly thick last whorl and thickened spire that enhance defense against crushing predators and a greatly expanded interior living space for the animal. The molluscan shell has gained prominence in recent years as an especially favorable system for the analysis of biomineralization processes (1-4). Much less attention has been paid to shell dissolution, a continuing, permanent, and profound process that alters exterior and interior surfaces of the shell in certain prosobranch gastropods (5-7). In the genus Conus, dissolution of the internal walls of the shell is particularly striking while shell material is added from within to thicken regions of the shell some distance from its growing edge. Although these renovations have not been studied previously, the resulting very thin inner wall structure has long been known (8) and was used as the primary character separating subfamilies of the Conidae in an early classification (9).In this study we addressed the following questions: (i) What regions and layers of the shell are involved in dissolution and thickening? (ii) Of the shell material deposited by the animal during its life, how much is later dissolved? (iii) How much of the animal's living space within the shell does dissolution produce? (iv) What is the adaptive significance of interior shell remodeling?
MATERIALS AND METHODSWe selected Conus lividus Hwass in Bruguiere, an inhabitant of tropical Indo-West Pacific coral reefs, as representative of the genus. Shells collected in Hawaii were filled with resin to preserve the integrity of the inner whorls and then were sectioned axially or transversely to permit measurements of shell thickness. Etched, polished, gold-coated sections were used to aid visualization of boundaries between shell layers. For scanning electron microscopy, pieces of fractured shell were cleaned in dilute sodium hypochlorite and coated with gold or gold/ palladium.
RESULTSIn C. lividus as well as most species in the genus, both spire and last whorl are conic; thus, the whole shell appears biconic, with a long, narrow aperture (Fig. 1). As the shell grows, its shape remains constant (10).To appreciate dissolution, one must understand how the shell is produced. Histochemically differentiated regions of outer mantle epithelium and underlying calcium gland cells secrete illustrating terms used. The outer lip is at right (also the animal's right side); the anterior end is at the bottom. Outermost calcified layer 1 appears white but contains a yellow pigment in C. lividus. Layer 2 contains dark purple to brown pigment visible in the photograph. Inner layers 3 and 4 appear white and indistinguishable from each other. Lines mark percentages of total aperture height and correspon...
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