INITIATION, CALCIFICATION, AND FORM OF LARVAL ARCHAEOGASTROPOD SHELLS

The coiled shell of gastropods begins as a cap-shaped lens of organic and calcified material that covers the posterior dorsal side of the la rva. During development the cap enlarges to cover the larval visceral mass. Marginal growth then produces the characteristic coiled shell. O ne model of the initiation of shell coiling in ''archaeogastropods'' r equires that the shell remains flexible and uncalcified until after to rsion, and that muscle contraction during torsion deforms the shell. W e describe early shell calcification and tested this requirement of th e model for the patellogastropod limpets Tectura scutum and Lottia dig italis, the trochids Calliostoma ligatum and Margarites pupillus and t he abalone Haliotis kamtschatkana. We determined the stage of initial calcification by staining larvae with the fluorescent calcium marker c alcein and observing them with bright field, crossed polarizing filter , and fluorescence microscopy. In T. scutum the earliest observable sh ell was calcified and calcium was sometimes detected even before the i nitial shell was visible. Larvae of the other species deposited a nonc alcified matrix that was subsequently calcified, and in C. ligatum and M. pupillus this initial calcification was distinctly spotty. Shells of both patellogastropods and the abalone were demonstrably rigid prio r to torsion while the shells of the trochids were not. These results suggest that shell coiling in patellogastropods and abalone is not ini tiated by contraction of the larval. retractor muscle during torsion; in trochids this mechanism is possible. However, analysis of camera lu cida drawings of pre- and post-torsional shells of T. scutum and C. li gatum did not detect shell shape changes during torsion. (C) 1998 Wile y-Liss, Inc.