How Single Cells Form Shells: Maturation and Secretion of Lorica‐Forming Material in the Tintinnid <i>Schmidingerella</i> (Alveolata, Ciliophora)

Tintinnid ciliates are distinguished by their loricae (shells), the key synapomorphy of these mainly marine planktonic protists. They can divide daily, with a considerable portion of biomass stored in the loricae. During each division, lorica-forming material (LFM) is produced and afterwards used by the proter (anterior division product) to construct a new lorica, while the opisthe (posterior division product) retains the parental one. Many aspects of lorica formation remain unclear, and no study describes the entire process from material maturation via secretion to assembly. Here, we present the first thorough investigation at cellular and sub-cellular levels, employing light microscopy on dividers and postdividers, as well as transmission electron microscopy on primarily cryofixed specimens from a Schmidingerella culture. Our study reconstructs LFM maturation, identifying two main developmental stages: the morula-shaped precursor granules and the mature granules. The latter cluster in the proter's ventral portion with a peripheral longitudinal strip of small granules embedded in large ones. Ultrastructurally and chemically, the mature granules of both size classes are identical. Through detailed live observations, we followed and documented, for the first time, the process of cell division, the behavior of the proter, the release of LFM granules, and features of lorica formation.