Reproduction and Development in Calcareous Sponges: A Panorama of the Last Two Centuries.

Nearly 200 years ago, scientists began examining reproductive cells in calcareous sponges (Calcarea, Porifera). Since then, 238 studies have been published, encompassing descriptions of gametes and embryos (including ultrastructural studies), life history, asexual reproduction, regeneration, and molecular aspects of development. Here, we present a comprehensive synthesis of two centuries of research on the development of Calcarea and review the different steps of the reproductive processes within this class. We analyzed temporal trends, thematic foci, authors, and geographic distribution of the studied species, and the reproductive characteristics of both subclasses (Calcinea and Calcaronea). Bibliographic productivity increased in gradual waves, from classical histology through ultrastructural and experimental embryology to advanced live imaging and molecular approaches. The past half-century has shown acceleration, yet with a persistent geographic bias toward the Mediterranean and North Atlantic species. Molecular datasets are beginning to link phylogenetic context and gene expression during development, but remain fragmentary and lack standardization within the class. We outline priorities for the coming years and identify critical unanswered questions. While significant challenges remain, confronting them actively advances research on Calcarea reproduction and development and firmly establishes it as a distinct field within sponge reproductive science.