Evolutionary consequences of tubular shells in a miniaturized-bodied gastropod: synchrotron imaging of Caecum laqueatum (Caenogastropoda: Truncatelloidea)

Tubular shells have evolved repeatedly across Mollusca, constraining both shell form and internal organization of the soft body. This convergent morphology is often linked to sessile or endolithic life, but the minute gastropod Caecum laqueatum C. B. Adams, 1852 remains mobile in interstitial habitats. Using high-resolution synchrotron microtomography, we reconstructed the soft anatomy of this species, resolving delicate systems, such as the nervous system and glandular oviduct. We identify several structures previously unreported in the family Caecidae, including a mantle (hypobranchial) gland, nephridial gland, and paired oesophageal glands, and we confirm a reduced monopectinate ctenidium. These results align C. laqueatum with other Truncatelloidea and provide a new anatomical reference point for the clade. More broadly, we show how tubular shell morphology interacts with small body size to shape internal anatomy, highlighting consistent patterns of elongation, organ reduction, and glandular rearrangement. Molluscan taxa with tubular shells, living and extinct, show recurrent solutions to mechanical and spatial constraints. In Caecum, the tubular form might itself be an adaptation that facilitates functional organization at small body size, enabling retention of a long gut and complex reproductive system. Once obscure and poorly studied, Caecum now provides a key anatomical reference point for recognizing convergence and recurring evolutionary patterns across Mollusca.