Morphogenesis and chemical composition of spicules in Doridina (Gastropoda: Nudibranchia)

Among molluscs, hard skeletal elements are widespread. These structures are typically external and synthesized extracellularly, as seen in Polyplacophora, Caudofoveata, and Solenogastres. However, certain heterobranch gastropods—such as Nudibranchia, Acochlidimorpha, Rhodopemorpha—have subepidermal spicules that are synthesized intracellularly and remain within cells throughout ontogeny. Here, we present new data on the morphology and formation of such spicules in four nudibranch species from three families: Acanthodoris pilosa (Abildgaard in Müller, 1789), Adalaria proxima (Alder and Hancock, 1854) (both Onchidorididae), Cadlina laevis (Linnaeus, 1767) (Cadlinidae), and Palio dubia (M. Sars, 1829) (Polyceridae). Using a combination of morphological methods and experimental techniques—including micro-computed tomography, light and confocal laser scanning microscopy, and scanning and transmission electron microscopy—we identified common patterns of spicule maturation and biomineralization, which affect spicules’ internal structure, and delineated their growth zones. Raman spectroscopy further revealed variability in the mineral composition of spicules across different dorid groups.