Streptoneury Is Independent From Ontogenetic Torsion in the Caenogastropod Snail Marisa Cornuarietis

A hallmark in snails’ anatomy is the conspicuous crossing of the pleurovisceral nerve cords present in all but the most derived gastropod clades. This feature is called streptoneury and hitherto near-universally believed to derive from the process of torsion which is, ontogenetically, visible by a 180° rotation of the visceral sac relative to the cephalopodium, being also responsible for the formation of a cranially bent gut and the location of gills in a mantle cavity that opens to the anterior. However, the mechanical link between the ontogenetic rotation of the visceropallium and streptoneury has never been demonstrated directly. After suppressing ontogenetic torsion in the freshwater apple snail Marisa cornuarietis, we could show in a 3D reconstruction based on serial sectioning that the nervous system of the non-torted snail almost identically mirrored the classical organization of a normally developed individual and showed all features of streptoneury in this species. Furthermore, confocal laser scanning microscopy revealed the pleurovisceral cords not to be fully shaped after completion of ontogenetic torsion. We therefore conclude that, ontogenetically, and potentially also phylogenetically, torsion is not an implicit prerequisite for streptoneury, thereby fundamentally challenging a century-old ‘certainty’ in molluscan developmental biology and evolution.