Modern insights on gastropod development: Reevaluation of the evolution of a novel body plan.

More than a century of speculation about the evolutionary origin of the contorted gastropod body plan has been inspired by adult anatomy and by long-standing developmental observations. The result has been a concept of gastropod torsion that I call the "rotation hypothesis." Under the rotation hypothesis, gastropods originated when all components of the visceropallium (shell, mantle, mantle cavity with contained structures, and viscera) rotated by 180° relative to the head and foot. This evolutionary rotation is echoed during early development of patellogastropods and vetigastropods and occurs to some extent during development of more derived clades. However, comparative developmental data on ontogenetic torsion are minimal and I argue that the rotation hypothesis is a tautological argument. More recent studies on representatives from 3 major clades of gastropods suggest that the highly conserved aspect of gastropod development is not synchronous rotation of all components of the visceropallium relative to the head and foot but rather a state of anatomical organization in which the developing mantle cavity is on the right but the shell coil is posterior (endogastric orientation). This conserved state of developmental anatomy has inspired an alternative hypothesis for the evolutionary origin of the gastropod body plan, the "asymmetry hypothesis." Under the asymmetry hypothesis, the gastropod mantle cavity originated from 1 side only of a bilateral set of mantle cavities. The asymmetry hypothesis does not require a saltation event to explain the origin of gastropods, nor does it require that the ancient molluscan precursor of gastropods carried the shell coil over the head (exogastric orientation).