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

More than a century of speculation about the evolutionary originof the contorted gastropod body plan has been inspired by adultanatomy and by long-standing developmental observations. Theresult has been a concept of gastropod torsion that I call the"rotation hypothesis." Under the rotation hypothesis, gastropodsoriginated 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 evolutionaryrotation is echoed during early development of patellogastropodsand vetigastropods and occurs to some extent during developmentof more derived clades. However, comparative developmental dataon ontogenetic torsion are minimal and I argue that the rotationhypothesis is a tautological argument. More recent studies onrepresentatives from 3 major clades of gastropods suggest thatthe highly conserved aspect of gastropod development is notsynchronous rotation of all components of the visceropalliumrelative to the head and foot but rather a state of anatomicalorganization in which the developing mantle cavity is on theright but the shell coil is posterior (endogastric orientation).This conserved state of developmental anatomy has inspired analternative hypothesis for the evolutionary origin of the gastropodbody plan, the "asymmetry hypothesis." Under the asymmetry hypothesis,the gastropod mantle cavity originated from 1 side only of abilateral set of mantle cavities. The asymmetry hypothesis doesnot require a saltation event to explain the origin of gastropods,nor does it require that the ancient molluscan precursor ofgastropods carried the shell coil over the head (exogastricorientation).