Developing nervous systems in molluscs: navigating the twists and turns of a complex life cycle.

Molluscs constitute a richly diversified phylum, containing abundant species that have successfully invaded a variety of habitats. Despite the long-standing importance of its various members as model species for neurobiology, research on the development of the molluscan nervous system has lagged behind that on several other phyla. Recent studies, however, have begun to sketch an overview of neural development during the complex life cycles of these animals, involving multiple larval and postlarval stages and often including processes of torsion and occasionally detorsion affecting the entire body plan. The first neurons appear early in life and innervate a variety of larval organs. The central and peripheral neurons that comprise the adult nervous system generally appear later in larval life. Metamorphosis involves the loss of many neurons and the gain of others, and yet more neurons change to accommodate transitions in modes of locomotion, feeding and habitat. But such large-scale transitions do not stop at metamorphosis as massive changes in body size and behavior occur during juvenile and adult stages, and the nervous system must change accordingly to meet the demands of expanding target tissues and the need to generate new behaviors. Work over the years has gradually revealed some of the genes important in molluscan neural development, but recent whole-genome, EST and microarray projects are now allowing much more rapid progress and providing a valuable molluscan perspective for understanding broader issues concerning the evolution of the nervous system across the animal kingdom.