Metamorphosis in the Marine Snail Ilyanassa obsoleta, Yes or NO?

Metamorphosis is a crucial life-history event that can change an organism's form, function, behavior, and ecological interactions. In the Mollusca, several neurotransmitters and neuromodulators play inductive or inhibitory roles in the pathways that govern larval metamorphosis. Nitric oxide (NO) has been implicated in developmental processes in vertebrates and arthropods, but not previously in molluscs. We determined that NO donors block pharmacologically induced metamorphosis in the mud snail Ilyanassa obsoleta, whereas injections of inhibitors of nitric oxide synthase (NOS) allow competent larvae to become juveniles. We describe a new developmental role for NO, as an endogenous inhibitor of molluscan metamorphosis.