Larval settlement and metamorphosis are regulated by nitric oxide (NO) signalling in a wide diversity of marine invertebrates (-). It is surprising, then, that in most invertebrates, the substrate for NO synthesis – arginine – cannot be biosynthesized but instead must be exogenously sourced . In the sponge Amphimedon queenslandica, vertically-inherited proteobacterial symbionts in the larva are able to biosynthesize arginine (,). Here we test the hypothesis that symbionts might provide arginine to the sponge host so that nitric oxide synthase expressed in the larva can produce NO, which induces metamorphosis , and the byproduct citrulline . First, we find support for an arginine-citrulline biosynthetic loop in this sponge larval holobiont using stable isotope tracing. In symbionts, incorporated ¹³C-citrulline decreases as ¹³C-arginine increases, consistent with the use of exogenous citrulline for arginine synthesis. In contrast, ¹³C-citrulline accumulates in larvae as ¹³C-arginine decreases, demonstrating the uptake of exogenous arginine and its conversion to NO and citrulline. Second, we show that while Amphimedon larvae can derive arginine directly from seawater, normal settlement and metamorphosis can occur in artificial sea water lacking arginine. Together, these results support holobiont complementation of the arginine-citrulline loop and NO biosynthesis in Amphimedon larvae, suggesting a critical role for bacterial symbionts in the development of this marine sponge. Given that NO regulates settlement and metamorphosis in diverse animal phyla (-) and arginine is procured externally in most animals , we propose that symbionts may play a equally critical regulatory role in this essential life cycle transition in other metazoans.