BACKGROUND Invertebrates express opiate receptors and synthesize opiate alkaloids such as morphine and morphine-6beta-glucuronide. Most of this work has been demonstrated in immune and neural tissues of various invertebrates. We hypothesized that morphinergic signaling may also take place in Mytilus edulis gill since they are innervated, in part, with dopamine nerves. MATERIAL/METHODS Ciliary activity from excised gills was evaluated via stroboscopic synchronization of metachronal wave formation before and after drug exposure. Nitric oxide was determined in real-time via an amperometric probe following drug application. Real-time RT-PCR was performed on excised gill tissue to confirm the presence of the mu opiate receptor transcript. RESULTS Incubation of M. Edulis excised gill filaments reveal spontaneously lateral cilia beating in a metachronal wave of about 600 beats per minute, which was significantly decreased by morphine in a concentration dependent and naloxone reversible manner. Exposure of the spontaneously beating cilia to SNAP, a nitric oxide donor, also diminished the beating rate in a concentration dependent manner. Exposing the cilia to L-NAME blocked the morphine induced cilio-inhibition, demonstrating that morphine was working to inhibit the cilia via NO. Furthermore, the gill tissue contained mu opiate receptor transcripts, which was mu3 in nature. CONCLUSIONS As in mammals, opiate signaling is not confined to neural tissues. This report demonstrates the occurrence of opiate signaling for the first time in an invertebrate's respiratory tissue.