We studied here neuron ultrastructure, synaptic plasticity and subcellular localization of NADPH-diaphorase (NADPH-d), a cytochemical marker for nitric oxide syntase, in the pedal ganglia of the Gray mussel Crenomytilus grayanus sampled from the polluted and reference sites in Amursky Bay (Sea of Japan) at lower and higher water temperature (in the beginning and the end of August, respectively). At lower temperature, neuroplastic changes in mussel ganglia prevailed: a sharp increase in the number of cytosomes in NADPH-d-positive neurons and a sharp decrease in the number of mitochondria in both NADPH-d-positive and NADPH-d-negative neurons. At higher temperature, neurodegenerative changes prevailed: disruption of a part of NADPH-d-negative axons and interneuronal contacts, formation of concentric lamellar structures in the neuropils, and accumulation of autophagosomes in NADPH-d negative neurons. The results suggest that the stress-induced production of nitric oxide in cytosomes of mussel neurons and plasticity of gap junctions have a neuroprotective effect.