Aquatic animals employ different sensory modalities to extract information from their environment and have developed various specialized behavior patterns to adapt to their diverse watery habitat. The aim of the biennial symposia 'Behavior and Nervous System of Aquatic Animals' is to discover new principles underlying neurological foundations of aquatic animal behavior. Providing a collection of articles from researchers attending the latest symposium, this special issue discusses topics ranging from hydra to fish, and from anatomy and physiology to behavioral studies. The contributions report new data and points of view on brain morphology and ecological niche, the evolution of the forebrain, sympathetic, sonic and bilaterian nervous systems, the central mechanisms of swimming, the ocular melatonin rhythm, and the significances of unique Na+ channels. Researchers in neuroscience, particularly comparative and evolutionary neuroscientists, will find useful new information on aquatic animals that enhances the understanding of the biological significance of recent discoveries in the non-aquatic neuroscience.