outside of a few basal animals that have no brain or a very simple one, no animals have been found to date that satisfy any of these criteria. while some varieties of shark, such as great whites and hammerheads, must remain in motion at all times to move oxygenated water over their gills, it is possible they still sleep one cerebral hemisphere at a time as marine mammals do. however it remains to be shown definitively whether any fish is capable of unihemispheric sleep.