sleep in fish is subject of current scientific research. typically fish exhibit periods of inactivity but show no significant reactions to deprivation of this condition. some species that always live in shoals or that swim continuously (because of a need for ram ventilation of the gills, for example) are suspected never to sleep. there is also doubt about certain blind species that live in caves. other fish seem to sleep, however. for example, zebrafish, tilapia, tench, brown bullhead, and swell shark become motionless and unresponsive at night (or by day, in the case of the swell shark); spanish hogfish and blue-headed wrasse can even be lifted by hand all the way to the surface without evoking a response. a 1961 observational study of approximately 200 species in european public aquaria reported many cases of apparent sleep. on the other hand, sleep patterns are easily disrupted and may even disappear during periods of migration, spawning, and parental care.